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Chain and Ring Phosphorus CompoundsЧAnalogies between Phosphorus and Carbon Chemistry.

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[44] A. Fischli, M. Klaus, H. Mayer, P. Schonholzer, R. Ruegg, Helu. Chim.
Acra 58 (1975) 564.
[45] a ) S. Terashima, S. Yamada, M. Nara, Tetrahedron Len. 1977, 1001; b)
M. Nara, S. Terashima, S. Yamada, Tefrahedron 36 (1980) 3161.
[46] a) M. Gill, R. W. Rickards, J. Chem. SOC.Chem. Commun. 1979. 121: b)
TefrahedronLett. 1979. 1539: c) R. M. Christie, M. Gill, R. W. Rickards,
J. Chem. SOC.Perkin Trans. 11981, 593.
1471 M. Gill, P. Bainton, R. W. Rickards, Tetrahedron Lett. 22 (1981) 1437.
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SOC.Perkin Trans. 11981, 1783.
[49] J. C. Barriere, A. Chiaroni, J. Cleophax, S . D. Gero, C. Richer, M. Vuilhorgn, Helv. Chim. Acta 64 (1981) 1140.
[ S O ] K. Ogura, M. Yamashita, G. Tsuchihashi, Tetrahedron Lett. 1976. 759.
(511 a) T. Tanaka, S. Kurozumi, T. Tom, S. Miura, M. Kobayashi, S. Ishimoto, Tetrahedron 32 (1976) 1713; b) ibid. 32 (1976) 1893.
1521 V. Osterthun, E. Winterfeldt, Chem. Ber. 110 (1977) 146.
1531 Y. Koksal, V. Osterthun, E. Winterfeldt, Liebigs Ann. Chem. 1979.
1300.
[54] Planned Dissertation R. Walenta, Universitat Hannover.
[55] H. Schneider, Diplomarbeit, Universitat Hannover 1978.
[56] H. Schneider, Dissertation, Universitat Hannover 1980.
I571 M. Harre, E. Winterfeldt, Chem. Ber.. in press.
[SS] P. Raddatz, Dissertation, Universitat Hannover 1981.
1591 Forschungspraktikum M. Bohnenpoll, lnstitut fur Organische Chemie
der Universitat Hannover 1981.
(60) Forschungspraktikum A. Meier, lnstitut fur Organische Chemie der
Universitat Hannover 1980.
[611 J. B. Heather, R. Sood, P. Price, G. P. Peruzotti, S. S. Lee, L. H. Lee, C.
J. Sih, Tetrahedron Lett. 1973, 2313.
[62] P. Raddatz, Diplomarbeit, Universitat Hannover 1979.
[631 J. Schreiber, D. Felix, A. Eschenmoser, M. Winter, F. Gautschi, K. H.
Schulte-Eke, E. Sundt, G. Ohloff, J. Kalvoda, H. Kaufmann, P. Wieland, G. Anner, Helu. Chim. Acta 50 (1967) 2101.
1641 E. J. Corey, R. H. Wollenberg, Tetrahedron Lett. 1976, 4705.
[65] D. Seyfarth, J. Am. Chem. SOC.79 (1957) 2133.
1661 a) S. Yamaguchi, H. S. Mosher, A. Pohland, J. Am. Chem. SOC.92 (1972)
9254: b) S. Yamaguchi, H. S. Mosher, J. Org. Chem. 38 (1973) 1870.
[67] R. S. Brinkmeyer, V. Kapoor, J. Am. Chem. SOC.99 (1977) 8339.
[68] a ) C. A. Brown, A. Yamashita, J. Am. Chem. SOC.97 (1975) 891; b) M.
M. Midland, R. L. Halterman, Tetrahedron Letf. 22 (1981) 4171.
[69] E. J. Corey, R. H. Wollenberg, Tetrahedron Lett. 1976, 4701.
[70] W. C. Still, Tetrahedron 37 (1981) 3981.
1711 Y. Koksal, Dissertation, Universitat Hannover 1980.
1721 T. Mukaiyama, M. Usui, K. Saigo, Chem. Lett. 1976, 49.
[73] a) K. F. Burri, R. A. Cardone, W. Y. Chen, P. Rosen, J. Am. Chem. SOC.
100 (1978) 7069; b) P. Raddatz, E. Winterfeldt, Angew. Chem. 93 (1981)
281; Angew. Chem. Int. Ed. Engl. 20 (1981) 286.
[74] 0. Mitsunobu, Synthesis 1981. 1.
I751 P. A. Bartlett, F. R. Green, J. Am. Chem. SOC.100 (1978) 4858.
1761 Y. Koksal, P. Raddatz, E. Winterfeldt, Angew. Chem. 92 (1980) 486; Angew. Chem. Int. Ed. Engl. 19 (1980) 472.
[771 A sample of natural brefeldin A was kindly put at our disposal by Dr. A.
von Wartburg and Dr. J. Hauth, Sandoz AG, Basel.
Chain and Ring Phosphorus CompoundsAnalogies between Phosphorus and Carbon Chemistry**
By Marianne Baudler”
Dedicated to Professor Klaus Weissermel on the occasion of his 60th birthday
U p to about 15 years ago compounds with a skeleton of phosphorus chains or rings were regarded as “exotic” in the field of nonmetal chemistry. Aside from a number of examples of
molecules with two P atoms directly bonded to each other and a few sporadically discovered monocyclic ring compounds, only solids of undefined composition and structure were
known. Since then the state of our knowledge in this sector has made considerable progress: between PH3 and its derivatives on the one hand, and the high-molecular modifications of elementary phosphorus on the other, an unexpected variety of well defined compounds have been discovered, showing many similarities to the analogous compounds of
carbon. However, surprises can still occur even with “small” phosphorus-containing molecules, as shown by the likewise recently discovered field of phosphorus three-membered
ring compounds.
1. Introduction
Compounds with a skeleton of trivalent phosphorus
atoms are generally very reactive: they can easily be attacked by oxidation on the free electron pair of the P
atoms, and disproportionate like other nonmetal compounds containing homonuclear element-element bonds.
Thus, until recent times only a limited number of such
compounds were known[’], except for the much earlier in[*I Prof. Dr. M. Baudler
[**I
492
lnstitut fur Anorganische Chemie der Universitat
Greinstrasse 6, D-5000 Koln 41 (Germany)
Contributions to the Chemistry of Phosphorus, Part IlS.-Part
Baudler, Y. Aktalay, 2. Anorg. Allg. Chem., in press.
0 Verlag Chemie GmbH, 6940 Weinheim. 1982
117: M.
vestigated tetraorganodiphosphanes R2P-PR2. In the last
decade advances in preparative techniques[’’ and in the
ability of spectroscopic methods to characterize reactive
species led to a considerable broadening of our knowledge
of chain and ring-type phosphorus compounds, and concomitantly some striking analogies to carbon chemistry
have come to light:
The existence of constitutional and configurational isomers;
valence tautomerism in phosphorus compounds;
stable phosphorus three-membered ring compounds;
with
the existence Of phosphorus
ring systems.
0570-0833/82/0707-0492 $02.50/0
Angew. Chem. lnr. Ed. Engl. 21 (1982) 492-512
In this review article we shall demonstrate the similarities between phosphorus and carbon chemistry, discussing
the various classes of compounds with P, skeletons from
several points of view.
2. Phosphorus Hydrides (Phosphanes)
All compounds having a skeleton of trivalent phosphorus atoms can be regarded as derivatives of the corresponding phosphorus hydrides (phosphanes). Since 1965
an unexpectedly wide variety of binary phosphorus-hydrogen compounds have been found, in addition to the already long-known hydrides PH3 and P2H4, in the volatile
hydrolysis products of calcium phosphide or in the thermolyzates of dipho~phane[~"-~j.
The present situation is
shown in Table 1.
The general formula of the individual phosphanes can
be deduced with certainty from their mass spectra. To exclude disproportionation as far as possible during the investigation, the specimens of the substances are introduced
directly into the ion source of the mass spectrometer at
various vaporization temperatures ( - 196°C to 5OoC)
by means of a special injection
The ionization
takes place by bombardment with 10-12 eV electrons or
by field ionization. The identification of the low-hydrogen
phosphanes relies on the dependence of the field ionization spectra on the vaporization temperature of the specimens and the different proportions of the individual compounds present in different fractions. The correlation is
fully confirmed by means of electron bombardment excitation, by the constant intensity ratios of the corresponding
peaks with respect to those of other molecule-ions when
+
Table 1. Phosphorus hydrides (phosphanes) with more than two P-atoms in the molecule
Triphosphanes
Tetraphosphanes
Pentaphosphanes
Hexaphosphanes
Heptaphosphanes
Octaphosphanes
Nonaphosphanes
Decaphosphanes
Undecaphosphanes
Dodecaphosphanes
Tridecaphosphanes
Tetradecaphosphanes
Pentadecaphosphanes
Hexadecaphosphanes
Heptadecaphosphanes
Octadecaphosphanes
By analogy with the boranes, the individual compounds
are characterized by the number of phosphorus and hydrogen atoms, for example P3Hs is triphosphane(5), P3H3
is triphosphane(3), etc.
The phosphanes make up a number of homologous series with general formulas P,,H,+2, P,,H,,, PnH,.-2, . .. up to
P,,H,,_ ,4. The first series comprises the open-chain phosphanes, the second series the monocyclic compounds (and
with more than five P atoms, probably also single rings
with side chains), while the other series comprise the lowhydrogen hydrides with condensed ring systems. As can be
seen from the synopsis in Table 1, as the phosphorus content of the compound increases, the tendency to form
chain-type phosphanes P,,Hnc2 as opposed to ring compounds shows a marked decrease. From n = 5 upwards first
the monocyclic phosphanes P,,H,, and from n = 7 upwards
the polycyclic hydrides P,rH,,-2,, ( m = 1-7) are clearly
preferred.
Up to now it has been possible to prepare triphos~hane(5)'~', pentapho~phane(5)[~~,and
heptaphosphane(3)I6] in pure form. The other hydrides have only
been isolated and studied in the form of sometimes
strongly enriched mixtures. The reason for this is that the
very similar properties of the individual compounds, and
particularly their marked tendency to disproportionate,
makes them very difficult to separate.
Angcw. Chcm. In!. Ed. Engl. 21 11982) 492-512
the ionization voltage is reduced from 70 eV to the appearance potential. The occasional complete absence of lowphosphorus disproportionation products such as PH3,
P2H4, P3H3, and P3H5 shows that the phosphorus-rich
compounds are not formed thermally or by electron bombardment within the mass spectrometer itself. If the measurement conditions mentioned are observed, then clearly
the results will hardly be influenced by any simultaneous
disproportionation of the phosphanes in the mass spectrometer. In contrast, if the specimen temperature is increased to above 100°C, or if the ionization voltage is
raised above 15 eV, rearrangement or decomposition reactions clearly become predominant.
In addition to this, triphosphane(5), tetraphosphane(6),
pen taphosphane(7), and pentaphosphane(5) have been
characterized by a complete analysis of their phosphorus
nuclear magnetic resonance spectra (see Sections 2. I to
2.4). Compared to mass spectroscopy, this method has the
advantage that the phosphane specimens can be investigated below room temperature and possibly in solution, so
that disproportionation reactions can be reliably excluded.
However, in the case of compounds with high phosphorus
and low hydrogen contents, the applicability of nuclear
magnetic resonance spectroscopy for characterization purposes is often limited by unduly low solubilities[91.Satisfactory agreement is generally obtained between the composi493
tions determined for phosphane mixtures by mass spectrometry and by NMR spectroscopy.
The higher phosphanes with more than two phosphorus
atoms in the molecule are generally relatively volatile.
Their spontaneous combustibility in air gradually decreases with increasing phosphorus content. In contradiction to many statements in the literature and in textbooks,
diphosphane and its higher homologs are not explosive:
they burn quietly in atmospheric oxygen, giving a white
smoke of phosphorus pentoxide. However, explosions
with phosphanes can arise in two ways: the product mixture contains hydrogen at the same time, as is possible in
the volatile hydrolysis products of metal phosphides; then,
in presence of oxygen an explosive gas mixture is produced, which is ignited by the spontaneously flammable
phosphane. Also, when phosphanes disproportionate one
of the end products is PH3, whose boiling point lies at
- 88 " C ; thus, sealed glass ampoules containing phosphane specimens can suddenly be destroyed by explosion,
even when cooled in dry ice, owing to the build-up of PH,
pressure. O n the other hand, the disproportionation, which
takes place to an appreciable extent already above
- 30"C, and which is greatly accelerated by heat and light,
gives rise to phosphorus-rich, solid, yellow decomposition
products. The phosphanes of the series P,,H,,+, and P,H,,
from n = 3 upwards can be dissolved in diphosphane,
molten white phosphorus, cyclooctatetraene, and the hydrocarbon mixtures I-methylnaphthalene/phenanthrene
and tol~ene/phenanthrene~'~~~.
