Ruthenium-Catalyzed Transfer Hydrogenation of Imines by Propan-2-ol in Benzene

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Abstract
Transfer hydrogenation of a variety of different imines to the corresponding amines by propan‐2‐ol in benzene catalyzed by [Ru2(CO)4(μ‐H)(C4Ph4COHOCC4Ph4)] (1) has been studied. The reaction is highly efficient with turnover frequencies of over 800 per hour, and the product amines were obtained in excellent yields. A remarkable concentration dependence of propan‐2‐ol was observed when the reaction was run in benzene as cosolvent. An optimum was obtained at 24 equivalents of propan‐2‐ol to imine, and further increase of the propan‐2‐ol led to a dramatic decrease in rate. Also the use of polar cosolvents with 24 equivalents of propan‐2‐ol gave a low rate. It was found that ketimines react faster than aldimines and that electron‐donating substituents on the imine increase the rate of the catalytic transfer hydrogenation. Electron‐withdrawing substituents decreased the rate. An isomerization was observed with imines having an α‐hydrogen at the N‐alkyl substituent, which is in accordance with a mechanism involving a ruthenium–amine intermediate. It was demonstrated that the ruthenium–amine complex from α‐methylbenzylamine, corresponding to the postulated intermediate, can replace 1 as catalyst in the transfer hydrogenation of imines. A primary deuterium isotope effect of kCH/CD=2.7±0.25 was observed when 2‐deuterio‐propan‐2‐ol was used in place of propan‐2‐ol in the transfer hydrogenation of N‐phenyl‐(1‐phenylethylidene)amine.

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