Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh1; 2) intramolecular isomerization from κ1-(N)-coordination to η2-(C, C)-coordination of the allylamine; 3) oxidative addition of C1-H to form a distorted octahedral η3-allyl complex of RhIII; and 4) hydrogen transfer to C3 (reductive C3-H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol-1. For the reductive elimination, the value in solvent is 28.6 kcal mol -1, very similar to the oxidative addition barrier. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.