A kinetic study of the asymmetric titanocene-catalyzed imine hydrogenation has revealed the rate law to be rate = k(obs)[Ti][H-2]. for cyclic imine 2 and acyclic imine 4. This rate law is consistent with a mechanism in which the imine reacts with a titanium hydride in a fast 1,2-insertion step, to form a titanium amide intermediate, followed by slow reaction of the amide complex with hydrogen to produce the amine and regenerate the titanium hydride. Labeling studies for the hydrogenation of 2 and studies using enantiomerically enriched aldimine 6 indicate that beta-H elimination is also slow, relative to hydrogenolysis, for both 2 and 4. The enantiomeric excesses for the hydrogenation of 2 were found to be essentially insensitive to changes in reaction conditions. However, for imine 4, the ee's were dependent on several variables, most significantly hydrogen pressure. This-phenomenon has been explained on the basis of the interconversion of the syn and anti isomers of 4 during the hydrogenation. It has been shown that syn-4 reacts faster than anti-4, a necessary condition for the explanation presented to hold true. A stereochemical model based on steric and electronic considerations has been proposed to account for the observed selectivity. This model can aid in predicting the absolute configurations of the amines farmed in this process.
“Catalytic Asymmetric Hydrogenation of Imines with a Chiral Titanocene Catalyst: Kinetic and Mechanistic Investigations”, J. Am. Chem. Soc., 1994, 116(26), 11703-11714.