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Novel Conjunction of Hetero(macro)cycles and a Pentamolybdodiphosphonate Cage.

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Angew. Chrm. I n / . Ed. Engl. 1990, 29, 183; c) P. Buzek, P. von R . Schleyer. S
Sieber. W. Koch, J. W de M. Carneiro, H. Vani-ik, D. E. Sunko. J. Chrm. S o ( .
Cheni. Comniun. 1991, 671; d ) P. Buzek. P von R. Schleyer, H. VanEik. D. E .
Sunko, rhid. 1991, 1538.
a) W. Hehre, L. Radom, P. von R. Schleyer, J. A. Pople. A h Inrrro M o k u l u r
Orbitui 717eori. Wiley. New York, 1986; b) B. A. Hess, Jr., L. J. Schaad. P
Carsky. R. Zahradnik. Chem. R e v 1986, 86. 709.
a) H. VanEik, V. Gabelica, D. E. Sunko. P. Burek. P. von R. Schleyer, J. Ph,w
Org. Chem. 1993. 6,421: b) W. Koch. B. Liu. D. J De Frees, J Am. ('hein. So<.
1988, i i o . 7325.
a) H:U. Siehl. F. P. Kaufmann, Y. Apeloig. V. Braude. D. Danovich. A .
Berndt, N. Stamatis, Angen.. Chem. 1991. 103, 1546. A n p . . Chem. In(. El/.
EngI. 1991, 30, 1479; b)G. A. MeGihbon, M. A. Brook, J. K . Terlouw. J.
Chem. Soc. Chrru Commun. 1992. 360; c) Y Apeloig in Th1, Chemrstri. of
Orgunic Sdrron Compounds (Eds.: S. Patai. 2.Rappaport); Wiley, Chichester.
1989; d) P. von R. Schleyer. P. Burek, T. Miiller, Y. Apeloig. H.-U. Siehl.
Angcw. Chrm. 1993, lf)S, 1558; A n p i . Chrm. I n / . Ed. Engl. 1993. 32, 1471
P. von R. Schleyer. T. M. Su, M. Saunders, J. C. Rosenfeld. J A m . Chern. So1
1969, 91. 5174.
It has not been possible t o observe the allyl cation by NMR starting with
cyclopropyl bromide. New experiments carried out at Tiibingen hy Prof H -U.
Siehl, Martin Fuss, and Bernhard Miiller using the matrix cocondensation
technique (ref. [I g] p. 31) only gave the broad, featureless spectra characteristlc
of polymers. The 'H N M R spectra were not clean enough to allow identification of signals expected for the allyl cation. N o peak near 6 = 227 in the j 3 ( 1
N M R spectrum. as predicted from CIAO-MP2 calculations tor I . could he
detected.
Very good agreement is found between the calculated and
experimental frequencies for 1 (Table 2). The graphical comparison (Fig. 4) demonstrates that the experimental frequencies
correspond excellently to the calculated values.
2500
t
s (exp)
[ cm
-' j
-
'OoO-
1500 1000
1000
I500
2000
S (calcd) [ cm -'I
2500
3000
+
Fig. 4. Correlation of the experimental and calculated (CISD,631G**) IR frequencies of the ally1 cation I . cc = correlation coefficient. rn = slope.
The allyl cation 1 is now the smallest long-lived carbocation
observable in a solid SbF, matrix. Our findings provide solid
evidence that the allyl cation was formed under cryogenic conditions by ionizing cyclopropyl bromide. Unlike the unsaturated
precursors of 1, cyclopropyl bromide evidently does not react
with 1 during generation. The IR asymmetic stretching vibration at about 1575 cm-' characterizes 1 as well as allyl cations
in general. Although accompanied by side reactions, 1 apparently also formed from allyl chloride and bromide, and possibly
BGW's NMR evidence
from trimethylsilylpropyne as
for 1 is equivocal. In light of the difference of A8 = 10 in the 13C
chemical shifts of the zeolite and the GIAO-MP2 I3C chemical
shifts, confirmation is desirable.
