HOMOGENEOUS CATALYSIS - MECHANISM OF DIASTEREOSELECTIVE HYDROACYLATION OF 3-SUBSTITUTED-4-PENTENALS USING CHIRAL RHODIUM CATALYSTS

Catalytic hydroacylation of 4-pentenals using [Rh(diphosphine)](+) catalysts leads to the formation of cyclopentanones. Catalytic kinetic resolution of racemic 3-phenyl-4-pentenal using the chiral catalyst, [Rh((S)-binap)](+), leads not only to the diastereoselective formation of beta-phenylcyclopentanone but also to the diastereoselective production of 4-phenyl-4-pentenal. The kinetics of the formation of these two products, together with deuterium distribution studies, indicates that hydroacylation proceeds by a variety of reversible steps. The 4-phenyl-4-pentenal is formed by carbonyl deinsertion-insertion steps. Although the steps appear to be reversible, equilibrium is not established. It is concluded that asymmetric hydroacylation is not governed by a single enantioselective step, but rather that the enantioselection is controlled by a number of reversible steps involving reaction intermediates.