4424
The Stereochemistry of Electroreductions. I. Cyclopropyl Halides’ Raymond Amino, Ronald E. Erickson, John Michalovic,2 and Bruce McKay2 Contributionf r o m the Department of Chemistry, Canisius College,
BuffaIo, New York. Received May 12,1966 Abstract: The electrochemical behavior of optically active l-bromo-2,2-diphenylcyclopropanecarboxyl~c acid, its methyl ester, and l-bromo-l-methyl-2,2-diphenylcyclopropane is reported. Reduction of the acid or its methyl ester yields products of partially inverted configuration, whereas the reductions of the carboxylate ion and l-bromo-lmethyl-2,2-diphenylcyclopropaneproceed with partial retention of configuration. The stereochemical results correspond closely with those observed in zinc metal reductions of the same compounds. They are readily interpreted in terms of a mechanism involving initial attack by the electrode or metal surface on the halogen side of the carbon-
halogen bond.
In
recent years there has been a great deal of interest shown in the mechanism of the electroreduction of the carbon-halogen bond. Elving and Pullman’s generalized reaction mechanism scheme follows3 RX
+ e ----+[ R . ] + X[R .] ----+R . 2R.
----+R-R
+ e ----+(R-) + solvent ----+R H
[R.] (R-)
where [R . ] and (R-) represent electrode complexes. In view of the large potential gradient in the immediate vicinity of the electrode, about IO7 v/cm,4 the initial step in this mechanism has been assumed to be attack of the electrode on the carbon atom of a polarized carbonhalogen bond. Controlled-potential electrolyses of benzyl halides have given evidence for the involvement of both carbanions and radicals as intermediates; carbonation of the electrolyzed benzyl chloride has yielded phenylacetic acid,j whereas 4,4-dinitrobibenzyl has been isolated from the electrolysis ofp-nitrobenzyl bromide.6 Many mechanistic implications have been drawn by correlating changes in polarographic half-wave potentials with changes in structure. Streitwieser and P e r r i r ~ by , ~ use of linear free energy relationships, have shown that the transition state for the polarographic reduction of substituted benzyl chlorides possesses considerable radical character. Lambert* correlated half-wave potentials of alkyl halides with s N 2 reactivity and more recently has proposed that in certain sterically hindered bridgehead halides, an ionic process of dissociation to carbonium ions may be the potentialdetermining step.g (1) The support of this Rork by the National Science Foundation is gratefully acknowledged. ( 2 ) Abstracted in part from the M.S. thesis of J. M. and the senior thesis of B. M. (3) P. J. Elving and B. Pullman, Adran. Chem. P h j s . , 3, 1 (1961). (4) G. J. Hoijtink, Rec. Trac. Chim., 76, 885 (1957). (5) S. Wawzonek, R. C. Duty, and J. H. Wagenknecht, J. Electro-
chem. Soc., 111, 74 (1964). (6) G. Klopman, Hela. C h m Acta, 44, 1908 (1961). (7) A. StreitNieser, Jr., and C. Perrin, J . Am. Chem. SOC.86, 4938 (1964). (8) F. L. Lambert and I
