A Single diastereoisomer of the sulfur ylide is formed and this can exist in conformations A and B. It is proposed that reaction occurs via the more stable conformer A
One face of the ylide is then blocked from the incoming aldehyde by the a-alkyl group as shown. Attack to the Re-face of the aldehyde leads to the formation of the (R,R) enantiomer, which is the major enantiomer produced.
As the same enantioselectivity is obtained for sulfides with increasingly large substituents a to sulfur (R=Me, R=iPr, ee = 93% in both cases, this suggests that the minor enantiomer is not formed from approach of the aldehyde on the Si face of the ylide.
This implies that the minor enantiomer is formed from the other conformation of the ylide B.
By performing the catalytic cycle using mesityl diazomethane, confomation B was further disfavoured and an enantiomeric excess of 98% was obtained. This lends weight to the proposal that the ylide shows very high face selectivity and that the minor enantiomer comes from the reaction of the minor conformer of the ylide.