Mechanistic Insights into the Phosphine-Free RuCp~*-Diamine-Catalyzed Hydrogenation of Aryl Ketones:Experimental and Theoretical Evidence for an Alcohol-Mediated Dihydrogen Activation

摘要

The commercially available chiral diamine quincorine-amine,originally derived from quinine,was found to be a highly active catalyst for ruthenium-catalyzed hydrogenation of ketones.The complex formed between the quincorine-amine,containing both a primary and a quinuclidine amino function,and RuCp~*CI catalyzes the hydrogenation of aromatic and aliphatic ketones in up to 90% ee approximately 24 times faster than previously reported Ru-diamine complexes.The pseudo-enantiomer of the quincorine-amine,i.e.,quincoridine-amine,also showed high activity;however,the enantioselectivities obtained with this catalyst were lower.The reason for the lower,but opposite stereoselectivity seen with the quincoridine-amine,as compared to the quincorine-amine,was rationalized by a kinetic and computational study of the mechanism of the reaction.The theoretical calculations also revealed a significantly lower activation barrier for the alcohol-mediated split of dihydrogen,as compared to the nonalcohol-mediated process,a finding of utmost implication also for the diphosphine/diamine-mediated enantioselective hydrogenation of ketones.