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Allylic strain is an unfavourable steric interaction involving substituents around the region next to a carbon–carbon double bond. The double bond restricts rotation and makes the relative placement of nearby groups particularly important.

A rotation around an adjacent single bond can reduce crowding while changing which face of the alkene is exposed to a reagent. This provides a route from conformational preference to reaction stereoselectivity. The term should specify the interacting groups and conformation; it is not a universal fixed energy penalty for every allylic fragment.

Ryōji Noyori developed chiral catalysts for asymmetric hydrogenation, in which addition of hydrogen preferentially produces one enantiomer. This work was recognized by the 2001 Nobel Prize in Chemistry.^[ref]

The catalyst creates competing reaction paths with different activation free energies. It can thus favour one molecular handedness while being regenerated, linking catalysis with stereoselectivity.