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Complex-Stabilization of an Aminooxophosphane (Phosphinidene Oxide).

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''R2NP0', which proved to be the trimer (4) of the aminooxophosphane (phosphinidene oxide) (3).
Presumably, in the initial step of the reaction a [2+ 21-cycloaddition product is formed, which decomposes to kinetically labile (3). Compound (3) is stabilized by coordinative
saturation at the phosphorus to give the trioxatriphosphorin
(4)-a reaction which can be formally compared with the observed cyclization in the isoelectronic XSN-system
(3 XSN+(XSN)3[41).
o=s=o
+
I(
0-s =o
i P r )zN-&-iktBu]
( ;Pr)zN-P=NtBu
2
-
O=S=NtBu
(2)
+
[ (iPr),N-P=O]
cycle has almost ideal boat conformation in the crystal,
whereby the transannular P . . .O distance (3.01 A) between
the ring atoms in the mirror plane of the molecule can possibly be seen as an indication of a 1,4-dipole-dipole interaction. The ligands take up the equatorial position at the phosphorus atoms, in which both a 1,3 repulsion of the ligands is
minimized and a more effective p,(N)-d,(P)
interaction is
achieved. The transfer of electrons from the nitrogen to the
phosphorus manifests itself in the relatively short P-N
bonds (1.64 and 1.66 A) as well as in the planar arrangement
of the ligands at the nitrogen atoms.
The molecule (3) can be trapped by complexation['].
Procedure
About 180 mmol SO2, dried with CaCl,, is passed into a
solution of (1) (9.5 g, 47 mmol) in ether (15 ml) at -25°C
under NZ. Immediately after passage of SO, is complete the
excess SOz is removed under further cooling. The reaction
solution is then warmed up to room temperature, and the solvent and any (2) that is formed are removed. Finally, the solid product is recrystallized twice from benzene; yield 2.8 g (4)
(41%), colorless needles, m. p. 107 "C.
The phosphorus-oxygen heterocycle (4) is a colorless, crystalline solid (m.p. 109 "C), sensitive to atmospheric moisture
and oxygen. The composition and structure are secured by
elemental analysis, mass spectrum[51,and NMR data, as well
as by X-ray structure analysis (Fig. 1).
Received: October 18. 1979 [ Z 542a IE]
Publication delayed at authors' request
German version: Angew. Chem. 92. 737 (1980)
[I] H. Staudinger, E Hauser, Helv. Chim. Acta 4. 861 (1921).
[2] H. Quast. M. Heuschmann, 2nd Int. Symposium on Inorganic Ring Systems,
Gottingen 1978; Angew. Chem. 90,921 (1978); Angew. Chem. Int. Ed. Engl.
17, 867 (1978); H. Tomioka, Y. Izawa. J. Org. Chem. 42, 582 (1977); U .
Schmidt, Angew. Chem. 87,535 (1975); Angew. Chem. Int. Ed. Engl. 14, 523
(1975).
131 Carbonyl compounds react only under stronger conditions; so far only polymers could be obtained.
141 0. Glemser. H. Perf, Naturwissenschaften 48, 620 (1961); 0.Glemser, H.
Meyer, A. Haas. Chem. Ber. 98, 446 (1965).
[ S ] MS (70 eV)- m/e=441 (12%. M + ) ,89 (100%. OPNCHMe').
[6] The possible existence of at least one further isomer (t2%) which is in equilibrium with (4) could be ascertained "P-NMR spectroscopically.
[7] W. C Bentrude, Han- Wan Tan, J. Am. Chem. Sac. 95, 4666 (1973).
[S] E. Niecke, M . Engelmann, H. Zorn, B. Krebs. C Henkel, Angew. Chem. 92.
738 (1980); Angew. Chem. In!. Ed Engl. 19. 710 (1980).