Received: April 14, 1992
Revised: August 31, 1993 [Z 5303 IE]
German version: Angen. Chem. 1994, 106, 470
[I] a) N. C. Deno in Curhonium Ions, Yo/. 2 (Ed.: G . A. Olah, P. von R. Schleyer),
Wiley-lnterscience, New York, 1970, Chapter 2; b) G. A. Olah. P. R. Clifford,
Y. Halpern, R. G. Johanson, J. Am. Chem. Soc. 1971, 93,4219; c) G. A. Olah,
R. J. Spear, ihid. 1975, 97, 1539. 1845; d) G . A. Olah, J. S. Staral, R. J. Spear,
97, 5489; e ) G . A. Olah, H . Mayr, ihid. 1976. 98, 7333;
f ) H. Mayr, G. A. Oiah. ;hid. 1977.99. 510; g) Mefhoden
Or-g. ('hem. (Houhen-Wed) 41h ed. 1952-, Bd. E19c, 1990.
[2] G. A. Olah, M. B. Comisarow, J. A m . Chem. Soc. 1964. 86. 5682.
[3] a) H.-U. Siehl, C. S. Yannoni. G. A. Olah, personal communication; b) P.
Vogel. Curhocurron Chemistrj,, Elsevier, Amsterdam, 1985, p. 173.
[4] a ) G . A. Olah, E. B. Baker. J. C. Evans, W. S. Tolgyesi, J. S. McIntyre, I. J.
Bastien, J. Am. Chem. Sot. 1964.86, 1360; b) W. Koch, P. von R. Schleyer. P.
Buzek. B. Liu, Croal. Chem. Actu 1992, 65. 655.
[5] S. G. Lias. J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin, G . Hallard,
J Phw. Chein. Ref. Dutu Suppl. 1988, 17, 1
[6] a) G . I. Hutchings, D. F. Lee, C. D. Williams, .lChrm. Soc. Chem. Cummun.
1990. 1475; b) E. J. Munson, T. Xu. J. F. Haw, ibid. 1973, 75.
[7] A. I . Biaglow. R. J. Corte, D. White, J. Chem. SOC.Chrrn. Commun. 1993, 1164.
[8] M Schindler, J. A m . Chem. Sot. 1987. 109. 1020. IGLO = Individual Gauges
for Localized Orbitals.
[9] a) J. Gauss. J. F. Stanton, R. J. Bartlett. Chem. P h w . Let/. 1992. 191, 614; J
Chcw. Plzw. 1993. 99. 3629. The (GIAO)-Gauge-Including Atomic Orbital
MP2-Programm has been implemented into the ACES I1 program system (J. F.
Stanton. J. Gauss, J. D. Watts, W. J. Lauderdale, R. J. Bartlett, Quantum Theory Project. University of Florida, Gainsville. 1992). h) Basis sets are given in:
A. Schifer. H Horn, R. Ahlrichs, J. Cheni. Phys. 1992, 97, 2571.
[lo] a) R. Ditchfield. Mol. Phvs. 1974, 27, 789; b) K. Wolinksi. J. E Hinton, P.
Pulay. J. Am. Chern. Sor. 1990, 112, 8251 ; c) M. HBser. R . Ahlrichs, H. P.
Baron, P. Weis. H. Horn, Theor. Chim. ACIU 1992, 83. 455.
[ l l ] S. Sieher. P. von R. Schleyer, J. Gauss, J. Am. Chem. Soc. 1993, 115, 6987.
[12] a) H. VanEik. D. E. Sunko. J. Am. Chem. Soc. 1989, 111. 3742; b) W. Koch, B.