While the phosphorus-rich
hydrides are clearly stabilized in the white phosphorus and
the aromatic solvents, the solutions in diphosphane are extremely unstable and tend to decompose spontaneously. A
violent reaction sometimes takes place with chlorinated
hydrocarbons.
In 1968 Fehlner described a process for the gas-phase photolysis of diphosphane in a closed-circuit apparatus, by
means of which about 10 rng of 95%-pure 1 was obtained
in 100 min['4.'s1. The chain structure of 1 was established
in 1972, by analysis of the ,'P{ 'HJ-NMR
In recent years we were able to develop a reproducible
process for the production of nearly pure 1 in milliliter
quantities by the thermolysis of liquid diphosphane at normal pressure followed by fractionation of the thermolyzatel4]. In a typical experiment 81 mL[171of highest-purity
PzH4 was subjected to thermolysis for 25 h at 35 "C, yielding in the first instance 63 mL of clear yellow thermolyzate
with a content of 1 amounting to about 20% of the total
phosphorus in the reaction mixture (P-Yo). At this stage the
thermolysis was stopped to avoid any further reaction of 1
and formation of solid phosphorus hydrides, which initiate autocatalytic decomposition of the thermolyzate.
Compound 1 was then isolated by cocurrent distillation in
a mixture with P2H4 (pressure
torr, bath temperature
- 40 to -+ 90 C) followed by fractional condensation. At
-40°C 9.1 ml of a condensate of highly enriched 1 (86.9
P-%) was obtained, which still contained some P2H4 and
P4H6, and at - 196°C 45 mL of P2H4. By repeated distillation, the purity of 1 was improved to 96%. The degree of
difficulty of the process is comparable to that of the preparation of diphosphane"81.
1 is a colorless liquid which in the absence of air and
light remains stable for many days at -80°C. At room
temperature and in diffuse daylight it rapidly turns yellow
and later decomposes to give yellow solid products.
The triphosphane 1 can be regarded as a derivative of
monophosphane 2 and diphosphane 3, in which individual hydrogen atoms have been replaced by the bulkier PH,
group. This substitution increases the bond angle at the
O
2.1. Triphosphane(5)
The first indication of the existence of triphosphane(5) 1
was given by the presence of an excessive number of lines
in the Raman spectrum of diphosphane, P2H41"]. The increase in the intensity of these attendant lines when the
specimen was illuminated or warmed, with simultaneous
evolution of PH3, led to the conclusion that the diphosphane was undergoing disproportionation with 1 as the
product:
Mass-spectrometric identification of the compound was
achieved by Royen et a[. in 1964, who decomposed small
amounts of Mg3P, and Ca3P2 mixed with the corresponding arsenides, nitrides, or germanides with dilute hydrochloric acid, directly at the mass spectrometer['". Shortly afterward fractions strongly enriched in 1 could be isolated
on a preparative scale during the separation of the reaction
products of commercial calcium phosphide1'21with pure
water[3".'I. At approximately the same time, F e h l ~ e r [de'~~
tected the formation of 1 mass spectrometrically in the
thermolytic and photolytic decomposition of diphosphane.
494
phosphorus, as shown by the parameters of the 3'P-NMR
(see Table 2): the &downfield shift in the sequence 2-3-1
and the increase in the absolute value of
the negative 'J(PP) coupling constant following the transformation of 3 into 1 are, according to current experiencerZn1,attributable to a change in the hybridization of the
phosphorus-increasing s-character of the bonding orbitals. The relative high-field position of the PBsignal of 1
provides evidence that the hydrogen in the PH group is
more acidic.
Table 2. "P-NMR parameters of P,H5 (1). P2H4 (3), and PH, (2), solventfree, 1 and 3 at 213 K , 2 at 183 K [a).
Phosphane
1
3
2
6
'J(PP)
PA: -156.7
Pe: - 173.2
- 205.0
-235.1
-- 146.6
- 108.2 [b]
Ref.
141
(161
1161
[a] &Values with negative sign for upfield shift: coupling constants in Hz.
[b] Determined from the 'H-undecoupled room-temperature spectrum 119).
Angew. Chem. In/. Ed. Engl. 21 (1982) 492-512
Table 3. Raman spectrum of P3H5(l),solvent-free, 193 K. Frequencies, relative intensities (0 to 100) with polarization filter (parallel to the E-vector of
the laser light), degree of depolarization ppl, state of polarization, and assignment of the bands.
Frequency
[cm - '1
Int.
Assignment
y 2
I58
414
457
610
655
717
747
835
883
1062
2267
15
63
14
I
1
4
I
2
2
5
100
6(PPP)
V*(PP)
VsdPP,
6(HPP)
6(HPP)
6(H PP)
6(HPP)
6(H PP)
6(H PP)
6(H PH)
v(PH)
CP-H
y 2
H-Pa
I
I
H-Pa
CP-H
I
1
PH2
PH2
d. I-nPqH6
y 2
CP-H
I
CP-H
I
PH2
meso-nPqH6
phosphorus chains (4a and 4b). To the branched isomer
4b, which can be regarded as tris(phosphino)phosphane,
there corresponds an A3B spin system (Fig. le).
The broadening of the angle at the central phosphorus
atom has been confirmed from the Raman spectrum of 1l4]
(see Table 3). If the molecule is treated to a first approximation as a three-mass model of the Czv point group["',
the PPP valence angle is obtained as 104.5' ( Q HPH in 2 :
93.8°12'1;Q H P P in 3 : 95.2"1231).
The valence force constant
of the P-P bond amounts to 1.77 mdyn/A and the deformation constant of the PPP valence angle to 0.15 mdyn/
A.
2.2. Tetraphosphane(6)
Heating of liquid diphosphane and triphosphane(5)
gives rise to phosphane mixtures containing varying
amounts of tetraphosphane(6) 4[3.7.81.
Initially, the diphosphane and triphosphane thermolyzates consist only of
chain-type hydrides, which, when the thermolysis is continued, react further in part to form pentaphosphane(5) 5 .
Clearly, the formation of the cyclopentaphosphane is thermodynamically more favorable, and is in any case accelerated by autocatalysis. Phosphane mixtures with a content
of 4 of 35-40 P-Yo in addition to 1 (about 55 P-Yo), 3 (510 P-YO),and 5 (i
5 P-%) are obtained from diphosphane
thermolyzates (see Section 2.1) by removal of 2 and 3 a t
- 40 " C under high vacuum@I. In this way 100 m L of 3 can
b e made to yield about 15 mL of a markedly tetraphosphane-rich, solid-free, yellow phosphane mixture of medium viscosity. Further enrichment of 4 by distilling off 1
is impossible, because this causes further disproportionation, with the formation of 5. The highest content of 4,
amounting to 48 P-%, has been obtained by the thermolysis
of pure 1 at + 10°C under high vacuum[71.
Beginning with tetraphosphane(6), the structural problem of the chain-type phosphanes becomes increasingly
complex as a result of isomerism. If a n unbranched phosphorus chain is present, 4 should be characterized in the
3'P/'HI-NMR spectrum by an A A B B ' system for the two
different types of P atoms (in the middle and at the ends of
the chain). However, the spectrum obtained in practice
contains considerably more lines (Fig. 1). According to the
complete analysis of the spectrum[241,there are two reasons
for this: in the first place, when either 3 or 1 is subjected
to thermolysis, 4 is always formed as a mixture of the two
constitutional isomers, with unbranched and branched
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-512
Fig. I."PI'HJ-NMR spectrum of a PIHJ/P4Hd mixture at 190 K (36.434
MHz). a) Observed spectrum: the eight intense signals correspond to the A L B
system of P,H,. b) Calculated spectrum of P4H,. c)-e) Separate simulation
of the individual isomers (d.l-nP4H6,meso-nPnHs, iP4Ha).
O n the other hand, in isomer 4a both of the middle P
atoms are chiral centers, since each of them has three different substituents and a free electron pair. Assuming that
the inversion at the P atoms is slow in comparison with the
N M R timescale, which is plausible in the case of chaintype phosphanes below room temperature[251,there will
49 5
then be two configurational isomers that can be distinguished by NMR spectroscopy, namely the d,l- and the
rneso-forms. Both diastereomers are characterized by an
AABB' system (Fig. l c and d, respectively), that of the
rneso-isomer being shifted somewhat downfield[261.Superimposition of the calculated individual spectra (Fig. lb) reproduces the experimental P4H6spectrum very satisfactorily. At the same time, this provides a clear indication of the
existence of isotetraphosphane(6) and of the two configurational isomers of n-tetraphosphane(6). The relevant 3' PN M R parameters and the relative isomer frequency are
summarized in Table 4Iz4I,
P H
"-p3
I
"-p3
eryrhro
K-Pc3
I
JAB
=
=
6,
-131.3
-147.6
-178.7
=
6,
=
Jnn =
JAM
JKB=
JBR.
6 ~ =
. -137.5
6s. = -163.0
JAW
-
=
6A
68
K - P a
IB
-168.5
JAR.
= i24.2
= - 3.0
JAA,
JA.0
JBB.
+ P3Hs
-+
PzH4 + PzHs
3
1
+ P2H4
HzP-PH-PH-PHz
1
ID
15. 5
25. 1 %
Yo
y
-+
HZP-P-PH2
I
= -144.6
60, = -173.0
J A .=
~ .- 168.2
= -181.2
= JAW =
25.7
7.1
+
=+
of
of
3.
to
(4
+ PH3
2
PH2
4b
2.3. Pentaphosphane(7)
Phosphane mixtures of P2H4thermolyzates or fractions
of the diphosphane preparation with a high content of 1
and 4 show, in their "Pi 'HI-NMR spectra, additional lowintensity signal groups between - 100 and -180 ppm,
which correspond to pentaphosphane(7), 6l2'I. The proportion of 6 is at the most 5 - 10 P-%. Any further enrichment
leads to decomposition with the formation of cyclophosphanes. 6 is not stable for long periods even at -70°C.
Clear proof of the existence of 6 and of its structural arrangement is based on a complete analysis of the "P('H}NMR ~ p e c t r u m [ ~ 'According
-~~~.
to the latter, all four of the
possible isomers distinguishable by NMR spectroscopy,
namely d,l-isopentaphosphane(7) 6b and the three diastereomers of n-pentaphosphane(7) 6a (erythro/erythro, erythro/threo, and threo/threo) coexist; of these, 6b shows the
highest relative frequency.
Together with the data concerning the low-phosphorus
homologs, the ,'P-NMR parameters of 6 make it possible
to derive general relationships for the chemical shifts and
the 'J(PP)-coupling constants of chain-type phosphanes as
496
8.4 yo
2
cP--H
P-P
iP5H7
H
6b
IC
HZ
51. 0 %
3
4a
6a
threo
=
The ratio of 4a to 4b is influenced by the method
production of (4): in the thermolyzates of 1 the fraction
4b is always smaller than that in the thermolyzates of
Very probably 4a is formed preferentially according
reaction (a), and 4b according to reaction (b).
P,Hs
1
threo
pH
D'
=
JAB =
- 163.8
nP5H7
I
C P - H
eryrhro
H~P~-PAH-PKH-P~.H~
meso-4a
d.I-4a
41.7%
28.1%
=
erythro
H-Pc3
Table 4. "P-NMR parameters of iP4Hb (4b), meso-nP,H6 (meso-4a), and d.C
nP,H,, @.I-4a) in a P,H,/P,H,, mixture at 190 K . J in Hz.
6~
68
P H
I*
P H
IA 2
2
IA
I
a function of their structural parameters (u, b, c: number of
phosphorus atoms in the a, p, and y positions)[291:
c~(~'P) =224.7 + 26.4a +33.76- 6.012
'J(PP)= -110.5-31.6a-13.0b+I.9~
Clearly, the chemical shifts depend strongly on the chain
length and the extent of the branching: c1 and p PH2
groups bring about a strong downfield shift, the p-effect
being still greater than the CL effect. y PH2 groups bring
about a slight upfield shift. It should be noted that in alkanes, though the displacement increments for a, p, and y
carbons[301are quantitatively smaller, they are of the same
sign as, and of similar magnitude ratios to the corresponding increments in the case of the chain-type phosphanes.