Complex-Stabilization of an Aminooxophosphane
(Phosphinidene Oxide)[**]
By Edgar Niecke, Monika Engelmann, Hans Zorn,
Bernt Krebs, and GeraId Henkel"'
[A]
Fig. 1. P,O,[N(i-C,H,),], (4) in the crystal, with average bond lengths
and
angles (u ca. 0.012 and ca. 1.0". resp.). (4): space group P2,/n, a = 10.092(4),
p=106.54(3)". Z = 4 (at -133°C).
b=36.848(15), c=7484(3)
A
A,
The 31P(1H]-NMRspectrum (30% solution in C6D6;
H3PO4 ext.; 25 "C) shows the preferred population of only
one isomer ( >98%)[@with two types of stereochemically
different phosphorus atoms ( 6 A = 140.3, 6Bz= 131.3,
*JPApB=
13.5 Hz), whose signals lie in the same region as
those of 2-amino-1,3,2-dioxaphosphorinanes~'~.
In accord
with the 3'P-NMR spectrum, the 'H-NMR spectrum (30%
solution in CH2Cl2:TMS int.; 25 " C )shows two multiplets,
each (2:1) for the methyl- and methine-protons: 6=1.21
(3JHH=6.8
HZ),
3.59
(I3&pB + 'J"pe I = 10.8
Hz
[PBN(CHMe,),]), 1.24 (35HH=6.8 Hz), 3.83 (3JHp,=11.5 Hz
[PAN(CHMe2),l).
In the solid state the heterocycle (4) has, to a close approximation, C,-symmetry. The sterically homogeneous molecules are disordered in the unit cell. Surprisingly, the hetero-
710
@ Verlag Chemie, GmbH, 6940 Weinheim, 1980
While stable methylenephosphanes (1) and iminophosphanes (2) have been reported recently, the existence of the
phosphorus-oxygen (p-p), bond, such as in (3), has not been
verified, although trapping reactions would indicate the for-p=c'
-p=N-
(1)
(2)
-P=O
(3)
mation of such a species[*].Recently, reaction of aminoiminophosphane (4) and sulfur dioxide yielded the trioxatriphosphorine (6), which is presumably formed by stereospe-
['I
Prof. Dr. E. Niecke, M. Engelmann, D1pl:Chem.
Fakultat fur Chemre der Universitat
Postfach 8640. D-4800 Bielefeld 1 (Germany)
H. Zorn
Prof. Dr. B. Krehs, Dr. G. Henkel
Anorganisch-chemisches lnstitut der Universitat
Gievenbecker Weg 9, D-4400 Miinster (Germany)
["I Phosphazenes of Coordination Number 2 and 3. Part 19. This work was
supported by the Fonds der Chemischen 1ndustrie.-Part 18: 111.
0570-0833/80/0909-0710
S 02.50/0
Angew. Chem. Int. Ed. Engl. 19 (1980) No. 9
cific cycloaddition of the intermediate aminooxophosphane
(phosphinidene oxide) (5)I'I. By application of the SO,-reaction to the iminophosphane complex (7) we have now succeeded in trapping the aminooxophosphane as complex (8).
Table 2. "P-, 'H- and "C-NMR data of the compounds (7) and (8) [a].
S(3'P)
224.0
a('H) (JHPW I )
rBu
iPr
S(''C)
rBu
iPr
CO,,
(JCP
1.40
1.17, 3.77 [b] (15.3)
319.2
-
1.23. 3.25 [b] (13.5)
IHzl)
coax
33.5 (5.9), 57.0 (1.9)
22.2 (3.3), 48.9 (9.6)
215.1 (16.0)
220.3 (3.6)
-
22.4 (3.3). 50.2 (10.3)
213.9 (18.7)
21 8.2
[a] 20% solution in CDCI,; H1P04 ext. ("P) or TMS ('H, "C) mt.; 25°C. [b]
' J ~ , = 6 . 7 (7). 5.9 Hz (8).
The compounds (7) and (8) are pale yellow crystalline solids which are sensitive to atmospheric moisture and oxygen.
Their composition and constitution are confirmed by elemental analysis, IR, Raman, NMR and mass spectral data,
and by complete X-ray structure analyses.