Liu, D. J. DeFrees, D. E. Sunko. H. VanEik, Angew. Chem. 1990. 102, 198;
A i r g i , ~. ~Chmri. I n r . Ed. Enk.1. 1994, 33. N o . 4
Novel Conjunction of Hetero(macro)cycles
and a Pentamolybdodiphosphonate Cage**
Mark P. Lowe, Joyce C. Lockhart,* William Clegg,
and Kelly A. Fraser
Macrocycles having separate additional functionality are much
sought after for the development of supramolecular chemistry.[']
Polyoxometalates with separate, additional functionality are also
in demand in materials sciences for the development of difunctional catalystsr2]and a whole gamut of other applications.[31
The conjunction of these two major areas of current research has
been achieved with the synthesis of the first examples of polyoxomolybdate cages derivatized with hetero(macro)cycles. The
potential of these novel conjugates for a variety of applications
from catalysis to nuclear medicine is under investigation.
Incorporation of the heterocycles as their methylphosphonates was achieved by mixing phosphonate (1 or 2) and molybdate according to Equation (a) at pH 2-5. The mixture was
2 [C(NH,),]+
+ 2 ZP0,H + 5 MOO:- + 8 H' e
(a)
z+=1,2
heated to nearly boiling and allowed to cool slowly to give
colorless crystals suitable for X-ray ~rystallography.[~]
Since heterocycle Z has a protonated nitrogen atom, ZPO,H
is a zwitterion. The structure of [C(NH,),], [3] 3 H,O consists
["I
[**I
Dr. J. C. Lockhart, M. P. Lowe, Prof. W. Clegg, K . A. Fraser
Department of Chemistry, University of Newcastle
Newcastle-upon-Tyne, NE1 7RU ( U K )
Telefax: Int. code + (91)261-1182
This work was supported by the Science and Engineering Research Council
and Courtaulds Coatings PLC. We thank C. J. Matthews for preliminary
"P NMR measurements and a particularly careful referee for his helpful
comments.
VCH Vrr/u~sk.e.rr/l.rchrrft
mbH, D-69451 Weinhc~irn,1994
0S70-0833/94~0404-0481
8 10.00+ .28!0
45 1
COMMUNICATIONS
H
Table 1. Selected bond lengths
[C(NHd.<I,PI 3 H20 [a].
[A] and
angles
["I
of the anion 3 in a crystal of
'
1
L
ligand 1: P O @ (El)
anion 3: z
'= 1
ligand 2: ZPO$
anion 4: z
'= 2
(2-2)
of guanidinium cations, water molecules of crystallization and
[(ZP),Mo,0,,J2- anions 3 (the protonated nitrogen atoms of Z
provide two additional internal counterions). Anion 3 lies on a
crystallographic C, axis passing through Mo3 and 0 7 . Its structure (Fig. 1 and Table 1 ) is similar to that well known for the
[P2Mo50,,l6- anionrs1in that it consists of a ring of five distorted MOO, octahedra. Four of the octahedra are joined by edge-
06
%3
Mol-02
Mol-07
M o l - 0 10
Mo2-04
Mo2-08
Mo2-030
Mo3-06
Mo3-011
PI-011
P1-c1
02-Mol-03
03-Mol-08
03-Mol-07
02-Mol-012'
08-Mol-012'
0 2 - M o l - 0 I0
OX-Mol-010
012'-Mol-010
04-Mo2-08
04-Mo2-09
08-Mo2-09
05-Mo2-010
09-Mo2-010
05-Mo2-011'
09-Mo2-011'