These relationships allow predictions concerning the appearance of the 3'P-NMR spectra of the various structurally isomeric phosphanes P,,H,l+2with n =6-9. This offers
the prospect of being able to characterize these extremely
unstable homologs, obtainable only in low concentrations,
not only by mass-spectrometry but also by 31P-NMRspectroscopy.
2.4. Pentaphosphane(5)
When phosphane mixtures with a high content of 1 and
4 are subjected to mild thermolysis (-2O"C, high vacuum), the main product formed is cyclopentaphosphane
5 [5l.
P3Hs
1
P4H6
4
+ P4H6
-+
4
+ P4Hcj
4
-+
PSHs + 2 PH,
5
2
PsHs
5
+ PzH4 + PH,
3
2
Since 2 and 3 are removed continuously from the reaction
system, the final residue of a low-temperature vacuum
thermolysis of this type consists of highly enriched 5
(about 80 P-YO),in addition to 2, 3, 1, 4, and other phosphorus hydrides, which give 3'P-NMR signals between
6 = + S O and -80 and very probably also have cyclic
Angew. Chem. Int. Ed. Engl. 21 11982) 492-512
structures. Largely pure 5 (96 P-%) can then be obtained as
a dilute and relatively stable solution by extraction with
aromatic hydrocarbons and subsequent removal of the
more volatile hydrides under vacuumf51.The methanolysis
of tetrakis(trimethylsily1)cyclotetraphosphane 7 also leads
to 5 oiu a complex reaction, the product being obtainable
in 97% p ~ r i t y ' ~ ' . ~ ~ ] .
The compound is positively identified from the mass spectrum (20 eV, 23°C) with a high relative intensity of the M +
ion ( m / z = 1 6 0 ; rel. int. 97%), and in particular from the
AABB'C spin system in the 3'P('H]-NMR spectrum (Fig.
2, Table 5)"'.
Table 5. "P-NMR parameters of PsHs (5) in I-methylnaphthalene/phenanthrene ( = 1%) at 300 K; J in Hz.
6A
6B
- 36.4
- 31.7
- - 9.9
= 6 ~ =,
=
6C
6e.
=
=
JAN
-234.6
J A ,=
~ -220.6
JBC = Jet, = -219.4
J A W 5 JA,B = + 21.5
JAC = JA.C =
31.6
JAB
JEW
=
=
+
+
44.3
2.5. Heptaphosphane(3)
Heptaphosphane(3) 8 is one of the phosphorus-rich hydrides with a particularly high tendency of being formed.
Thus, it can always be detected in considerable concentrations by mass spectrometry in fractions from the preparation of diphosphane and in P2H4therm~lyzates'~~'.
According to uon S ~ h n e r i n g [ ~it' ~is, also observed as the main
product in the mass spectrum of the hydrolysis products of
Ba3P14and Ba2P7Cl.Preparative amounts of pure 8 can be
obtainedL6]by mild solvolysis of the trimethylsilyl compound 9136,61.
In this case 8 occurs in the form of white flakes which turn
pale yellow on drying and persistently retain traces of the
solvent. On contact with atmospheric oxygen there appears
to be no change, but subsequent mass spectrometry reveals
the presence of P,H30[371.On prolonged exposure to water
a number of phosphorus acids are formed. The X-ray
amorphous substance is completely insoluble in aliphatic
and aromatic hydrocarbons, alcohols, ethers, acetonitrile,
carbon disulfide, dimethylformamide, dimethyl sulfoxide,
hexamethylphosphoric acid triamide, as well as in 3 and in
molten white phosphorus. This phenomenon is attributable to a still unexplained process of physical aggregation,
which is also observed in substituted polycyclic phosphanes P,,R, (m< n) with small organic groups R (see Section 6). The aggregation forces are relatively weak, since 8
occurs with a high intensity in the mass spectrum (12 ev)
already at a vaporization temperature of 60 C, and can be
transformed by metalation directly into derivatives with
discrete P7H,, P7H2-, and P:- anions (see Section 3).
The compound 8 has a remarkable thermal stability; it decomposes and turns red at about 300°C.
The tricyclic structure of 8 can be reliably ascertained
by analogy of the molecular vibration spectra[61with those
of metalated derivatives of known structures (see Section
3).
O
3. Metalated Phosphanes
Fig. 2. Observed (upper) and calculated (lower) "P('HJ-NMR spectrum of
PsHs in I-methylnaphthalene/phenanthrene(= 1%) at 300K (101.25 MHz).
From the calculated 3'P-NMR parameters it is evident
that the configurational isomer obtained is the one with
the maximum trans-arrangement of the adjacent free electron pairs or hydrogen atoms. 5 is the basic compound of
pentaorganocyclopentaphosphanes,
the
long-known
(PR)5111,
which according to the results of X-ray structure
analysis show the same configuration for R = CF3[331and
~ ~ ~ ~ [ 3 4 1 .
Angew. Chem. Inr. Ed. Engl. 21 (1982) 492-512
Since the structures of metal phosphides have already
been reviewed in recent y e a r ~ l ~ ' . ~in~ ]what
,
follows we
shall only deal with the compounds obtained by direct metalation of phosphanes. In common with the entire chemical behavior of the higher PH, homologs-except for their
self-ignition in air and their easy disproportionation-their
metalation has also never been studied until very recently.
3.1. Trilithium Heptaphosphide
3 reacts with n-butyllithium or lithium dihydrogen phosphide with simultaneous disproportionation to give trilithium heptaphosphide 10 and phosphane 2 ; in the first
case n-butane is also formed'61.
497
nonspecific broad signal. A further rise of the temperature
produces at 50 C as the average spectrum a singlet at
6 = - 119, which becomes completely sharp at 80 "C (in
monoglyme). The low-temperature spectrum reappears on
cooling.
+
The first step is undoubtedly the metalation of 3 to lithium
trihydrogen diphosphide 11, which immediately reacts further with an excess of 3 with disproportionation and transmetalation to give the triphosphide 12. The latter under-
O
+
I '
5OoC
LiH3P2+ P2H4 + LiH4P3 + PH3
11
3
12
goes an analogous transformation to that of 11, so that tetraphosphides, pentaphosphides, etc. are produced in succession. Since the PH groups in phosphanes are generally
more acidic than PH2 group^[^.*^^, and are therefore also
preferentially metalated, not only do chain branchings occur preferentially but also polycyclic compounds can be
formed in addition to monocyclic ones.
A similar complex reaction results in the nucleophilic
cleavage of white phosphorus with lithium dihydrogen
phosphide, in which once more 10 is produced as the end
product in a yield of 95%[391.
3 P4 + 6 LiPHz + 2 Li3P7+ 4 P H 3
10
- 60°C
10 is stabilized by the addition of three solvent molecules (tetrahydrofuran, monoglyme), whose removal leads
to decomposition. With chloro(trimethyl)silane the silyl
compound 9, obtained by Fritz et al. in another
and with methyl bromide the methyl derivative P7Me3
131401,
are produced.
The structure of the P7 skeleton has already been elucidated by uon Schnering by X-ray structure analysis in the
case of the compounds Sr3P1J4'1,Ba3P1J421,
and 9[431:
it is a
tricyclic cage, analogous to P4S3.
The low-temperature 3'P-NMR spectrum of loi6](Fig. 3 )
shows three separate groups of signals at 6 = - 57, - 103,
and - 162 in the intensity ratio of 1 : 3 :3, corresponding to
the three different types of phosphorus atoms (at the apex,
in the negatively charged bridges, and in the three-membered ring). According to the result of a complete spectrum
analysisL441,
the finely split "doublet" in the high-field region and the low-field "quartet" correspond respectively
to the P atoms in the three-membered ring and to the single atom at the apex, while the pseudo-triplet is produced
P73
-
CIOHlO
by the P-nuclei bound to the lithium. Surprisingly, increasing line-broadening sets in above -35"C, until at room
temperature the only remaining observable feature is a
498
-3600
-1800
-70
-80
-5200
- 120
HZ
-160
PPm
Fig. 3. "P-NMR spectrum of Li,P, in [D&tetrahydrofuran (saturated at 195
K) as a function of temperature (36.434 MHz).
The reversible temperature variation of the 3'P-NMR
spectrum of 10 is caused by a dynamic process known as
reversible valence tautomerism (degenerate Cope rearrangement)1451
and has been thoroughly investigated in carbon chemistry in the case of b~llvalene[~~"-"~.
In fact, the
P:- ion and the CloHIohydrocarbon have three essential
structural elements in common: they each contain a threemembered ring which markedly reduces the activation energy for the Cope rearrangement, and in addition a threefold symmetry axis and three easily-movable electron
pairs.
In the course of the intramolecular rearrangement of the
P:- ion P-P bonds are continually broken and new ones
formed, and on account of the threefold symmetry axis,
the anion is repeatedly re-formed.
By means of three reversible Cope rearrangements of
equal statistical weight, each involving the breaking of one
bond in the three-membered ring ( - 1,2 or - 2,6 or - 1,6)
and the formation of a new P-P bond (+ 3,7 or 3,5 or
+ 5,7), the P atoms in the three-membered ring are successively transformed into bridge phosphorus atoms, while
the original bridge atoms change into three-membered ring
+
Angew. Chem. Inr. Ed. Engl. 21 (1982) 492-512
3
2 LiH,P5
15
atoms with migration of the electron pairs. Each of the
seven phosphorus atoms can end up in any of the possible
positions by appropriate further rearrangements, so that in
all there are 7!/3 = 1680 valence-tautomeric forms. At
room temperature this dynamic process is rapid in comparison with the N M R time scale, so that all seven P atoms
are equivalent. The activation enthalpy (AH* = 59.12 kJ/
''
well with the temmol) calculated by Gleiter et ~ i . ' ~agrees
perature-dependent "P-NMR studies, and is of the same
order of magnitude as in bullvalene (AH' = 53.53 kJ/
m ~ l ) [ ~ *Accordingly,
].
the P:- ion is a typical fluctuating
molecule.
The valence tautomerism is naturally limited to the heptaphosphide ion. In the methyl compound 13 the substituents are fixed by covalent bonds at the bridge phosphorus
atoms. Since the configuration of the latter is not specific
in the methylation of 10, 13 is obtained as a mixture of
13.
3
0
Li,HP,
-PZH4, PH,
-Li,P,
- LIX
14
10
p, H3'
P?H3
8
8
Lithium dihydrogen heptaphosphide 16 is also produced under modified stoichiometric conditions in the liThe best way to
thiation of 8 or the protolysis of
prepare it is by reacting 8 with Li3P, at - 78 " C , when it is
obtained as a bright orange, amorphous solvent ad-
- 2 LIX
Li3P,
10
The composition and the structure of 14 and 16 are established by a complete analysis of their 3'P-NMR spectra.
In the case of 16 it was only possible to analyze the spin
As
system for the symmetric isomer 16a (see Table 6)'49.501.
shown by the NMR parameters, the P, cage in 14 and 16
13b
two isomers (13a and 13b), of which, for steric reasons,
13a is preferred over a statistical frequency distributi~nl~~].
3.2. Dilithium Hydrogen Heptaphosphide and
Lithium Dihydrogen Heptaphosphide
In the metalation of 3 with n-butyllithium a red-orange
oil appears before the formation of 10; this consists essentially of dilithium hydrogen heptaphosphide 14'491.
Pure 14 can be obtained by reacting 3 with Li3P7(4.5 : l),
and is isolated by low-temperature crystallization as a
bright orange solvent
14 is also formed in the
partial metalation of P,H3 8 with LiPH, or 10, and in the
decomposition of lithium tetrahydrogen cyclopentaphosphide 15 and the partial protolysis of
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-512
16
16
is clearly distorted compared with 10 (shown by the broken lines), as a result of the partial hydrogen substitution.
Like dihydrobullvalene, 14 should be capable of a degenerate Cope rearrangement between two valence tautomers. According to temperature-dependent 3'P-NMR
coalescence signals develop as the temperature
is raised, thus confirming such a process.
At room temperature 14 and 16 decompose with the
formation of dilithium hexadecaphosphide 17, which can
be isolated in the form of large octahedral crystals as a solvent adduct. The P
:
, ion shows the constitution recently
determined by oon Schnering et ul.[38.5'1
in the case of the
Ph4P+ salt by X-ray structure analysis, as is evident from
the six signal groups (intensity ratios 2 : 1 : 1 :1 : 1 :2) in the
31P-NMRspectrum of 17 (Fig. 4).
