Heterogeneous catalytic hydrogenation of allenes over supported palladium: selectivity, stereoselectivity, and regioselectivity

Abstract

The palladium-catalysed hydrogenation of twelve allenes has been studied, from the point of view of product composition, over the complete range (0–2 mol. equiv.) of hydrogen absorption. Selectivity, stereoselectivity, and regioselectivity have been estimated with special reference to the semihydrogenation point. The relative haptophilicities of the four possible approaches of suitably substituted allenes to the catalyst surface show that steric effects play a major role in determining which orientations shall lead to product formation. In the case of conjugated allene esters and acids however, parallel orientation of the ester and acid groups is unexpectedly disfavoured: this may imply repulsive electronic contributions.