Homogeneous Catalysis. Transition-Metal-Catalyzed Claisen Rearrangements

Article abstract of DOI:10.1021/ja00293a041

Palladium(0), palladium(II), rhodium(I), and iridium(I) complexes catalyze the rearrangement of allyl imidates to allyl amides under mild conditions.The palladium(II) catalysis is characterized by exclusive <3,3> regioselectivity and high steroeselectivity whereas the palladium(0), rhodium(I), and iridium(I) catalysts generally give both the <3,3> and <1,3> rearrangement products and although the palladium(0) catalyst can give high stereoselectivity, the rhodium(I) and iridium(I) catalysts are nonstereoselective.After a series of experiments using chiral substrates and substrates with specific deuterium labels, the mechanisms of these catalytic reactions have been elucidated.The palladium(II) catalysis is proposed to proceed via cyclic carbonium ion intermediates, and the mechanism resembles the thermal uncatalyzed Claisen rearrangement path.The palladium(0) catalysis is a form of catalytic allylation involving oxidative addition followed by nucleophilic attack on a ?-allyl intermediate.The mechansim of the rhodium(I) and iridium(I) catalysis was not as fully investigated as the others, but it appears to involve carbonium ion intermediates formed by cleavage of the allyl-oxygen bond.