The mass spectrum (Varian 31 IA, 70 eV, direct sample inand of (8)
troduction) of (7) (L=(iPr),NPNtBu)
(L = (iPr),NPO) show the molecular ion LCr(C0); [m/
e=394 (3%) and 339 (4%), resp.] and the fragments
LCr(C0); (n = 4-0) resulting from successive cleavage of
CO, with the LCr+ fragment (m/e=257 and 199, resp.) as
base peak. In the IR and Raman spectra (Table 1) of (7) and
(8) five bands appear in the expected v(C0) range which can
be considered as resulting from a strong perturbation of the
local C4" symmetry of the Cr(CO), group. The frequency
shift to high wave numbers observed in the case of (8) is possibly due to the stronger m-acceptor properties of the aminooxophosphane ligand.
coordinated, as it is in the complexes (C0)5CrL
[L = (Me,Si),NP==NtBu, ~BUHNP==NS~M~,][~'],
whose
structures were analyzed previously. The extremely short
P-N,,,,,
distance"] and the P==O bond length, which is
comparable with that in trimethylphpsphane oxide"]', are
interaction.
compatible with a pseudoallylic R2N-P-0
Table 1. Characteristic IR [a] and Raman data [b] of compounds (7) and (8).
(8)
f 71
IR
Raman
IR
Raman
2072 s
1984 s
1960 sh
1952 vs
1940 sh
I340 m. br.
2072 m-s
1986 vs
1960 m
1950 sh
1937 s
1344 s. br. [c]
2080 s
1995 m
2082 m-s
1996 vs
1965
1935 vs
1198 m
1963 s
1940w
1203 s [d]
[a] Nujol mull. [b] Solid. Ic] u(P-N).
For
(C4"j
A/
B,
[d] v(P=O)
The broad intense Raman line at 1344 cm-' in (7), as in
the case of other iminophosphane complexes'31,can be ascribed to strengthening of the P=N-bonding [in (2): 1242
cm-''41] by coordination. In (8) a Raman band of comparable intensity appears at 1200 cm-l; accordingly, it should arise from a stretching vibration with predominant P=O character.
The 'H- and l3C [ 'H) -NMR spectra of (7) and (8) show
the expected groups of signals (Table 2). In the "P-NMR
spectrum of (7) the coordination of the iminophosphane results in an upfield shift of the 31P resonance ~igna11~J
[6 = 224.0; for comparison, 295.2 in ( 9 1 . On the other hand,
the replacement of the tert-butyl imino-group in (7) by oxygen as in (8) leads to strong deshielding of the phosphorus
atom (6=319.2), from which it can be concluded that the
phosphorus in (7) is considerably more electrophilic than in
(4). These findings are consistent with an extremely reactive
"free" aminooxophosphane.
Figure 1 shows the molecular structure of the complex
@)I6].
The central phosphorus atom is trigonal-planar
Angew Chem. Inr. Ed Engl. 19 (1980) No. 9
Fig. I . Molecular structure of (8) (without H atoms; vibration ellipsoids with 50%
probability) with important bond lengths (pm; 0 = 0 . 2 pm). Further bond data:
0-P-N
120.6(1)", 0-P-Cr
111.4(1)', N-P-Cr
128.0(1)"; Cr-C,,
188.4(2) pm, Cr-C,, 190.2-191.4(2) pm.
This is in accord with an almost planar arrangement of the
C2N-P=O moiety; the dihedral angle between the C2Nand NPO-plane is only 1.6". The P-Cr distance is significantly shorter than in the corresponding iminophosphane
which should be attributed to better n-acceptor properties of the aminooxophosphane ligand. These properties result from the higher orbital electronegativity of the
phosphorus in (8) in comparison with that in the iminophosphanes.
Procedure
All reactions are carried out in anhydrous solvents under
N2. (7): A solution of (4) (4.4 g, 22 mmol) and Cr(C0)6 (5.0
g, 23 mmol) in tetrahydrofuran (THF) (250 ml) is irradiated
in a photoreactor at - 5 "C until about 2/3 of the calculated
amount of CO has been formed. The solution is then concentrated to 50 ml and unreacted Cr(CO), is filtered off. The residue after removal of solvent is recrystallized twice from nhexane; yield 2.8 g (32%) (7), m.p. 95-98 "C (dec.).
(8): About 15 mmol SO2, dried over CaC12, is passed into a
solution of (7) (2.1 g, 5.3 mmol) in ether (20 ml) at - 30 "C.