499
Table 6. "P-NMR parameters of Li,P7 (10) [44], Li2HP7 (14) and LiH2P,
(symmetrical isomer 16a), in each case in tetrahydrofuran (saturated), 10
and 14 at 213 K, 16a at 193 K ; J in Hz.
o a
0
6~
- 57.0
- 67.5
6,
6t
- 103.0
-
6.4
6e
6F
9.0
- 119.4
fic
- 161.7
JCD
JDE - 265.3
JDG
- 79.2
83.7
-
0.0
0.O
45.4
3.3. Lithium Tetrahydrogen Cyclopentaphosphide,
Lithium Trihydrogen Diphosphide, and
Lithium Tetrahydrogen Triphosphide
In the low-temperature metalation of 3 with nBuLi or
LiPH2 (-7O"C), the first reaction product that can be detected by 3'P-NMR spectroscopy is lithium tetrahydrogen
cyclopentaphosphide 15'"]. Its formation undoubtedly
takes place in a rapid series of metalation and disproportionation reactions in which the highly reactive monolithiated chain-type phosphanes LiP,H,,+ I are first formed,
which cannot exist in the presence of 3. As in the case of
the hydrides, when the chain length is n = 5 cyclization to
the thermodynamically more favorable five-membered
ring compound takes place.
6(PdDex)
6(pbr,d%)
- 145.2
-215.9
134.8
- 113.1
-
- 199.8
- 199.8
6(Pxhree-mcmhered
ring)
-277.6
-304.7
-316.9
-226.6
226.6
-366.8
-
'Jpp(apex-bridge)
-398.9
-355.1
-370.2
-399'6
-315.1
-315.1
'Jpp(bridge-three-membered
LiH3P2
JAG
Jer.
-378.7
JEF
ring)
+ P A
+
+
38.6
-
15.3
Jvt
JCE
Jcc
56.5
15.4
59.5
+
+
19.6
'Jpp(bridge-bridge)
JAC
JAE
2Jpp(bridge-three-membered
JCF
3.4
+
+
7.4
9.4
5.0
7.6
2.9
9.3
+
+
+
+
+
JFG
91.9
28.6
28.6
+
-
- 5.2
+
+
+194.6
5.5
5.5
- 18.0
JEG
JBE
JBG
+
+
+
+
14.1
14.1
17.5
8.1
17.5
8.1
'JPp(apex-three-membered
ring)
LiH6P,
-PHx
+ P2H4-,LiH4P5 + 2 PH3
3
JA D
+ P,H*
12
11
LiH,PS
JtlD
LiH4P3 -P
+ PxHn
HI LiH5P4
-PHI
15
15 can be obtained pure in solution, and by analysis of the
"PI'HJ-NMR spectrum it has been clearly characterized as
the monolithium salt of pentaphosphane(5) 5 (Table
7)[5*'.
Table 7. ?'P-NMR parameters of LiH4P, in tetrahydrofuran (saturated) at
203 K ; J in Hz.
ring)
8~ = -101.9
60 = - 1.4
6c = - 79.3
6~ = - 50.6
6~ =
15.7
H
\
+
+
+
+
+
L
c'
Li
JAB
= -360.8
= -352.0
J B c = -240.3
Jco = -237.4
J D E = -219.6
JAC =
5.5
JAD= 12.8
JBD
=
25.9
JBE
=
80.0
JCE= i- 3.9
JAE
5
Above -30°C 15 disproportionates to form 14, for
which the marked stability of the tricyclic P7 skeleton is responsible.
-
2 LiH4P5
15
i
Li,HP,
If further amounts of the metalation agent are added to
the reaction system, also small quantities of lithium trihydrogen diphosphide 11 (6,= - 167.0, 6,= -271.7,
JAB=- 205.3 Hz) and lithium tetrahydrogen triphosphide
12 (6A= - 123.0, 6,= -251.7, J A B = -266.8 Hz; all at 203
K) are detectable by 3'P-NMR spectroscopy'521.
H2PA-PBH Li
11
8000
0
-8000
60
0
-60
-16000
-120
Hz
H2PA-PB(Li)-PAH2
12
4. Homocyclic Phosphorus Three-Membered Ring
Compounds
PQm
Fig. 4. "P-NMR spectrum of Li7P16in dimethylformamide (saturated) at 243
K (121.497 MHz).
500
+ PzH4+ PH3
14
Ever since the mass-spectrometric detection of the exisas the first member of the
tence of tripho~phane(3)'~"I
Angew. Chem. h i . Ed. Engl. 21 (1982) 492-Ji12
monocyclic phosphane series P,,H,,, the synthesis of isolable compounds with a phosphorus three-membered ring
skeleton has become a challenge for preparative nonmetal
chemistry. Although P3H3 can be separated in small
amounts from phosphane mixtures by gas chromatograp h ~ its~preparation
~ ~ ~ , on a preparative scale is made difficult by the extremely high rate of rearrangement into the
five-membered ring compound 5 . However, since the reactivity of phosphorus hydrides can generally be reduced by
suitable substitution of the hydrogens, the possibility of
synthesizing relatively stable triorganocyclotriphosphanes
(PR), did not seem to be excluded.
The first derivative of P3H,, the pentafluoroethyl compound 18, was described by Cowley et uZ.[54a1
in 1970. In
the reaction of the diiodophosphane 19 with mercury a
mixture of the corresponding cyclotriphosphane and cyclotetraphosphane was obtained, which could be separated
by fractional condensation. 18 was then identified by mass
spectrometry and by gas density-molecular weight determination.
However, a little later West et al.154b1
reported that in the
reaction taking place the five-membered ring compound
(PC2FJ5 was produced in place of 18. This controversy
persisted for over half a decade, until in 1976 Mills et
~ l . [ ' ~were
' ~ able to ascertain the identity of 18 by 31PNMR spectroscopy.
At the same time and independently, our Cologne group
succeeded after prolonged efforts[551in synthesizing the
phenyl compound 20, whose formation during the cyclocondensation of the diphosphane 2lcs6Iwith phenyldichlorophosphane was unambiguously demonstrated by 31PNMR spectroscopy[571.However, 20 cannot be isolated in
the pure state, since even in solution and below room temperature it transforms relatively rapidly via the cyclotetraMe,Si(Ph)P-P(Ph)SiMe,
21
A
(PPh),
22
-
+ PhPQ
(PPh)3
20
(PPh)S
23
phosphane 22 into the thermodynamically stable fivemembered ring compound 23.
4.1. Syntheses
It has been shown in recent years that triorganocyclotriphosphanes may have considerable kinetic stability, depending on the substituent R on the three-membered ring,
so that they are formed in various reactions and can often
be isolated in the pure state[3c1.
A general route for the generation of a phosphorus
three-membered ring skeleton is the [2 I]-cyclocondensation of a bifunctional 1,2-diorganodiphosphane with an organodihalophosphane. This reaction path leads to uniformly or mixed-substituted cyclotriphosphanes.
X(R')P-P(R')X
+ RZPYz
-
(PR')2PR2
-2XY
Up to now the following possibilities have been verified:
a) for X = SiMe3, Y = C1: R' = R2= Ph["]; R' = Ph,
R2 = Et(5'bl. R'= ph, R2 = MelS7bl; R' = ph, R2 = ipr[581;
R ' = Ph, R2= ~ B U [ b)
~ ~for
] ; X = K, Y = C1: R' = R'= t B ~ [ ' ~ l ;
R' = Ph, R2= ipr[58];R' = tBu, R' = ipr[60].
A comparison of the leaving groups X on the diphosphane shows that the formation of potassium chloride generally takes place already at considerably lower temperatures than does the cleavage of chloro(trimethy1)silane. The
latter is sterically hindered, particularly with bulky substituents R', and can thus be achieved only above 0°C or
room temperature, which is a disadvantage for the generation of thermally labile cyclotriphosphanes. On the other
hand, the basic phosphide attacks the already-formed
three-membered ring compound and favors a rearrangement into the comparatively more stable cyclotetra- and
cyclopentaphosphanes. Thus, the cyclocondensation of the
salts can only be successfully achieved in practice if the action of the educt on the reaction product is prevented by
carrying out the reaction heterogeneously in a nonpolar
solvent.
A synthetic route for cyclotriphosphanes that is nonspecific for the size of the ring is the dehalogenation of an
organodihalophosphane by a metal.
RPXz
+ MI' (2 MI)
-+
1
n
- (PR),,
+ M"X2
(2 MIX)
n=3,4, 5
The same reaction has long been used for the preparation of organo cyclophosphanes with n>3[']. Thus, it is
only preparatively efficient for cyclotriphosphanes when
these possess a considerable stability and show an appreciable tendency to form. This is the case for R=C2F5[54a1,
~ B U [ ~ 'sBuI~~],
. ~ ~ ] , iPrr6*I, hex^^^], and iC3F71641.
In spite of
this, one always obtains a mixture of oligomers of various
ring sizes, which has to be separated by fractional distillation or cry~tallization[~'-~~~.
However, this process is particularly suitable for making kinetically stable cyclotriphosphanes on a preparative scale.
Not significant from the preparative standpoint, but interesting on systematic grounds, is the cyclization of I ,3-dihalo-1,2,3-tri-tert-butyltriphosphanes24 which have recently become available1651to the tert-butyl compound
25[661.This reaction corresponds fully to the classical synthesis of cyclopropanes by dehalogenation of 1J-dihalogenpropanes; the use of lithium hydride instead of a metal
is advantageous, merely because of the temperature-sensitivity of 24.
X(rBu)P-P(rBu)-P(rBu)X
24, X = Ha1
+ 2 LiH
-
(PrBu),
25
+ 2 LiX + Hz
+
Angew. Chem. Inr. Ed. Engl. 21 (1982) 492-512
Surprisingly, the cyclohexyl compound 26 is obtained in
good yield by the action of carbon disulfide on the corresponding tetraphosphide 27. The first step in this complex
501
reaction is very probably the insertion of a CS2 molecule
into the metal-phosphorus bond.
K(cHex)P-(PcHex),-P(cHex)K
% (PcHex),
phanes of other ring sizes[721.
Accordingly, the 6(,'P) values
are influenced by the three-membered ring structure in the
same direction as the 6(,'C) values of the cyclopropanesi7']. The even more marked "small ring effect" observed with phosphorus compounds can be attributed to
the predominant s-character, and the consequently enhanced shielding effect of the free electron pairs on the
phosphorus atoms of the cyclotriph~sphanes[~~~~.
The uniformly substituted cyclotriphosphanes (PR),
show in each case an A2B spin system (Table 8), which
means that the substituents are undoubtedly arranged on
both sides of the ring plane. More precisely, the 6(,lP) values are determined not only by the endocyclic bond angles
but also by the exocyclic angles, the dihedral angle between the free electron pairs of adjacent P atoms, and the
o-effect[20al.
+ (PcHex)l + .
26
27
Finally, triorganocyclotriphosphanes are also formed in
the thermolysis of cyclophosphanes with n > 3, as has been
demonstrated for R = Et[s7a3671
and for R = Me[20a.681.
In agreement with the known stabilization of small ring
systems by bulky substituents, it has been possible so far to
isolate in pure form the uniformly and mixed-substituted
cyclotriphosphanes (P~BU),~'~.~'],
( P S B U ) , ~ ~(PiPr)3i621,
~~,
( P ~ H e x ) , [ ~(PiC3F7)3[641
~l,
and (PtBu),(PiPr)[""l, all as compounds stable at room temperature.
Of particular interest as a synthon is the monometalated
cyclotriphosphane 28. This is formed-together with other
open-chain and cyclic tert-butyl phosphides-in the ring
cleavage of the cyclophosphanes 29[691
or 25[701
with potas-
-
fBu,
+K
(PtBu),
29: n = 4
25: n
tBu
Y-f
R
t
K(fBu)P-P(tBu)K
P
&
/
3
28
A
tBu
tBu
b-P/
I
H
Table 8. "P-NMR parameters of the cyclotriphosphanes (PR),; J in Hz.
tBu,
+
.
68
6A
Ref.
