Development and Mechanistic Investigation of a Highly Efficient Iridium(V) Silyl Complex for the Reduction of Tertiary Amides to Amines

Abstract

The cationic Ir(III) acetone complex (POCOP)Ir(H)₂(acetone)⁺ (POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) was shown to catalyze the reduction of a variety of tertiary amides to amines using diethylsilane as reductant. Mechanistic studies established that a minor species generated in the reaction, the neutral silyl trihydride Ir(V) complex (POCOP)IrH₃(SiEt₂H), was the catalytically active species. High concentrations of this species could be conveniently generated by treatment of readily available (POCOP)IrHCl with tert-butoxide in the presence of Et₂SiH₂ under H₂. Thus, using this mixture in the presence of a trialkylammonium salt, a wide array of tertiary amides, including extremely bulky substrates, are rapidly and quantitatively reduced to tertiary amines under mild conditions with low catalyst loading. A detailed mechanistic study has been carried out and intermediates identified. In brief, (POCOP)IrH₃(SiEt₂H) reduces the amide to the hemiaminal silyl ether that, in the presence of a trialkylammonium salt, is ionized to the iminium ion, which is then reduced to the tertiary amine by Et₂SiH₂. Good functional group compatibility is demonstrated, and a high catalyst stability has provided turnover numbers as high as 10 000.