Origin of the Diastereoselection in the Indium-Mediated Addition of Haloallylic Sulfones to Aldehydes

The R1 substituents at C(2) of the haloallylic sulfones 1 play a pivotal role in controlling the diastereoselectivity of the indium-mediated
addition reaction to benzaldehyde to produce the homoallylic alcohols 3. The R1 Me group of 1 prefers the chair form in the In-coordinated
six-membered cyclic transition state to give anti-3a, and the R1 Ph group of 1 favors the twist boat form to give syn-3n, both in a high 13:1
selectivity.