06-Mo3-06'
06-Mo3-09
06-Mo3-011
09-Mo3-011
0 1 1-Mo3-011'
Mo2-08-Mol
P1-010-Mo2
Mo2-010-Mol
P1-011 -MOB
PI-012-Mol'
012-P1-010
012-P1 -c1
010-P1-C1
1.701(3)
1.947(2)
2.380(3)
1.702(3)
1.919(3)
2.251(3)
1.723(3)
2.366(3)
1.539(3)
1.826(4)
102.1(2)
100.13(14)
100.88(12)
83.27(13)
77.10(11)
169.20(12)
70.44(11)
88.90(10)
99.93( 14)
100.22(14)
149.07(12)
91.1 j(13)
80.23(12)
161.45(12)
72.22(11)
in4.2(2)
102.21(14)
87.02(13)
80.92( 12)
82.39(14)
122.6(2)
130.6(2)
93.35(10)
123.7(2)
115.8(2)
109.2(2)
110.312)
106.9(2)
Mol-03
Mol-08
Mol-012'
Mo2-05
Mo2-09
Mo2-011'
Mo3-09
Pi-010
P1-012
C1-N1
02-Mol-08
02-Mol-07
08-Mol-07
03-Mol-012'
07-Mol-012'
03-Mol-010
07-Mol-010
04-Mo2-05
05-Mo2-OX
05-Mo2-09
04-Mo2-010
08-Mo2-010
04-Mo2-011'
08-Mo2-011'
010-Mo2-011'
06-Mo3-09
09-Mo3-09'
06-Mo3-011
09'-Mo3-011
Mol'-07-Mol
Mo3-09-Mo2
PI-010-Mol
PI -011-Mo2'
Mo2'-011 -Mo3
012-PI-011
011-PI-010
011-PI-CI
N 1-C1 -P1
[a] Symmetry transformation for primed atoms:
Fig. 1. Structure o f the anion 3 in a crystal of [C(NH,),],[3]
'
3 H,O
sharing oxygen atoms; the ring is completed and the symmetry
broken by one corner-sharing oxygen atom between two molybdenum atoms. Two ZPO, tetrahedra cap the top and bottom of
the ring; one oxygen atom is shared with only one MOO, octahedron (the corner-sharing one) and the other two shared with
two MOO, octahedra each. As a result the ten-membered (Moo), ring has an envelope-type conformation, whereby one
molybdenum atom is bent out of the plane formed by the other
four. This is reflected in the Mo-0-Mo bond angles. The angles
at the four edge-sharing linkages are roughly 123", whereas the
angle at the shared corner is 148". Extensive intra- and intermolecular hydrogen bonding is evident throughout the crystal.
The morpholine ring bends over the molybdenum cage and its
protonated nitrogen atom forms an intramolecular hydrogen
bond to a terminal molybdenum oxygen atom (N-H . . . O =
2.043 A, angle = 106.4"). This is reflected in the terminal
M o = O bond lengths: M o - 0 groups not involved in hydrogen
bonding have a M o = O bond length of 1.700 A ; however, the
452
fi?
VCH l4~rlugsgeselisrhuft mhH. 0.69451 WXnheim.1994
--I
+ 1, y.
1.709(3)
1.922(3)
2.343(3)
1.715(3)
1.950(3)
2.349(3)
1.891(1)
1.543(3)
1.506(3)
1.498(5)
ino.44(14)
102.66(14)
144.52(14)
174.34(13)
79.28(9)
85.51(1 3)
83.07( 12)
106.4(2)
100.58(14)
95.96(14)
162.28(13)
73.53(11)
90.10( 13)
84.53(1 I)
73.07(11)
99.1 5( 14)
144.9(2 )
167.55(13)
72.78( 12)
147.7(2)
123.1(2)
134.7(2)
129.2(2)
9 1 . m 10)
113 l(2)
112.6(2)
104.5(2)
113.1(3)
--I
+ 312.
group forming a hydrogen bond has one of 1.715 A. Although
this difference is hardly significant, the overall pattern of bond
lengths for terminal M o = O bonds in the anion (see below) does
show a real correlation with the hydrogen bonding.
