Is amine addition vital for highly enantioselective hydrogenation of α,β-unsaturated carboxylic acid over cinchonidine-modified palladium?

Benzylamine (BA) is a commonly employed additive for the hydrogenation of α,β-unsaturated acids over the cinchonidine-modified palladium catalyst, but the BA addition effects largely depend on the reaction solvent. In toluene, BA is not required to obtain the high enantioselectivity. Quicker desorption of the product due to weaker acidity in toluene than in a commonly used wet dioxane reasonably explains both the kinetic properties and the product selectivities.