J48
~~
Me
Et
cHex
iPr
Ph
fBu
- 171.0
- 157.3
- 145.0
- 157.3
- 170.8
- 178.5
- 185.0
- 141.7
- 138.9
- 132.3
- 128.6
- 147.2
- 184.8
-108.1
-201.1
-131.7
- 71.9
1681
I671
I631
I621
I571
I611
- 186.9
,tBu
+ UeOH
31
- UeSiOUe
P
I
SiMe,
30
Table 9. "P-NMR parameters of the cyclotriphosphanes (PR')2PR2;J in
HZ.
sium. By direct protolysis or-which is better from the preparative standpoint-uia the silyl derivative 30, earlier obtained by Fritz et al.[7'1in another way, this compound can
be converted into the partially hydrogen-substituted cyclotriphosphane 31['01. Compared with 25, 31 is kinetically
destabilized and even below room temperature a rearrangement takes place to form the four-membered ring
compound 32, with simultaneous partial decomposition.
R'
6~
R2
fBu iPr a
b
Ph iBu a
b
Ph
Ph
iPr a
b
Et a
b
68
6,
JAB
JAC
- 68.5 - 127.8
- 192.1
- 109.0 - 112.5 - 99.5 - 188.7 -205.1
- 137.4 - 162.9
- 185.6
- 149.9 - 148.7 - 140.9
- 140.3 - 134.1
- 152.3 - 123.6 - 136.9
- 138.7 - 151.8
- 148.3 - 141.6 - 138.8
- 180.0
- 195.3
JET
1601
-262.2
I581
-216.0
- 189.9
- 185.3
- 184.1 - 226.9
- 186.3
- 182.7
Ref.
- 184.8 -220.0
[SSl
I57bl
4.2. Properties and Structural Characteristics of
Triorganocytlotriphosphanes
The triorganocyclotriphosphanes made on the preparative scale up to now are colorless, somewhat viscous liquids or crystalline solids oxidizing easily in air. They are
readily to very readily soluble in ethers and in aliphatic
and aromatic hydrocarbons, polar solvents favoring the
rearrangement into cyclophosphanes with n > 3. Decomposition takes place with halogenated hydrocarbons.
All cyclotriphosphanes are characterized in their
31P('HJ-NMR spectra by chemical shifts at high fields
compared to open-chain triphosphanes or to cyclophos502
The configuration of the triorganocyclotriphosphanes
has been confirmed by crystal structure analysis of the tertbutyl compound 25[741.
Figure 5 shows the arrangement of
the substituents of the two independent molecules in the
crystal association.
In the mixed-substituted cyclotriphosphanes (PR'),PR2
there are two diastereomers. The symmetric isomer a with
identical substituents on one side of the ring shows an A2B
system in the 3'P(lH}-NMRspectrum, and the asymmetric
isomer b an ABC system (Table 9). If R' and R' are not
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-SJ2
bulky groups, both isomers will be formed in the statistical
ratio of a : b = 1 :2[3c1.
?
n > 3. Thus, depending on the substituents R and the polarity of the solvent, they undergo rearrangements at various rates to form the stable four-membered or five-membered ring phosphanes@'I. The individual reaction steps do
Fig. 5 . Structure of the two independent molecules of (PtBu), in the crystal
1741.
Fig. 6. Structure of (PtBu),Cr(CO), [77].
4.3. Reactions of Triorganocyclotriphosphanes
Insofar as the reaction behavior of triorganocyclotriphosphanes is known at all, one can distinguish between
reactions in which the P3 ring skeleton is preserved and
ones in which it is changed.
4.3.1. Reactions in which the P3 Ring Skeleton is Preserved
As a rule, additions on the X3 phosphorus in cyclotriphosphanes proceed with opening of the three-membered
ring structure. However, it has proved possible to make the
monothiocyclotriphosphane 33 by the action of sulfur on
the tert-butyl compound 25 under mild reaction conditions, and to enrich this compound up to 50 mol-% in the
reaction mixture[751.The compound is thermally unstable
and rapidly rearranges at room temperature into the
heterocyclic four-membered ring compound 34[751.
18,
r p
-2% /p,t
/p\
P-P
I
1Bu
25
tBu
> + 4oc
,
P-P
I
tBu
I
tBu
P
I
tBu
33
34
Trivalent phosphorus compounds are generally outstanding ligands in the chemistry of complexes. Thus, cyclotriphosphanes react as expected with metal carbonyls,
to form mononuclear and binuclear cyclotriphosphanecarbonylmetal complexes (35 and 36), which are remarkably stable both thermally and to oxidation1761(Fig. 6"'l).
The P3 ring here functions-as in hexamethyltetraphosphatri~ilanortricyclene~~~]
but differently from the triphenylcyclotriphosphane complex 37 obtained in another
~ a y [ ~ ~ I -aa2s or a 4-electron donor["].
not proceed radically via free phosphinidenes PR, but
rather, most probably, by a four-center mechanism between two P-P g r o ~ p s [ ~
as ~has
~ ,also
~ ~been
~ , assumed for
the formation of the mixed-substituted cyclopentaphosphane~['~].
The corresponding transition state requires a
trans arrangement of the free electron pairs on the phosphorus atoms of the participating P-P bonds. The resultant dimerization which takes place first leads to a sixmembered ring compound, which is not, however, observed to be the main rearrangement product of the cyclotriphosphanes. Enrichment of this species is opposed by its
comparatively high reactivity, which is a result of the conformational mobility and hence the ability of the free electron pairs on adjacent P atoms to assume a trans position.
The same is true of cyclophosphanes with n > 6. Thus, subsequent reactions first produce still larger rings, such as cyclonona- and perhaps also cyclododecaphosphanes, which
then break up again into smaller oligomers. In the course
of these rearrangements there is a progressive enrichment
of the thermodynamically favored four-membered or fivemembered ring compounds['*'.
Mild halogenation of the tert-butyl compound 25 leads
to 1,3-dihalo-l,2,3-triorganotriphosphanes38, which are
of considerable interest as synthetic reagents for the preparation of new open-chain and ring-type tert-butylphosp h a n e ~ l ~38a
~ ] . is a remarkably stable open-chain triphosphane.
(PrBu),
25
+ Xz
Triorganocyclotriphosphanes are only metastable in
comparison with the corresponding cyclophosphanes with
Angew. Chem. I n t . Ed. Engl. 21 (1982) 492-512
X(tBu)P-P(rBu)-P(tBu)X
38a: X = C I
38b: X = B r
3 8 ~ X: = I
If phosphorus(ii1) bromide is used for ring-opening with
halogenation, the subsequent reaction consists in the insertion of the intermediate bromophosphanediyl into 38b
with the formation of the mixed-substituted tetraphos(PtBu),
4.3.2. Reactions in which the P3 Ring Skeleton is
Changed
+
,P(fBu)Br
PTP(fBu)B r
P(IBu)Br
40
-
+ PBr3
25
-I.[
(PtBu),Br,
+ 3Br
Y
38b
( r Bu) P-P(tBu)-P(Br)-P(t Bu)Br]
39
503
phane 39. As expected, the latter is unstable and rearranges to form the constitutional isomer 40[x41.
The light-sensitive compound 40 is the first derivative of
isotetraphosphane(6) that is stable at room temperature in
the absence of air'8s1(see Section 2.2).
Ph
Ph
tBu
43
tBu
44a: R = H
44b: R = H, M e
44c: R = M e
5. Heterocyclic Phosphorus Three-Membered Ring
Compounds of the Types (PR'),ER2 and (PR),E
tBu
tBu
R'
'R
46a: R = H
46b: R = M e
46c: R = 4-C1C H
6 4
Three-membered ring compounds with only one phosphorus atom in the ring have been known since the work
of Goldwhite et ai.[sxl
in the 'sixties and have already been
reviewed in recent timesfsg1.In contrast, three-membered
phosphorus heterocycles with two P atoms have generally
been thought to be extremely reactive and hence not preparatively accessible because of the thermal instability of
heterocyclopropanes with elements of the second short per i ~ d [ ~ However,
'~.
in recent years it has become possible to
make a broad spectrum of three-membered ring heterocyclophosphanes with carbon, silicon, germanium, tin, boron, nitrogen, arsenic, antimony, sulfur, and selenium as the
heteroatoms, and in large part also to isolate them in the
pure ~ t a t e [ ~ ~ , ~ , ~ ' ] .
A suitable route for the generation of a P2E ring skeleton
(E = heteroatom) is the [2 + 11-cyclocondensation of a bifunctional 1,2-diorganodiphosphane with a geminal nonmetal or metal compound. Assuming, that the leaving
groups on the phosphorus and on the heteroatom will all
cause the requisite polarization of the remainder of the
molecule and will combine to form a stable leaving compound, this reaction has a very broad field of application.
The tert-butyl-substituted diphosphanes 41[691and 42Ig2]
are particularly well suited synthetic reagents for the preparation of kinetically stable phosphorus three-membered
ring heterocycles.
X(R')P-P(R')X
+ GEY2
methylenediphosphiranes 46a-c are formed, but because
of their enhanced reactivity only 46b and 46c can be prepared in the pure ~ t a t e ~ ~ ~
The
, " ~diphosphirane
.
structure
of all these compounds is ascertained unambiguously from
the high-field position of their 3'P-NMR signals, which occur in the cyclotriphosphane region (see Table 10).
5.2. Diphosphasiliranes, Diphosphagermiranes, and
Diphosphastanniranes
The reactions of 41 with diorganodichlorosilanes, diorganodichlorogermanes, and diorganodichlorostannanes
lead to the heterocyclic compounds 47-49[3C.d.96.971
. The
reactivity of the compounds increases with increasing covalent radius of the heteroatom and with decreasing bulk
of its substituents. Thus, the diphosphasilirane 47a is appreciably less stable than the diphosphirane 4412, and the
diphosphastannirane skeleton can only be stabilized sufficiently by rert-butyl groups on the tin atom, while with
smaller substituents dimerization takes place at once.
(PR')ZER:
X = K , Y = C I : E = C , Si, Ge, Sn, B, As, Sb
X=CI, Y=Me,Sn:E=S, Se, N
K(rBu)P-P(tBu)K
41
Cl(tBu)P-P(tBu)CI
42
5.1. Diphosphiranes (Diphosphacyclopropanes)
Among the three-membered ring heterocyclophosphanes
the diphosphiranes are of particular interest. Their successful synthesis completes an unbroken transition in the
series of three-membered ring compounds from cyclopropane to cyclotriphosphane.
The first spectroscopic detection of the diphosphirane
43[931stimulated efforts to synthesize kinetically more stable representatives. The reactions of 41 with dichloromethane, 1,l-dichloroethane, or 2,2-dichloropropane give the
diphosphiranes 44a-c, which can be isolated by distillation as oxidation-sensitive
While 44c is surprisingly stable in the absence of air, and can even be
heated to 200°C for a short time without decomposition,
44a dimerizes already quantitatively to 45 within a few
days at room temperature. With 1,l-dichloroolefins the
504
47a: R
47b: R
48a: R
48b: R
= Me
= Ph
= Et
= Ph
49: R = tBu
As has been shown by the investigation of the crystal
structure of 47b'981,
the replacement of a >PtBu ring member in the cyclotriphosphane 25 by a >SiPh2 group leads
U
Fig. 7. Structure of (PtBu)2SiPh2 (981.
'
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-512
to little if any distortion of the ring skeleton, so that the endocyclic bond angle at the phosphorus and silicon is in
each case 60" (Fig. 7). This is surprising, since silicon generally shows a considerably greater tendency than phosphorus to keep the tetrahedral angle constant.
In diphosphagermirane 48a the greater covalent radius
of the germanium atom compared to phosphorus results in
a reduction of the interior ring angle at the heteroatom to
58", but the deviations compared to an equilateral threemembered ring are not large[3'.991.
5.4. Diphosphaarsiranes and Diphosphastibiranes
The heterocycles 51['021and 52[3d, obtained by reacting 41 with tert-butyldichloroarsane or tert-butyldichlorostibane are both formed as mixtures of the diastereomers
A and B, of which the isomer A , with trans-oriented tertbutyl groups at the phosphorus atoms, is more stable.
While 51 can be isolated in the form of colorless, light-sensitive crystals, 52 rearranges already at -78°C into larger
phosphorus-antimony rings.