The C-H protons of the morpholine ring and of the NCH,P
group are extremely close to the terminal (Mo=O) and bridging
(Mo-0-P)
oxygen atoms on the adjacent molecule. The
0 . .. H-C distances are in the range 2.3-2.5 A. The guanidinium counterions form hydrogen bonds to the oxygen atoms in
the molybdenum cage, the morpholine oxygen atoms, and the
water molecules with 0 . ' . H-N distances of 2.2-2.3
The
water molecule, as well as being involved in hydrogen bonding
with the previously mentioned guanidinium ions, is also hydrogen bonded to just one terminal molybdenum oxygen atom
(Mo3-06) with 0-H . . ' 0 distances of approximately 2.3 A.
The combination of two water molecules forming hydrogen
bonds to this oxygen atom results in an increase in the Mo=O
bond length from roughly 1.700 A to 1.723 A.
The cage anions were characterized in solution by 31PN M R
spectroscopy. Individual species have been assigned in systems
of pentamolybdodiphosphonates (and other systems) by Pettersson et a1.16' by using electromotive force (EMF) and spectroscopic methods including 31PN M R . These studies indicate that
at defined ratios of P:Mo in defined p H ranges, signals maximize in sequence according with species postulated based on the
results of E M F and crystallographic studies.
A selection of "P N M R spectra at a P:Mo ratio of 2 3 and
pH 1-6 (for phosphonates 1 and 2) is shown in Figure 2. In
057U1-0833;94/0404-0452$ 10.00f .25/0
A.
Angew. Chmi. I n t . Ed. EngI. 1994. 33. N o . 4
COMMUNICATIONS
~
PH
PH
1.00
1.00
L1.90
2.01
3.10
2.89
3.95
c
LL
3.98
B
5.02
from the spectrum of the filtrate; however, no attempt was made
to observe the reestablishment of the equilibrium, The signals
broadened at lower pH values indicating fast exchange processes not quite at the fast-exchange limit on the NMR timescale.
Cage rearrangements are a recognized phenomenon in polyoxometalate chemistry and have been studied in much detail by
NMR spectroscopic methods."]
The plot of signal intensities in Figure 3 indicates that the concentration of the [(ZP),MO,O,,]~- anion should be maximized
at pH = 2-3. The plot also indicates (as noted by other workers[*])that the most abundant species in solution is not necessarily
the one that precipitates. Indeed the 2: 5 compound is never the
most abundant species here. Also clear from the plot is the total
incorporation of the macrocycle into some form of molybdenum cluster at pH < 3 .
Intramolecular hydrogen bonding between the protonated
nitrogen atom of the hetero(macro)cycle and the terminal M o - 0
oxygen atom of the MOO, octahedra is an integral part of these
structures, and the bending of the hetero(macro)cycle to form
this hydrogen bond can clearly bring an attached macrocyclic ring
and associated metal ions into close proximity to the MoOP cage.
Since analogous phosphonomolybdates with rings containing
N, S, and 0 donors have been successfully synthesized in this
work, an entire range of metal ions from hard alkali metals to
soft thiophiles could be incorporated. In view of the great potential of these conjugates at the interface of two burgeoning fields,
we are seeking to extend the range of cages and macrocycles that
may be joined, especially by using organic solvents.
L
A
B
L
A
*
A
4.98
6.10
20
10
20
0
10
0
t 6
t 6
Fig. 2. Change in the 'IP NMR s~gnalswith pH in mixtures of [2] and [MOO,]'(left). and [I]and [ M o O J - (right). in both cases at a ratio of 2:s. Chemical shifts:
B: 6 =11.80-12.80, C:
free ligand L: 6 = 6.50-7.60, A : 6 =10.95-11.10,
6 =15.60-17.70. H,PO, (impurity marked with *): 6 = 3.30-3.50.
