On the basis of the Felkin-Anh model, trends of
selectivities obtained in a large number of nucleophilic additions to R-chiral aldehydes can be convincingly explained (Table 1).14-16 An increase in the
steric bulk of the nucleophile renders its addition
more selective toward the Felkin product, as depicted
in the reaction of 13 with various Grignard reagents
(Scheme 6), since the nucleophile is “more aware” ofthe difference of S andMupon attacking the carbonyl
group.17 It is important to note that the phenyl group
must be regarded quite similar in its steric size to a
methyl group, as seen by comparison of the selectivities
obtained with phenyl- and methylmagnesium
bromide. The fact that the phenyl group is regarded
in 13 as the substituent L and the methyl group as
M might be due to stereoelectronic reasons with
phenyl being a weak electron acceptor. However, the
balance of the factors seems to be quite delicate, and
examples in which methyl rather than phenyl takes
the role of L are also encountered.