Unreacted SO2 is removed at - 30 "C and the reaction solution warmed to room temperature. The crude product [ca.
80% (8)] after removal of solvent and any tBuNSO that is
0 Verlag Chemre, GmbH, 6940 Wernherm. 1980
0570-0833/80/0909 0711
3 02.50/0
71 1
formed is purified by sublimation at 5OoC/0.1 torr; yield 0.9
g (43%) (S), m. p. 93-96 "C (dec.).
Received: January 9, 1980 [Z 542 b IEI
supplemented: April 8, 1980
German version: Angew. Chem. 92, 738 (1980)
CAS Registry numbers:
(4). 63950-84-5; (6). 74563-07-8: (7), 74592-15-7; 181, 74592-16-8; SO>,7446-09-5,
t-BuNSO, 38662-394; Cr(CO)6, 13007-92-6
111 E. Niecke, H.Zorn, B. Krebs, C. Henkel, Angew. Chem. 92, 737 (1980); Angew. Chem. Int. Ed. Engl. 19, 709 (1980).
[2] H. Quast, M . Heuschmann, 2nd Int. Symposium on Inorganic Ring Systems.
Gottingen 1978; Angew. Chem. 90, 921 (1978); Angew. Chem. Int. Ed. Engl.
17, 867 (1978). and references cited therein.
(31 E. Niecke, R. Kroher. G. Ringel. Chemiedozententagung, Marburg 1977; C
Ringel, Dissertation, Universitat Gottingen 1977; S. Pohl, J. Organomet.
Cbem. 142, 185, 195 (1977): E. Niecke, G. Ringel, S. Pohl, unpublished.
141 H.Zorn, Diplomarbeit. Universitat Gottingen 1975.
[5] 0. J. Scherer, N. Kuhn, H . Jungmann, 2. Naturforsch. B 33, 1321 (1978).
[6] (8) crystallizes monoclinically, space group P2,/n, a = 1126.2(3),
b= 2186.4(5), c=632.6(2) pm, p = 100.65(3)", V = 1530.8 x 10' pm', 2 = 4 (at
- 130°C). Refinement with the structure factors of 2661 observed reflections
converged to the unweighted R value of 3.5%.
[71 A comparable bond length IS observed in ions of the type [ N-P-.N
] +:
S. Pohl, Z . Naturforsch. B32, 1342(1977); A. H . Cowley, M . C. Cusher, J. S
Szobota, J. Am. Cbem. SOC.100, 7784 (1978).
[8] D. E. C. Corbridger The Structural Chemistry of Phosphorus. Elsevier, Amsterdam 1974.
By Mohamed Hassen Khalga, Giinther Jung, and Anton
Riekerl'l
In acetonitrile, sterically hindered phenols of type (1) can
be electrochemically oxidized to phenyloxylium ions which
form p-quinols or p-quinol derivatives in the presence of nucleophiles (water, alcohols and amines)[l.21.The specificity
and yield of this synthesis are usually far higher than normally attained in chemical oxidation.
We now report the application of electrochemical oxidation of 2,4,6-trisubstituted phenols to the synthesis of new Nprotected amino acids and peptide derivatives. A suitable
reagent is 3,5-di-tert-butyl-4-biphenylol
(la) which is readily
accessible by reaction of 2,6-di-tert-butyl-l,4-benzoquinone
with phenylmagnesium bromide and subsequent reduction
with z i n ~ / H C l [ ~ Introduction
.~].
of the new protecting group
3,5-di-tert-butyl-4-oxo-l
-phenyl-2,5-cyclohexadienyl(PChd)
OH
I .
R NH-CR'R2-COOR3
(3)
['I Dr. M.H. Khalifa, Prof. Dr. G. Jung, Prof. Dr. A. Rieker [ '1
Institut fur Organische Chemie der Universitat
Auf der Morgenstelle 18, D-7400 Tiibingen 1 (Germany)
[ '1 To whom correspondence should be addressed.
This work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie.