Experimental Procedure
All chemicals were obtained from Aldrich except the phosphonic acids, which were
synthesized by using modifications to the Moedritrer-Irani reaction [9].
addition to signal L of the phosphonate reagent, which disappeared around pH 3, three prominent signals marked A, B, and
C were observed for both phosphonates in the range 6 = 10.911.1 (A), 11.8-12.8 (B) and 15.6-17.7 (C) within the pH range
1.5-4.5. That signal C pertains to the cage is confirmed for
phosphonate 1 : upon addition of guanidinium chloride the salt
of anion 3 precipitates and concomitantly signal C disappears
[C(NH2)JI[3] . 3 H,O: Na,MoO,. 2 H,O (3.00 g, 12.40 mmol) and the hydrate of
ligand l ( l . 0 0 g, 5.02 mmol) were dissolved with stirring in H,O (50 mL). Guanidine
carbonate (0.70 g. 3.89 mmol) was neutralized with HCI (11.34 M) and added dropwise to the stirred solution. The pH was lowered to 3.5 with HCI (3 M). The white
precipitate that formed was redissolved by heating the solution to 90'C. On
slow cooling large colorless crystals formed (yield: 1.1 g, 35%); IR (KBr):
C[cm-'] = 925, 897 [Mo-O(terminal)]; 690 [Mo-O(bridging)], 1151. 1067. 978
(P-0): 1296, 1263 (P-C); correct C,H,N analysis.
N M R Titration of ligand2 with the molybdate anion: A solution containing
Na,MoO, . 2 H,O (0.095 g, 0.39 mmol) and the sodium salt of ligand 2 (0.054 g.
0.16 mmol) in H,O (1 mL) was titrated with HCI (1 1.34 M). No correction was made
for the 7 % volume increase during the titration. The ) ' P N M R spectra of samples
were recorded at regular intervals of pH [lo]. The resultant spectra are shown in
Figure 2, and a plot of the relative intensities of the signals is shown in Figure 3 .
Corresponding spectra for ligand 1, shown in Figure 2 for comparison, were obtained similarly by using a solution in H,O (10 mL) containing Na,MoO, ' 2 H,O
(0.45 g, 1.88 mmol) and the hydrate of ligand 1 (0.15 g. 0.75 mmol).
Received: August 18,1993 [Z6296 IE]
German version: Angew. Chrm. 1994, fU6. 463
2
3
4
PH
-
5
6
7
+ L t A + B - - e C
Fig. 3. Mole fraction of phosphorus F (derived from the "P N M R spectra of
ligand 2 by using the relation F = (integral of one P signal)/(total integral of all P
signals) for each of the species A. B, C. and L for L = 2 at different pH values.
A n x e ~Chrnz.
.
I n / . Ed Engl. 1994. 33. No. 4
[l] a) C. Seel. F. Vogtle, Angew. Chem. 1992, 104. 542; Angew. Cheni. fnr. Ed. Engl.
1992, 31, 528: b) Fronriers in Supramolrrulur Organic Chrnzisrrj und Photoc.hemisiry (Eds.: H.-J. Schneider, H. Durr), VCH, Weinheim. 1990: c) JLM.
Lehn, Angew. Chem. 1990, 102, 1347; Angrw. Chewi. In!. Ed. Engl. 1990. 29.
1304.
[2] A. Proust, P. Gouzerh, F. Robert, Angeiv. Ch1.m. 1993, 105, 81, Angew. Chem.
I n r . Ed. Engl. 1993, 32. 115.
[3] a) V. W. Day, W. G . Klemperer, Science 1985, 22X, 533; b) M. T. Pope, A.
Miiller, Angeiv. Chem. 1991. 103.56; Angebv. Chem. Inr. Ed. Engl. 1991,3(1. 34.
[4] [C(NH,),],[3] 3 H,O: M , =1254.16, monoclinic, C2:c, ( I =16.347(5).
b =13.776(5),r=16.132(5)A,~=102.71(4)". V = 3544(2)A3.Z=4,eCrlcd =
2.351 gcm-', F(000) = 2464, Mo,, radiation, E. = 0.71073 A, [i = 1.92 m m - l .
