LUCY MILNER, GARAZI TALAVERA, HANS GÜNTER NEDDEN*
*Corresponding author
Johnson Matthey Plc, Catalysis and Chiral Technologies, Cambridge, United Kingdom

Abstract

The transfer hydrogenation of ketones and imines using RuII, RhIII and IrIII complexes with beta-amino alkoxide and beta-amino sulphonimide ligands has been used for the last 20 years as a practical and powerful tool for the synthesis of alcohols and amines. When compared to Noyori hydrogenation under H2 pressure, e.g. catalysts of the type [(diphosphine)RuCl2(diamine)], which uses basic co-catalysts and requires access to pressure equipment, transfer hydrogenation is much more versatile and often more cost effective. The aim of this review is to highlight the large number of available process conditions that promise to give cost-efficient reductions of complex substrates, demonstrating the incredible versatility and effectiveness of this technology

INTRODUCTION

The power of ketone and imine reduction by transfer hydrogenation lies in the versatility and breadth of reaction conditions that can be applied to the production of industrially relevant products. Our literature survey shows that only a limited number of catalyst structures cover the majority of industrial applications, namely Ru-, Rh- and Ir-based catalysts, and first- and second-generation tethered catalysts (Scheme 1). Therefore, considering the large diversity of substrates, the availability of the best catalyst for screening and development/production is critical (1).

In contrast to previous reviews on different catalyst structures or specific catalyst applications (2-5), this article instead focuses on the wide range of process conditions that can be fine-tuned for maximum catalytic efficiency. Mechanistically, all catalysts in this review work via a bifunctional outer-sphere mechanism with the active participation of the amino (NH) moiety of the ligand, which is coordinated to the metal centre.

CATALYSIS USING ALCOHOLS AS HYDROGEN DONORS

In the presence of catalysts bearing eith ...