712
0 Verlag Chemre, GmbH, 6940 Weinheim, 1980
Table 1. Yields and physical data of selected PChd-protected amino acids (3a)
and peptides derivatives.
N-PChd derivative
PChd-Gly-OH
PChd-t-Ala-OH
PChd-Aib-OEt
PChd-Aib-OH
PChd-L-Leu-OBzI
PChd-L-Leu-OH
PChd-L-Pro-OBzl
PChd-L-Pro-OH
PChd-L-Leu-Gly-NH2
PChd-L-Pro-L-Leu-Gly-NH2
Yield
[%I
M.p.
I"C1
90 tai
94 la1
97 Ibl
94 14
89 Ibl
60 Icl
59 1'4
57 Icl
85 Id1
75 [d]
167-168
181-1 83
88-89
169-171
86-87
131-133
144-145
81-82
148
116-1 18
la]::
RF
[gI
(el
-
0.30 (11)
0.45 (11)
0.26 (I)
0.48 (11)
0.41 (I)
0.69 (11)
0.29 (I)
0.56 (11)
0.58 (11)
0.54 (11)
- 125.9
~
- 169.5
- 185.3
+
55.3
+
+
51.3 [Q
70.5
+ 183.8
[a] Based on (la) after hydrolysis of (3). [b] Based on ( l a ) (on reaction of H-AibOEt with (lo)in a molar ratio of 1 : I the yield is 82%). [c] Based on hydrolysis of
(3). [d] Based on coupling of PChd-Leu-OH and PChd-Pro-OH, respectively. [el
c= 1 , ethanol. [Q c=O.4. [g] (I) Dichloromethane/light petroleum (3:2), (11)
chloroform/methanol (8: 1).
The alkali-stable PChd group can be quantitatively removed by 50% trifluoroacetic acid (TFA) in dichloromethane within 15 min at 25 "C. The protecting group regenerates a phenol (4)['] which can easily be separated from the
amino acid ester (5) by extraction with diethyl ether.
An Amino-Protecting Group for Use in Peptide
Synthesis Which Can Be Introduced
Electrochemically[**J
[**I
is accomplished by anodic oxidation of (la) in dichloromethane in the presence of the free amino acid esters (2) at an
anode potential of + 1300 mV us Ag/O.Ol M Ag@at a platinum electrode in an undivided cell.
The N-(PChd)-amino acid esters (3a) (Table 1) are usually
formed cleanly, with high selectivity, and without racemization. Polyfunctional amino acids have to be completely
blocked except for the amino group to be protected.
TFA/CH2C12
(301
H,C, NH-CR'R2-COOEt
(4i
$"
C6H5
+
F3
H2N-CR'R"COOEt
*
TFA
(5)
These mild acidolysis conditions permit selective use of
this protecting group alongside benzyloxycarbonyl, benzyloxy, and benzyl ester protecting groups, even in syntheses involving repeated coupling and deblocking steps.
Particularly useful is the quantitative hydrogenolytic removal with Pd/C (10%)in methanol which regenerates the
protecting group in its original form (la). This method of removal permits its use alongside all hydrogenolysis-stable
protecting groups, e. g. the Boc group.
The applicability of the PChd protecting group in peptide
synthesis has been tested in the case of the MSH inhibitor HL-Pro-L-Leu-Gly-NH,["I (see Table I). Coupling of PChd-LLeu-OH with H-Gly-NH2 by the DCC/HOBt method afforded Pchd-~-Leu-Gly-NH~.
The H-L-L~U-GIY-NH~
generated both by acidolysis as well as hydrogenolysis was then
reacted with PChd-L-Pro-OH by the same method to give
well-crystalline, unequivocally characterized PChd-L-Pro-LLeu-Gly-NH, which, on hydrogenolysis, gave chromatographically pure, racemate-free H-L-Pro-L-Leu-Gly-NH2.Thus
the new protecting group fulfils the requirements of unequivocal peptide syntheses[']. The PChd group also facilitates
ready fast monitoring of the course of reaction by means of
UV detection. Easy recovery of the phenol (la) after hydro-
0570-0833/80/0909-0712
d 02.50/0
Angew. &hem Int. Ed. Engl. 19 (1980) No. 9
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