5.3. Diphosphaboriranes
41 reacts with diorganoaminodichloroboranes to give
the diphosphaboriranes SOa-SOg, while with organodihaloboranes only larger phosphorus-boron rings are produced' 'I.
tBu
tBu
\
/
P
P
-
\
/
\\ //
B
I
50a:
... R = Et
~
50b: R = iPr
50~R
: = M e . nBu
5 0 d : R = Me, tBu
50e: R = Me, cHex
50f : R = Me, P h
5Og: R = P h
d N \ R
The P2B heterocycles are stable at room temperature if
the substituent on the boron contains two secondary Catoms or one primary and one tertiary C-atom in a position to the nitrogen (50b and 50d). If both of the alkyl
, if the electron
groups are unbranched (50a and ~ O C )or
density on the nitrogen is lowered by phenyl substitution
(50f and SOg), dimerization will take place relatively rapidly. The occurrence of an AB-spin system in the "P-NMR
spectra of 50c-50f indicates the presence of a partial double bond between the boron and the nitrogen atoms. In
contrast, corresponding to the high-field position of the
signals, there is no appreciable phosphorus-boron TC interaction in the ring (see Table 10). According to the results
of X-ray structure analysis carried out on 50a, the endocyclic bond angle is 54" at the phosphorus, and 72" at the
sp2-hybridized boron (Fig. 8)["11.
il
The remarkable stability of 51 also stimulated the synthesis of pho~phadiarsiranes[''~~
and of tri-tert-butylcyclotriar~ane["~I,so that in the series of three-membered ring
compounds from cyclotriphosphane to cyclotriarsane examples are known for each stage.
5.5. Thia-, Selena-, and Azadiphosphiranes
Heterocyclic phosphorus three-membered ring compounds with the electronegative ring members sulfur, selenium, and nitrogen have been obtained by the condensation of the diphosphane 42 with the stannyl compounds of
the corresponding elements['06,'071.The thiadiphosphirane
53 is also produced in the thermolytic ring contraction of
larger phosphorus-sulfur heterocycles, and in the reaction
of sulfur dichloride with 1,2-di-tert-butyIdiphosphaneor
with the diphosphasilirane 47b[Io6]. Azadiphosphiranes
with bulky amino groups on the phosphorus have been
by the dehydrohalogenation
prepared by Niecke et UZ.['~~],
of a phosphorane with a P-P bond.
53: E
iPr
= S
55a
54: E = S e
-
,NRz
(Me,Si),N,
7II 7
(Me & )zN-P-P-NRz
Me,SiN
+ CH,Li
- CH4, - LiF
7-7
iv
I
SiMes
55b: R = SiMe,
5%: R = CHMez
Fig. 8. Structure of (PtBu)>BNEt2[loll.
The diphosphaboriranes are at the same time the firstknown monocyclic three-membered ring boron compounds, since no boracyclopropane has been known up to
now.
Angew. Chem. Int. Ed. EngI. 21 (1982) 492-512
Collectively, these heterocyclic compounds can be isolated by distillation and show considerable kinetic stability. In contrast to the aziridines, the appearance of their
"P('HJ-NMR spectra down to -140°C is not temperature-dependent, so that the nitrogen has a planar surrounding, most probably because of the formation of dative n-bonds to the phosphorus atom.
505
5.6. A Spirocyclic Phosphorus Three-Membered Ring
Compound
As early as in 1896 it had proved possible to link two cy-
clopropane rings at a common carbon atom to form spiro[2.2]pentane"091.It was therefore a challenge for preparative chemists to combine two P2E rings at the common heteroatom E into a spirocyclic compound.
The synthesis of the tetraphosphasilaspiro[2.2]pentane
56 could be recently achieved in our group by cyclocondensation of silicon tetrachloride with the diphosphide
41["01.At the same time, 56 is the first spirocyclic compound made from two three-membered rings with silicon
as the central atom.
Table 10. "P-NMR parameters o f the three-membered ring phosphorus heterocycles (PR')2ER.t and (PR),E; J in Hz.
Compound
(PPh)2CMe2
(P~Bu)~CR,
R=H
R = H , Me
R=Me
(PfBu)2C=CR2
R= H
R=Me
R = 4-CICeH4
(PtBu),SiR,
R=Me
R=Ph
(PtBu)2GeR2
R=Et
R=Ph
(PfBu)2SntBu2
6A
43
- 122
44a
- 168.8
44b
44c
-129.4
- 91.7
46a
46b
46e
-143.2
-139.1
-149.7
47a
47b
-132.7
-122.4
48a
48b
-126.8
-113.3
- 73.3
49
6e
JAB
Ref.
[931
-138.5
- 162.4
194al
[94b]
[961
1961
(P ~ B uB) N
~ R2
mu
,tBu
'P
P
R=Et
R=iPr
R = Me, nBu
R = Me, rBu
R = Me, cHex
R = Me, Ph
R=Ph
( PtBu)2Asf Bu
A
B
The compound 56 is formed as a mixture of the isomers
A and B, which can be isolated separately and which differ from each other in the relative arrangement of the
trans-oriented tert-butyl groups at the two three-membered
rings. The sterically less favored isomer B slowly rearranges at room temperature into the more stable isomer A.
According to the results of X-ray structure analysis""], in
the individual, mutually perpendicular P2Si rings the endocyclic bond angles at the phosphorus and silicon atoms are
ca. 60".
5.7. ."P-NMR Parameters of the
Phosphorus Three-Membered Ring Heterocycles
(PR')2ER: and (PRkE
As in the case of the homocyclic phosphorus three-membered ring compounds, so with the heterocyclic compounds the signal positions in the "P-NMR spectra are
determined mainly by the size of the ring.
Table 10, which is arranged according to substance
classes, illustrates the dependence of the 6(3'P) values on
the individual heteroatoms. The dominant influence is that
of the covalent radius, while the electronegativity is only of
secondary significance: an increase in the P-E
bond
length widens the endocyclic bond angles at the phosphorus atoms and thus produces a shift in the downfield direction; at the same time, the absolute value of the negative
'Jppcoupling constant increases. Moreover, the S(3'P) values of the three-membered ring heterocycles are influenced by the same parameters as those of the homocyclic compounds.
506
5Oa
50b
50c
50d
50e
50f
5Og
-152.5
-159.2
-145.7
- 139.8
-147.3
-141.8
- 151.4
-150.3
-168.8
- 157.0
-155.4
- 75.9
- 77.2
-
-
75.8
79.4
51
52
(PtBu)2S
(PtBu)2Se
(PfBu)2NiPr
54
- 91.5
- 16.9
55a
-
-
[IOOb]
[IOObj
[IOOb]
[IOOb]
86.0
-230.5
- 69.3
-245.9
-
51.5
11031
- 40.0
- 42.5
53
[lOOa]
[IOOb]
[I021
- 45.1
Isomer A
Isomer B
(PI BuhSbtBu
Isomer A
Isomer B
(PI Bu)&
Isomer A
Isomer B
[IOOb]
67.0
56
- 152.3
- 145.1
6. Polycyclic Organophosphanes
Phosphorus compounds with the compositions P7R3,
P,&, and P9R3, in which the hydrogens of polycyclic hydrides have been replaced by alkyl or phenyl groups, have
already been observed several times in the thermolysis of
open-chain or cyclic phosphane derivatives and identified
by mass ~ p e c t r o ~ c o p y. [H~owever,
~ ~ ~ ~their
~ " isolation
~~
and
structural characterization are opposed by considerable
preparative difficulties. Since a better knowledge of these
compounds was expected to yield the decisive key to understanding the complex structures of the polycyclic hydrides PnH,-2, (n 2 4 , m = 1-7), the Cologne group has,
for some time, devoted considerable attention to this class
of compounds.
6.1. Syntheses
The first polycyclic organophosphane prepared was trimethylheptaphosphane(3) 13, obtained by the reaction of
trilithium heptaphosphide 10 with methyl b r ~ m i d e [ ~ . ~ ' ~ .
The formation of the P,, skeleton of 10 takes place-as in
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-512
the case of the polycyclic hydrides in Table 1-by uncontrolled disproportionation (see Section 3.1).
+
Li3P7 3 MeBr
P7Me3
10
13
The breakthrough in the production of polycyclic organophosphanes on a preparative scale came with the discovery of directed and generally applicable synthetic
methods. Since organodichlorophosphanes give monocyclic phosphorus compounds on dehalogenation"', in order
to obtain polycyclic organophosphanes it is necessary to
introduce into the reaction additional phosphorus atoms
not substituted with organo-groups. Corresponding to this
concept, substances of the general composition P.,+,R, can
be made by any of the reaction paths (c) to (e).
x RPCIZ
+
y PC13
X(PR),
+
y PC13
xRPCi2
+
f
9
T
P,
+
+
A3
Mg
% Mg
x Mg
-
PxfyRx
Px+,,Rx
2 x + 3y
7
MgClZ
+
+
PxfyRx
+
2
2
(C)
MgCIZ
(d)
x MgClZ
(e)
Other dehalogenating agents, such as lithium hydride,
can be used instead of magnesium["31. In all cases the
overall composition of the reaction product, i. e. the number of organo-substituted P atoms compared to the number
of P atoms connected only to other phosphorus atoms, can
be established from the molar ratio of the two starting
phosphorus-containing compounds. However, whether the
stoichiometry observed corresponds to a definite organophosphane or only to a mixture of substances with the
same overall composition will depend on the thermodynamic and the kinetic stability of the compounds involved.
Since the stoichiometric composition of various polycyclic
organophosphanes can be very similar to one another, a
product mixture of several compounds is always obtained
which must be separated chromatographically.
The structures of these compounds have been determined
by NMR spectroscopy and confirmed by a complete analysis o f the 31P{'H}-NMRspectra of 57a, 57b, and 58b"'61.
According to this, the compounds 57 are 2,3,5,6,7-pentaorganobicyclo[2.2. llheptaphosphanes with a skeleton analogous to that of norbornane, while the 58-type compounds
are
2,3,4,6,7,8-hexaorganobicyclo[3.3.0]octaphosphanes
with a structure analogous to pentalane. In the case of
57a-c, apart from the dominant configurational isomer
with the maximum trans arrangement of the substituents
other isomers occur with a lower relative frequency.
The compounds 58a-c are always formed together with
the corresponding compounds 57, in spite of the maintenance of the required stoichiometry. This can clearly be attributed to the fact that both the octaphosphanes(6) and
the heptaphosphanes(5) contain a bicyclic molecular skeleton consisting of two phosphorus five-membered rings. It
follows from the product distribution in each case that
with small substituents (Me, H) the P7-skeleton with a oneatom bridge has an appreciably greater tendency to form
than the P8-SkeletOn with a zero bridge. With increasing
bulk of the substituents (Et, iPr), however, the stability difference between the two bicyclophosphane skeletons is
changed, since the steric interactions of the substituents on
P6 and P7 in 57 are considerably greater than those of the
cis substituents in 58, so that the 58 :57 ratio increases.
On going over to R=tBu both the norbornane and the
pentalane skeletons are destabilized: while no corresponding heptaphosphane(5) has so far been obtained["'], the
relatively stable octaphosphane(6) shows the unusual
structure of a 2,2',3,3',4,4'-hexa-tert-butyl-l,lf-bicyclotetraphosphane 59[1181
(Fig. 9). All the P-P bond lengths
within the nonplanar four-membered rings and in the
bridge are approximately the same. Rotation around the
exocyclic P-P bond is already frozen-in at room temperature['I9]. The mixed bicyclic compound P6As2tBu660,accessible according to reaction (c) with arsenic trichloride
instead of phosphorus trichloride, has the same structure,
with arsenic atoms in the bridgehead
6.2. Bicyclophosphanes: P7R5,PsR6,P6R4
Dehalogenation in accordance with (c) or (d) at moderate temperature leads to heptapho~phanes(5)"'~~
or octapho~phanes(6)["~~,
of which 57a, 57b, 58b, and 58c have
been isolated as oily liquids.
Fig. 9. Structure of P8fBu6.
'7%
57a: R = M e
57b: R = E t
57c: R=iPr
Angew. Chem. Inl.
8' R6
58a: R = M e
58b: R = E t
58c: R=iPr
Ed. Engl. 21 (1982) 492-512
Together with 59, a further bicyclic compound, namely
a hexaphosphane(4), is produced in accordance with reaction (c) in a wide range of mixture ratios of tert-butyl dichlorophosphane and PC13; surprisingly, this compound
has a phosphorus skeleton comprising a three-membered
ring and a five-membered ring. It is therefore a 2,3,4,6-tetra-tert-butylbicyclo[3.1.O]hexaphosphane 61
b1 (Fig.
507
10). The slightly distorted three-membered ring is nearly
perpendicular to the relatively flat five-membered ring. In
spite of this unusual structure, 61 is remarkably stable toward heat and atmospheric oxygen.
tive clarification of this question is difficult, because the
spectrum is extremely complex owing to the presence of
configurational isomers.
