Summary: 6536 J. Am. Chem. SOC.1989, 111, 6536-6544
in the literature, but there are no unambiguous examples sup-
porting the electron-transfer mechanism for the reaction of al-
kylcopper-Lewis acid systems.
Finally, a possible theoretical rationale for our findings is as
follows. The phenyl group, although larger than the methyl group,
is a weak -I group. It therefore has an electronic preference for
the outside position over the anti position, which contributes to
stabilize 17(ii) over 17(i). This somewhat compensates for the
repulsion between the phenyl group and the nucleophile. The latter
steric effect is presumably still dominant; however,the preference
for the anti position is reduced by the destabilizingelectroniceffect.
The methyl group prefers the anti position to the outside position
for both steric and electronic reasons and thus occupies the anti
position in preference to the phenyl group.
It is natural to ask whether a similar argument might then apply
to the reaction of the E isomers discussed earlier. Specifically,
it could be argued that structure 16(i) (Figure 21, R = C6H5))
is less stable than structure 16(iii) (R = C6H5),having the phenyl
group inside and the methyl group anti. Indeed, the latter con-