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

摘要

Transfer hydrogenation of a variety of different imines to the corre-sponding amines by propan-2-ol in benzene catalyzed by [Ru_2(CO)_4(mu-H)(C_4Ph_4COHOCC_4Ph_4)] (1) has been studied. The reaction is highly efficient with turnover frequencies of over 800 per hour, and the product amines wee obtained in excellent yields. A remark-able concentration dependence of prop-an-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 alpha-hydrogen at the N-alkyl substituent, which is in accordance with a mechanism involving a ruthenium-amine in-termediate. It was demonstrated that the ruthenium-amine complex from alpha-methylbenzylamine, corresponding to the postulated intermediate, can replace 1 as catalyst in the transfer hydrogenation of imines. A primary deuterium isotope effect of k_CH/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.