6.4. Tetracyclophosphanes: P9R3, P1 R5
Of the phosphorus hydrides with nine P atoms, nonaphosphane(3) is outstanding because of its marked stability and tendency towards formation13b1.In agreement with
this, organo-substituted nonaphosphanes(3) are generally
formed in the thermolysis of open-chain or ring-type phosphane derivatives140,60,112. I 2 1 4
Fig. 10. Structure of P,tBu,.
6.3. Tricyclophosphanes: P7R3,P9R5
If the dehalogenation according to reaction (d) is carried
out at an elevated temperature (boiling tetrahydrofuran),
then in addition to phosphorus-richer polycyclics the heptaphosphanes(3) with R = Me, Et, iPr are preferentially
formed['221.The methyl compound 13 is identical with the
reaction product obtained from 10 and methyl bromide in
all its properties, including the distribution of the isomers'"'"''. The compounds are 3,5,7-triorganotricyclo[2.2.1.02~6]heptaphosphanes,
with a skeleton analogous to
that of nortricyclene (see Section 3.1), as has been ascertained by a complete analysis of the "P{'H)-NMR spectra
of the two configurational isomers of 13['231.
Under suitable conditions of stoichiometry, reaction
temperature, and reaction time, nonaphosphanes(5) are
formed predominantly according to reactions (c) or (d), of
which the compounds 62a and 62b have been isolated as
analytically pure, high-viscosity oils["4,
62b: R = E t
62c: R=iPr
P9Rs
Their structure has been clearly shown from the
31P('H}-NMR spectra to be 2,4,6,8,9-pentaorganotricyclo[3.3. 1.03~7]nonaphosphanes
with a P9 skeleton analogous
to that of noradamantane. Since the >PR ring members in
these molecules are all separated from one another by unsubstituted P atoms, a deviation from the all-trans arrangement of the substituents does not lead to any drastic destabilization. As a result, in all cases mixtures of various configurational isomers with comparable frequency are
formed (e.g. 62a : 3 isomers in the ratio 40 :40 :20); this
constitutes the reason for the low tendency of these compounds to crystallize.
Furthermore, a constitutional isomer of 62b is formed in
accordance with reaction (e), and has been separated in
the pure form["51. Since its 31P{'HJ-NMRspectrum shows
no signal groups in the three-membered or four-membered
ring region, the P9 skeleton should have a structure analogous to the hydrocarbons brendane or brexane. A defini508
According to reaction (c) considerable portions of nonaphosphanes(3) are produced under suitable reaction conditions (excess of PCI3, prolonged reaction time at elevated
temperatures), often together with corresponding heptapho~phanes(3)['~~].
Separation of the product mixture is
difficult owing to the poor solubilities, but has recently
been achieved in the case of the tert-butyl compound
63d.
63a:
63b:
63c:
63d:
R=Me
R=Et
R=iPr
R=tBu
p9 R3
According to data from the "P('H}-NMR spectra of 63b
and 63d, these compounds are 5,8,9-triorganotetracyclo[4.3.0.02~4.0'~7]nonaphosphaneswith a P9 skeleton analogous to that of the hydrocarbon deltacyclane. Corresponding to the observed nonequivalence of all nine P-atoms, the
symmetry is of the C, type, while the symmetric triasterane
structure assumed by other a ~ t h o r s ~ ~is* ~inconsistent
~''~
with the spectroscopic findings. As expected, there are two
configurational isomers of different frequencies, which
differ in their spatial arrangements of the substituent on
P'. The deltacyclane skeleton is also present as a structural
element in the Pf, ion (see Section 3.2).
Together with the tricyclic nonaphosphanes(5), varying
amounts of the corresponding undecaphosphanes(5) are
formed in most c a s e ~ [ " ~.. Th
'~~
is ]suggests that the stabilities of the two classes of compounds are similar and that
their structures are closely related. In agreement with this,
the results so far obtained on enriched PllEt5f'251
suggest
that the structure is associated with the noradamantane (or
brendane or brexane) analogous P9(5) skeleton in a similar
manner as the P43) skeleton is associated with the P7(3)
skeleton, namely by an additional two-atom bridge, giving
a new five-membered ring.
6.5. General Properties of Polycyclic Organophosphanes
The polycyclic organophosphanes so far obtained on a
preparative scale are colorless to yellow oily liquids or solids, which only crystallize when they do not consist of
diastereomeric mixtures. Their solubilities in the usual solvents become lower with increasing number of phosphorus
atoms and decreasing number of organo groups. Within a
Angew. Chem. Int. Ed. Engl. 21 (1982) 492-512
given class of compounds, the solubility increases with increasing number of C-atoms in the substituents. In the
ethyl, and particularly methyl, compounds there is often a
marked reduction of the solubility on allowing the solution
to stand or after complete removal of the solvent. This
physical aggregation process, which is limited to small
substituents and which is still unexplained, is also responsible for the extremely sparing solubility of the underlying
polycyclic phosphorus hydrides. The stability of the organophosphanes to atmospheric oxygen increases with increasing phosphorus content and with increasing bulk of
the substituents.
7. General Structural Principles of
Polycyclic Phosphanes and their Derivatives
The structures established for the organophosphanes
with condensed ring systems constitute an essential key to
understanding the structures of the polycyclic phosphorus
hydrides, which because of their low solubilities and poor
crystallizability have not yet proved amenable to NMR or
X-ray investigation. As shown by the synopsis in Figure
11, a variation of the substituents in the range of relatively
small alkyl groups brings about no alteration of the phosphorus skeleton, so that even when the organo groups have
been replaced by hydrogens a structural change can be reliably excluded. This leads to definitive conclusions for a
whole range of phosphorus hydrides:
P7HSis a bicyclo[2.2.1]heptaphosphane;
P7H3 is a tricycl0[2.2.1.0~~~]heptaphosphane;
PsH6 is a bicyclo[3.3.0]octaphosphane;
P,H, is a tricyclo[3.3.1.03.']nonaphosphane;
P9H, is a tetracyclo[4.3.0.02~4,03~7]nonaphosphane.
Small differences between the hydrogen- and the methylsubstituted phosphanes are only expected in the frequency
distribution of the various configurational isomers, since
with trimethylheptaphosphane(3) 13 there is already a
sterically determined deviation from the statistical distribution (see Section 3.1).
Significant changes in the skeleton structures characteristic for the individual compound classes-as in the case of
P8rBu,,59 - may be compelled by special steric conditions;
P7R5
'sR4
P7R3
Me
bQ
Et
iPr
tBu
0
I
Fig. 11. Phosphorus skeleton in polycyclic organophosphanes.
Angcw. Chem.
Inr. Ed. Engl. 21 (1982) 492-512
I
I
however, these only represent nature's emergency measure,
as shown by the fact that the element-homologous compound As8tBu6has the same structure as the sterically unhindered octaphosphanes(6)~''8"~
"'I.
Whether the unusual
skeleton in P6tBu4is only limited to bulky substituents will
be shown by further investigation of the compounds P6%
(R= Et[lz4I,Me[''71, H[3b1)already identified by mass spectroscopy. In general, the formation of different skeleton
structures within one and the same class of compounds
should mainly be reckoned with in the bicyclophosphanes,
where steric interactions between the substituents play a
far greater role than in tri-, tetra- or pentacyclic compounds.
The P,, skeletons of the various classes of compounds
show close family relationships. For example, the double
ring in P6tBu4can be obtained formally from the tricyclic
P7(3) cage if one of the three one-atom bridges is removed.
The P7(5) skeleton analogous to norbornane and the P9(5)
skeleton analogous to noradamantane differ from the P7(3)
cage only by the opening of one bond in the three-membered ring or by the substitution of the three-membered
ring by a five-membered ring. The tetracyclic P9(3) skeleton can be obtained from the P7(3) skeleton by the condensation of a five-membered ring onto the latter structure
with the aid of a two-atom bridge. Moreover, the P7(5),
Ps(6), and P9(5) skeletons are essential structural unit
groups of Hittorf phosphorus['281.It is these structural relationships that are responsible for the easy reciprocal transformations and the simultaneous genesis of the various
species.
If the class of structurally ascertained molecular polycyclic phosphane derivatives is also taken to include the
"inorganically" substituted P,l(SiMe3)3, which according
to uon Schnering et U ~ . [ ' contains
~ ~ I
a symmetric skeleton of
six condensed five-membered rings, the outstanding
significance of the formation of five-membered ring structural elements in polycyclic phosphane skeletons becomes
clearly evident. This is an essential difference from carbon
chemistry, and among other things it explains why it has
not yet been possible to prepare a molecular decaphosphaadamantane[38.'301.The mass-spectroscopically identified compounds Plo& (R=iPr['l5], HL3d1)
must have some
other structure. In contrast to several three-membered and
six-membered partial frameworks, structure elements
made up of four-membered rings or from rings of more
than six P-atoms have not so far been encountered in sterically unhindered polycyclic phosphanes.
Summarizing, according to our present state of knowledge the structures of polycyclic phosphanes and their
sterically unhindered derivatives are determined by the following criteria:
1. In accordance with the empirical formula, fundamentally the same formally derivable polycyclic structures may
exist as in hydrocarbons of analogous composition, each
P-atom substituted by a monofunctional residue corresponding to a CH2 group and each P-atom bound only to
phosphorus corresponding to a CH group.
2. Of the constitutional isomers possible according to
point 1 above, those with the maximum number of fivemembered ring elements are formed.
3. If the stoichiometric composition requires other
509
structure elements in addition to five-membered rings,
then additional three-membered and six-membered rings
are incorporated in the phosphane structure, but not fouro r seven-membered rings.
4. Phosphane skeletons with three-membered rings are
generally more thermally stable than ones with six-membered rings.
5. The structures of higher-condensed phosphanes are
derived from those of lower-condensed phosphanes with
two fewer P-atoms, often by the condensation of a new
five-membered ring onto the latter structure via a two-atom
bridge with trans substituents.
6. Particularly phosphorus-rich structures are built u p
by the linking of large partial frameworks (P7-, P8-, P9-,
PI]-), either by an exocyclic P-P bond or via the zero
bridge of a commonly formed bicyclo[3.3.0]octaphosphane
partial structure.
7. Constitutional isomers may be produced if several
skeletons of similar structure and hence comparable stabilities are possible for one and the same empirical formula.
8. Of the various possible configurational isomers, the
one produced is always that with the maximum trans arrangement of the substituents. The formation of further
isomers is determined by the steric interactions when the
structure deviates from the “all-trans” arrangement.
Thus, whilst the formulas of the polycyclic phosphorus
hydrides listed in Table 1 may at first sight show no rhyme
or reason, the application of these structure principles
makes them largely comprehensible.
8. Summary and Future Prospects
Research in the last 15 years has led to the realization
that compounds with a skeleton of phosphorus chains and
rings are not in any way “exotic” in the field of nonmetal
chemistry. With modern preparative techniques such compounds can be handled with little or no problem and in
their structures they show many similarities to the corresponding hydrocarbons. This must be attributed to the
equal numbers of valence electrons in the -CH2-- and
-PH--, and in the >CH and >P: groups. Analogous
framework structures can therefore be produced. Certain
differences d o exist, due mainly to the following special
features of phosphorus:
1. P-P bonds enter easily into four-center reactions
owing to their free electron pairs. Disproportionations and
ring-rearrangement reactions therefore take place under
considerably milder conditions than in the case of hydrocarbons or of hydrogen-silicon compounds.
2. Phosphorus tends to form smaller bond angles than
carbon, and this can lead to different stability sequences
among the cyclic compounds of the two elements.
3. As a result of the trivalence of phosphorus, chiral
centers occur appreciably more often in open-chain and
ring-type phosphorus compounds than in the corresponding compounds of carbon.
The present review should make it clear that we just
stand at the beginning of an era of new and highly promising developments in the field of chain-type and ring-type
phosphorus compounds. If we consider the number of
phosphorus hydrides and organophosphanes that have al510
ready been identified by mass spectroscopy but not yet
been made on a preparative scale, and whose structures
still have to be confirmed experimentally, the scope and
diversity of work ahead of us in the near future becomes
very evident. Further stimulating investigations will arise
from the question whether one could carry out aimed
isomerizations from which information could be derived
about the relative stabilities of the individual constitutional isomers and about the mechanism of these reactions.
