Selectivity in nucleophilic additions to chiral aldehydes is often explained by the Felkin–Anh model (see figure). The nucleophile approaches the carbon of the carbonyl group at the Burgi-Dunitz angle. At this trajectory, attack from the bottom face is disfavored due to steric bulk of the adjacent, large, functional group.
The polar Felkin–Anh model is applied in the scenario where X is an electronegative group. The polar Felkin–Anh model postulates that the observed stereochemistry arises due to hyperconjugative stabilization arising from the anti-periplanar interaction between the C-X antibonding σ* orbital and the forming bond.
Improving Felkin–Anh selectivity for organometal additions to aldehydes can be achieved by using organo-aluminum nucleophiles instead of the corresponding Grignard or organolithium nucleophiles. Claude Spino and co-workers have demonstrated significant stereoselectivity improvements upon switching from vinylgrignard to vinylalane reagents with a number of chiral aldehydes.