Mechanistic Studies of Hydride Transfer to Imines from a Highly Active and Chemoselective Manganate Catalyst

We introduce a highly active and chemoselective manganese catalyst for the hydrogenation of imines. The catalyst has a large scope, can reduce aldimines and ketimines, and tolerates a variety of functional groups, among them hydrogenation sensitive examples such as an olefin, a ketone, nitriles, nitro groups, and an aryl iodo substituent or a benzyl ether. We could investigate the transfer step between imines and the hydride complex in detail. We found that double deprotonation of the ligand is essential and excess base does not lead to a higher rate in the transfer step. We identified the actual hydrogenation catalyst as a K-Mn-bimetallic species and could obtain a structure of the K-Mn complex formed after hydride transfer by X-ray analysis. NMR experiments indicate that the hydride transfer is a well-defined reaction, which is first order in imine, first order in the bimetallic (K-Mn) hydride, and independent in rate from the concentration of the potassium base. We propose an outer-sphere mechanism in which protons do not seem to be involved in the rate-determining step, leading to a transiently negatively charged nitrogen atom in the substrate which reacts rapidly with HOBu (2-methylpropan-2-ol) to produce the amine. This is based on several observations, such as no dependency of the reaction rate on the HOBu concentration, no observable manganese amide complex, and a high reaction constant in a conducted Hammett study. Furthermore, hydrogen transfer of the catalytic cycle was experimentally probed and monitored by NMR with subsequent quantitative regeneration of the catalyst by H₂.