The structure was determined by direct methods and refined o n F z for all 3095
independent reflections measured at 160 K with a Stoe-Siemens diffractoineter
(28,,
= 50.). with anisotropic atomic displacement parameters and with
isotropic hydrogen atoms: R = [ZufF:' F ~ ) ' / X W ' ( F ~=)0.0762
~ ] ~ ~ for
~ all
'
C VCH Verlugsgesellschaft m h H , 0-69451 Weinheim, 1994
-
0570-0833/9410404-0453 8 10.00+ ,2510
453
COMMUNICATIONS
(51
[6]
[7]
[8]
191
[lo]
data, conventional R [on F values for 2999 reflections with F,' P 2 0 ( F : ) ] =
0.0266. goodness of t i t on F 2 =1.199 for 272 parameters. Programs:
SHELXTL:PC and SHELXL-93. Ci. M. Sheldrick. Universitit Gottinpen.
Further details of the crystal structure investigation are avarlable on request
from the Director of the Camhridge Crystallographic Data Centre, 1 2 Union
Road. GB-Cambridge CB2lEZ (UK), on quoting the full journal citation.
a) R. Strandherg. .4clu Chon. Scimd. SiJr.A 1973.27, 1004: b) W. Kwak, M. T.
Pope. T. F. Scully. .I Ani. C/iiw/. Sot. 1975, Y7. 5735: c) J. K. Stalick. C . 0
Quicksall. / n o r E . Chein. 1976, 1.5, 1577: d ) D -G. Lyxell. R . Strandherg. Actu
C F J , T I U / Secr.
/ O ~ .C 1988. 44. 1535.
a ) A. Yagasaki, I. Andersson. L. Pettersson. h r g . Cliem. 1987. 26 3926; h) L
Pettersaon. I. Andersson. L . - 0 . Ohman. ;hid 1986. 15. 4726: c) A r m Chiw.
S c u d . Sw. A 1985. 39. 53.
W. G. Klemperer. C. Schwartr. D . A . Wright, J A n ! . C/wnz. SO(. 1Y85, 107.
6941
D . G . Lyxell. R . Strandberg. D. Bostrom, L. Pettersson. Acru C l i ~ m Scund.
.
Ser. A 1991. 4.5, 681
K. Moedritzer. R. Irani. J. 0i.g. C/win. 1966, 31, 1603.
The "P NMR spectra were recorded on a JEOL 90-FXQ spectrometer at
36.2 MHI with proton decoupling and referenced to external 8 5 % H,PO,
Before we were aware of this work we investigated two preparatively versatile reactions in static magnetic fields of varying flux
densities. These were the alkylation of aldehydes 1 with alkyl
magnesium halides and the reduction of prochiral ketones 2 with
complex metal hydrides. The dependence of the enantioselectivity
of these reactions on the magnetic flux density was studied.
It was found that a static magnetic field of flux density 0.22.1 T was sufficient to achieve considerable enantiomeric excesses in these reactions.[l6]Thus the addition of dimethylmagnesium, methyl magnesium iodide, or ethylmagnesium t-butylalcoholate to benzaldehyde ( I a) or naphthalene-2-carbaldehyde (1 b) in a static magnetic field of flux density 1.2 T leads to
( R ) - or (S)-1-phenylethanol (3a), -1-(2-naphthyl)ethanol (3b),
or -1-phenylpropanol (3c), respectively, with enantiomeric excesses of 65, 98, and 57%, respectively. The reduction of phenones 2a-c with lithium aluminum hydride at 1.2 T yielded the
( R ) - or (S)-1-arylethanols 3a-c with similar selectivity (5598 YO r e ) . Significantly lower enantiomeric excesses (1 1 YO,
