Enantioselectivity

Enantioselectivity is preferential formation or reaction of one enantiomer over the other. For a product containing two enantiomers, their imbalance is commonly reported as enantiomeric excess, the absolute difference between their amounts divided by their sum.

A product ratio of $90:10$ corresponds to $80\%$ enantiomeric excess, not $90\%$. A chiral catalyst can make the competing transition states diastereomeric and therefore unequal in free energy. Selectivity depends on the full reaction conditions and must be distinguished from the separate question of overall chemical yield.