Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

Abstract

The deaminative hydrogenation of amides is one of the most convenient pathways for the synthesis of amines and alcohols. The ideal source of reducing equivalents for this reaction is molecular hydrogen, though, in practice, this approach requires high pressures and temperatures, with many catalysts achieving only small turnover numbers and frequencies. Nonetheless, during the last ten years, this feld has made major advances towards larger turnovers under milder conditions thanks to the development of bifunctional catalysts. These systems promote the heterolytic cleavage of hydrogen into proton and hydride by combining a basic ligand with an acidic metal centre. The present review focuses on the computational study of the reaction mechanism underlying bifunctional catalysis. This review is structured around the fundamental steps of this mechanism, namely the C=O and C–N hydrogenation of the amide, the C–N protonolysis of the hemiaminal, the C=O hydrogenation of the aldehyde, and the competition between hydrogen activation and catalyst deactivation. In line with the complexity of the mechanism, we also provide a perspective on the use of microkinetic models. Both Noyori- and Milstein-type catalysts are discussed and compared.