2.1 T) were achieved in the reduction of butanone 2d to give ( R ) or (S)-2-butanol 3d.
Enantioselective Reactions in a Static
Magnetic Field
H
Guido Zadel, Catia Eisenbraun, Gerd-Joachim Wolff,
and Eberhard Breitmaier*
Absolute asymmetric synthesis (AAS)"] denotes an enantioselective synthesis from achiral starting materials without the
help of chiral reagents. An example is the synthesis of hepta- and
octahelicenes in circular polarized light with enantiomeric excesses up to 7.3 %.['. 31 Absolute asymmetric synthesis has been
discussed as the origin of optical activity in evolution.[4- ' 1
In 1975 P. Gerike introduced a new absolute asymmetric synthesis.['21 He carried out reactions in electric fields (158100 Vcm-') to which parallel, antiparallel, or orthogonal constant or alternating magnetic fields (8 x 103-1.17x l o 5 A m - ' ,
0-50 Hz) were superimposed. The formation of oxiranes, the
addition of bromine to C-C double bonds, and the alkylation
of prochiral ketones with Grignard reagents to form secondary
alcohols were studied. The enantiomeric excesses amounted to
at most 0.94% without preference for either ( R ) or ( S ) isomer.
However, the level of enantiomeric excesses contradicts the theoretical prediction of W Rhodes and R. C. Dougherty[13]that
enantiomeric excesses of no more than 3 x
YOshould be
achievable with magnetic flux densities of up to 1 T. R. C.
Dougherty et al. later studied the formation of oxiranes and
cyclopropanes at flux densities of "about 1 . I T"[14,151
without,
however, the simultaneous application of an electric field. Instead, they spun the reaction vessel at high speeds (600014000 rpm) in order to achieve the desired selectivity with the
additional gravitational field.[141As Gerike had reported, they
found an irregular preference for the ( R ) or ( S )enantiomer (ee
< 1 %), which they traced back to photochemical side-effects
(daylight, artificial light), after the influence of the earth's magnetic field had been eliminated by aligning the apparatus with
the compass.
[*] Prof. Dr. E. Breitmaier, D r . G. Zadel, Dipl.-Chem. C. Eisenbrdun
Institut fur Orgdnische Chemie und Biochemie der Universitit
Gerhard-Domagk-Strasse 1. D-53121 Bonn (FRG)
+
Telefax: Int. code (228)73-5683
Dr. G:J. Wolff
Bruker Analvtische Messtechnik
D-76287 Rhdinstetten (FRG)
454
:(~"VCH
Verlu~sResellschaftmhH, 0.69451 WiGnheirn, 1994
RMgX
1
3a
3b
3c
3
R
Ar
Me
Me
Et
Ph
X
So[TI
UMe
2-naphthyl
I
OtBu
Ph
LiAIH,
C,H,. Et,O
R'
I
R/'\O
R'
I
3
R
3b
3c
3d
65
98
57
ca. 2 0 T , 1.5h
2
3a
ee[./]
1.2
1.2
1.2
13'
Ph
2-naphthyl
Ph
Et
Me
Me
Et
Me
6, [TI
ee ("4
1.2
67
0.4
2.1
2.1
68
55
11
When the reactions were repeated under the same flux density
the enantiomeric excesses were always reproducible, although
which enantiomer dominated was unpredictable. However, in
the case of the crystalline 1-(2-naphthyl)ethanoI (3b), we could
repeatably demonstrate that by seeding the reaction mixture of
2 b in the magnetic field with the desired isomer of 3 b before the
start of the reaction, it was possible to produce the desired
isomer. Without the magnetic field the observed enantiomeric
excess corresponded to the amount of seed crystals added.[16]
The enantiomeric excesses were determined by specific rotation and also by 'Hand I3C NMR measurements in the presence of tns(3-heptafluoropropylhydroxymethylene)-~-~dmphoratoeuropium(Ir1) or -praseodymium(m) as chiral shift reagent
(Fig. 1). The absolute configurations of the products were determined by comparison with the known, published directions of
rotation. As shown in Figure 2 the enantiomeric excess increases
steadily with the magnetic flux density B, within the field range
investigated so Far.
0570-0833/94/0404-0454Si 10.00
+ .25/0
A n p w . Chrm. In[. Ed. Engl. 1994, 33, N o . 4
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