Another particularly fascinating subject is the valence tautomerism in phosphorus compounds. Furthermore, the
reactive potential of the phosphorus three-membered ring
compounds could prove to be useful in various preparative
contexts. Studies in the various areas mentioned are currently under way in our laboratories.
Special thanks are due to my co-workers cited in our own
contributions, for their dedicated and always friendly collaboration, and to Dr. M. FehPr and Dr. K . Glinka for valuable technical assistance. The work of our research group was
kindly supported by the Deutsche Forschungsgemeinschaft,
the Minister for Science and Research of the NordrheinWestfalen province, the Fonds der Chemischen Industrie,
and Hoechst AG. Frankfurt am Main.
Received: April 8, 1982 [A 418 IEI
German version: Angew. Chem. 94 (1982) 520
Translated by AD-EX Translations Ltd., London
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[I51 A considerable amount of the data quoted about the conditions of preparation, composition, and properties of the substances are not consistent with our experiences and findings, which are not only based o n
mass spectrometry but also o n ”P-NMR and Raman spectroscopy. The
discrepancies could arise, inter alia, from the fact that the phosphanes
examined by Fehlner were characterized only by mass spectra at 70
eV.
1161 P. Junkes, M. Baudler, J. Dobbers, D. Rackwitz, Z . Naturjorsch. B27
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[I71 Measurement of the amounts of the air- and temperature-sensitive
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weight.
1181 M. Baudler, H. Standeke, M. Borgardt, H. Strabel, J. Dohbers, A.
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I271
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[3l]
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(431 W. Honle, H. G. von Schnering, Z. Anorg. Allg. Chem. 440 (1978)
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606; Angew. Chem. Int. Ed. Engl. 20 (1981) 594.
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3852.
I551 M. Baudler, M. Bock, Z. Anorg. Allg. Chem. 395 (1973) 37; however,
the results obtained were not substantiated by checking the preparative
work "P-NMR spectroscopically, and therefore, according to more recent experience (see [57b]), cannot be regarded as unquestionable.
I561 M. Baudler, M. Hallab, A. Zarkadas, E. Tolls, Chem Ber. 106 (1973)
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[SS] M. Baudler, W. Driehsen, unpublished.
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Angew. Chem. Inr. Ed. Engl. 2l (1982)492-512
[62] M. Baudler, G . Fiirstenberg, H. Suchomel, J. Hahn, Z. Anorg. Allg.
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[67] M. Baudler, E. Clef, unpublished.
1681 M. Baudler, J. Hahn, E. Clef, Z . Naturforsch. 8. in press.
1691 M. Baudler, C. Gruner, G . Fiirstenberg, B. Kloth, F. Saykowski, U.
Ozer, Z. Anorg. Allg. Chem. 446 (1978) 169.
I701 M. Baudler, B. Makowka, K. Langerbeins, G. Fiirstenberg, Z. Nuturforsch. B., in press.
[71] W. Holderich, G . Fritz, Z. Anorg. Allg. Chem. 457 (1979) 127.
[72] Cf. e.g. [57a].
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[74] C. Kriiger (1977); see [ZOa].
1751 M. Baudler, E. Darr, unpublished.
[76] M. Baudler, F. Salzer, Z . Nuturforsch. 8. in press.
I771 K.-F. Tebbe, M. Feher, Z. Naturforsch. 8, in press.
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I791 G. Huttner, H. D. Miiller, A. Frank, H. Lorenz, Angew. Chem. 87(1975)
597; Angew. Chem. Int. Ed. Engl. 14 (1975) 572.
[SO] An earlier communication [Sl] o n triphenylcyclotriphosphane complexes of iron and nickel requires checking because of the unconfirmed
identity of the educt (see [SS]).
[81] M. Baudler, M. Bock, Angew. Chem. 86 (1974) 124; Angew. Chem. Inr.
Ed. Engl. 13 (1974) 147.
[82] On the ring size preferred by the individual substituents R, see [I].
[83] M. Baudler, B. Carlsohn, Chem. Ber. 110 (1977) 2404.
I841 M. Baudler, J. Hellmann, Z. Anorg. Allg. Chem.. in press.
[SS] The compounds P(PF,), [86] and P(PPh2), 184, 871 already described are
unstable at room temperature.
1861 D. Solan, P. L. Timms, J. Chem. SOC.Chem. Commun. 1968. 1540.
[871 H. Schumann, A. Roth, 0. Stelzer, Angew. Chem. 80 (1968) 240; Angew.
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1901 Cf. e.g. D. Seyferth, J. Organomet. Chem. 100 (1975) 237.
[91] M. Baudler, Lecture, 3rd Int. Symp. Inorg. Ring Syst., Graz 1981.
[92] M. Baudler, J. Hellmann, J. Hahn, Z. Anorg. Allg. Chem.. in press.
[93] M. Baudler, B. Carlsohn, Z. Nuturforsch. 832 (1977) 1490.
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Anorg. Allg. Chem., in press.
[95] M. Baudler, F. Saykowski, unpublished.
[96] M. Baudler, H. Jongebloed, Z . Anorg. Allg. Chem. 458 (1979) 9.
[97] M. Baudler, H. Suchomel, Z. Anorg. ANg. Chem.. in press.
[98] K.-F. Tebbe, Z. Anorg. Allg. Chem. 468 (1980) 202.
[99] B. Freckmann, K.-F. Tebbe, Z. Anorg. Allg. Chem.. in press.
[IOO] a) M. Baudler, A. Marx, J. Hahn, Z. Nuturforsch. 833 (1978) 355; b) M.
Baudler, A. Marx, Z . Anorg. Allg. Chem. 474 (1981) 18.
[loll M. Feher, R. Frohlich, K.-F. Tebbe, Z. Anorg. A&. Chem. 474 (1981)
31.
[lo21 M. Baudler, S. Klautke, Z. Nuturforsch. 836 (1981) 527.
[I031 M. Baudler, S. Klautke, Z. Nuturforsch. 8. in press.
[I041 M. Baudler, D. Habermann, Angew. Chem. 91 (1979) 939: Angew.
Chem. Int. Ed. Engl. 18 (1979) 877.
[I051 M. Baudler, P. Bachmann, Angew. Chem. 93(1981) 112; Angew. Chem.
I n t . Ed. Engl. 20 (1981) 123.
[ 1061 M. Baudler, H. Suchomel, G. Fiirstenberg, U. Schings, Angew. Chem.
93 (1981) 1087: Angew. Chem. I n t . Ed. Engl. 20 (1981) 1044.
11071 M. Baudler, G. Kupprat, Z . Nuturforsch. 837(1982) 527.
[IOS] E. Niecke, A. Nickloweit-Luke, R. Riiger, Angew. Chem. 93 (1981) 406;
Angew. Chem. In!. Ed. Engl. 20 (1981) 385.
[I091 G . Gustavson, J. Prakt. Chem. 12154 (1896) 97; 56 (1897) 93.
[ I lo] M. Baudler, T.Pontzen, Z. Anorg. Allg. Chem.. in press.
[ I 1 I] K.-F. Tebbe, T. Heinlein, unpublished.
[ I 121 W. Holderich, G. Fritz, Z. Anorg. Allg. Chem. 457 (1979) 127.
[ 1131 M. Baudler, B. Koll, K. Kazmierczak, unpublished.
[114] M. Baudler, Y. Aktalay, J. Hahn, E. Darr, Z. Anorg. Allg. Chem. 473
(1981) 20.
[ l 151 M. Baudler, Y. Aktalay, Z. Anorg. Allg. Chem.. in press.
[I161 M. Baudler, E. DBrr, unpublished; see also [114, 1151.
[ I 171 M. Baudler, Y. Aktalay, unpublished.
[I181 a) M. Baudler, J. Hellmann, P. Bachmann, K.-F. Tebbe, R. Frohlich,
M. Feher, Angew. Chem. 93 (1981) 415; Angew. Chem. I n t . Ed. Engl. 20
51 1
(1981) 406; b) M. Feher, R. Frohlich, K.-F. Tebbe, 2. Krisfallogr., in
press.
[I191 The line-broadening in the 'H{"P)-NMR spectrum below - 10°C is
contrary to our earlier interpretation [118a] - t o be attributed to freezing-in of the rotation of the terf-butyl groups.
[I201 M. Baudler, Y. Aktalay, T. Heinlein, K.-F. Tebbe, 2. Naturforsch. B37
(1982) 299.
[I211 a) M. Baudler, Y. Aktalay, K.-F. Tebbe, T. Heinlein, Angew. Chem. 93
(1981) 1020; Angew. Chem. Inf. Ed. Engl. 20 (1981) 967: b) K.-F.
Tebbe, T. Heinlein, Z. Kristallogr., in press.
[I221 M. Baudler, Y. Aktalay, B. Koll, unpublished.
I1231 M. Baudler, T. Pontzen, unpublished.
[I241 M. Baudler, B. Koll, Y. Aktalay, unpublished.
~
(1251 M. Baudler, V. Arndt, unpublished.
[I261 M. Baudler, K. Kazmierczak, B. Koll, Y. Aktalay, unpublished.
[I271 R. Frohlich, K.-F. Tebbe, Z. Kristallogr.. in press.
[I281 H. Thurn, H. Krebs, Angew. Chem. 78 (1966) 1101: Angew. Chem. I n t .
Ed. Engl. 5 (1966) 1047.
(1291 H. G. von Schnering, D. Fenske, W. Honle, M. Binnewies, K. Peters,
Angew. Chem. 91 (1979) 755: Angew. Chem. Int. Ed. Engl. 18 (1979)
679.
[I301 The compound PIoPh, described earlier by E. Wiberg et a/. [I311 could
not be confirmed by us.
[I311 E. Wiberg, M. van Ghemen, G. Miiller-Schiedmayer, Angew. Chem. 75
(1963) 814; Angew. Chem. Int. Ed. Engl. 2 (1963) 646.
Conformation and Biological Activity of Cyclic Peptides**
By Horst Kessler*
Dedicated to Professor Klaus Weissermel on the occasion of his 60th birthday
Cyclic peptides containing biologically active hormone sequences are suitable models for
studying conformation-activity relationships. In such models the usual flexibility of peptide
chains is reduced by their cyclic arrangement. However, conformational analysis of such
systems by experimental means is possible only if a single conformer predominates at equilibrium, and criteria for this are put forward. NMR spectroscopic methods, including many
recent advances, are discussed in relation to their ability to contribute to peptide conformational analysis.
Apart from consideration of the hydrogen bond, we organic chemists have really paid little attention to linkages other than the purely covalent. I believe that it will be the duty of organic chemists in
the furture to study the weak non-bonding interactions which are of
enormous importance in the large natural macromolecules. Such
studies will lead to a new blossoming of organic chemistry in the future.
Lord Alexander R . Todd"]
1. Introduction
The planned synthesis of a totally selective drug with no
side effects is the ambition of every pharmacologist and
drug chemist. Efforts towards realizing this goal of drug
design have been
and the search for relationships between structure and activity has been the principal
subject of activity in many industrial and academic laboratories. At the same time, the search for refinement of present-day concepts of the structure of organic compounds is
reflected in the work of chemists, biochemists, and pharmacologists alike. Initial efforts at finding a connection between structure and activity equated structure with constitution, but it soon became evident that configuration was
at least of equal importance. As a result, special emphasis
has been placed in recent years on stereospecificity in the
[*I
[**I
512
Prof. Dr. H. Kessler
lnstitut fur Organische Chemie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main 50 (Germany)
Peptide Conformations, Part 19.-Part 18: [106].
0 Verlag Chemie GmbH. 6940 Weinheim. 1982
synthesis of active substances. The importance of molecular conformation in both chemistry and biochemistry has
been evident ever since the pioneering work of Barton''];
yet only hesitant efforts have since been made to establish
a link between conformation and activity''].
Perhaps it is not difficult to see why. Conformers are
generally not isolable as stable compounds and therefore
cannot be tested separately for biological activity. The cyclic tetrapeptide [Sar'l-Tentoxin appears to be the only molecule known so farr9"]whose two conformers have been
separately examined-and here in fact they have been
found to differ in activity. Generally it is difficult enough
even to determine the most stable conformation in solution, and all the more to follow changes in conformation in
biological
The only way out of this problem
appears to be to effect conformational changes by altering
the constitution and/or configuration of an active substance, but it would then be highly difficult to distinguish
between the separate effects on biological activity of the
various changed parameters.
0570-0833/82/0707-0512 $02.50/0
Angew. Chem. Inf. Ed. Engl. 21 (1982) 512-523
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