Non-classical hydrosilane mediated reductions promoted by transition metal complexes

This article reviews the most recent advances on the study of non-classical mechanisms for the reduction of organic substrates with hydrosilanes catalyzed by transition metals. A wide variety of catalytic cycles that go beyond the classical steps described for Ojima, Chalk-Harrod and modified Chalk-Harrod mechanisms, as representative examples, have been proposed in recent years. In this review, these alternative mechanistic proposals have been analyzed and classified according to the type of substrate, focusing on the reduction/hydrosilylation of carbonyl compounds (ketones and aldehydes), carbon dioxide, silylesters, amides, N-heterocycles, alkyl halides, nitriles, alkenes and alkynes. In spite of the broad diversity of non-classical reaction mechanisms hitherto reported, the catalytic cycles described for each substrate have been arranged in different categories according to their characteristics. The epigraph dedicated to the first type of substrate (carbonyl compounds) comprises most of the mechanisms described in this review for the reduction of polar bonds, which, to some extent, show a relationship to those proposed for non-polar bonds (alkenes and alkynes). Remarkable types of reaction mechanisms that will be dealt with in this work are: ionic mechanisms, ligand-assisted mechanisms, nonhydride mechanisms, NHC-Cu-H mediated mechanisms, and silylene-mediated mechanisms. (C) 2019 Elsevier B.V. All rights reserved.