Page 263 - Mechanism and Theory in Organic Chemistry

and give racemic products on reaction with an electrophile. Attempts to prepare
optically active Grignard reagents have been unsuccessful, except in the case of a
cyclopropyl derivative such as 42, where the increase in strain associated with the
planar transition state (I strainlZ2) provides a sufficiently high barrier to inver-
sion to maintain the stereochemistry.lZ3 The lithio-, and even sodio-derivatives
of the cyclopropyl system also show sufficient stereochemical stability to give
optically active products.lZ4 There is a certain amount of covalent character in
the carbon-metal bonds, although it must be quite small in the organosodium
compounds. The highly covalent organomercurials are readily prepared in
optically active form without any special structural requirements.
Most of the work on stereochemistry of carbanions free of specific bonding
to metals has been done with molecules that include a stabilizing group. Exam-
ples are shown in Structures 43,44, and 45. In these cases, because of the stabiliza-
tion attained by favorable conjugation with the .rr electron systems, the ions are

probably planar.lZ5 The situation for these structures is much like that of the
carbocations, and we may expect ion-pairing phenomena to exert a strong in-
fluence on the stereochemistry. Indeed, the stereochemical consequences of
generating anions at chiral centers in conjugation with .rr systems depend strongly
on the base and solvent.lZ6
In the deuterated 9-methylfluorenyl system (46), Cram and co-workers
found retention of configuration in tetrahydrofuran with ammonia or a primary
amine as base.lZ7 Streitwieser has obtained similar results with benzyl systems in
cyclohexylamine with cyclohexylamide as base.lZ8 Cram's proposed mechanism
is shown in Scheme 6. In dimethylsulfoxide, the exchange and racemization

lZ2 (a) H. C. Brown and M. Gerstein, J. Amer. Chem. Soc., 72, 2926 (1950); (b) H. C. Brown, R. S.
Fletcher, and R. B. Johannesen, J. Amer. Chem. Soc., 73, 212 (1951).
lZ3 H. M. Walborsky and A. E. Young, J. Amer. Chem. Soc., 86, 3288 (1964).
Iz4 (a) H. M. Walborsky, F. J. Impastato, and A. E. Young, J. Amer. Chem. Soc., 86, 3283 (1964);
(b) J. B. Pierce and H. M. Walborsky, J. Org. Chem., 33, 1962 (1968).
lZ5 A fluorine substituent, however, has the opposite effect on geometry. Pyramidal ions are stabilized
by fluorine and planar ions destabilized; conjugation with the filled f orbitals on fluorine is un-
favorable. See A. Streitwieser, Jr., and F. Mares, J. Amer. Chem. Soc., 90, 2444 (1968). Chlorine,
bromine, and iodine apparently stabilize an adjacent carbanion more than does fluorine, presumably
because the destabilizing f orbital overlap is less effective with the larger halogens (see Section 5.2,
p. 227). J. Hine, N. W. Burske, M. Hine, and P. B. Langford, J. Amer. Chem. Soc., 79, 1406 (1957).
See Cram, Fundamentals of Carbanion Chemistry, chap. 111, for a summary of results and discussion
of mechanisms.
lZ7 (a) D. J. Cram and L. Gosser, J. Amer. Chern. Soc., 85, 3890 (1963); (b) D. J. Cram and L.
Gosser, J. Amer. Chem. Soc., 86, 5445 (1964).
Iz8 A. Streitwieser, Jr., and J. H. Hammons, Prog. Phys. Org. Chem., 3, 41 (1965).