Non-bifunctional Mn catalysts based on phosphine-phosphites for the hydrogenation of carbonyl substrates

Abstract

A series of Mn(I) complexes bearing phosphine-phosphite ligands (P-OP) of formula [Mn(Br)(P-OP)(CO)3] (1) have been prepared and characterised. These compounds exist in solution as a mixture of fac and mer isomers with fac/mer ratios from 18:1 to 2:1. The relatively low donor ability of P-OP compared to bis(trialkylphosphines) or bis-NHC ligands, typically found in non-bifunctional Mn catalysts reported to date, is evinced in IR spectra. Compounds 1 provide efficient catalysts for the hydrogenation of carbonyl substrates using KOtBu as a base. Notably, these complexes constitute a set of modularly designed precatalysts and optimisation of the P-OP ligand provided appropriate catalysts for the reduction of a wide variety of aldehydes and ketones under mild reaction conditions (S/C = 250-500, 20 bar H2, RT to 40 °C). A mechanistic study combining spectroscopic and computational results provides support for a catalyst activation process based on the nucleophilic attack of the alkoxide on a coordinated CO in [Mn(OtBu)(P-OP)(CO)3], as well as a non-bifunctional inner-sphere hydrogenation pathway in the representative hydrogenation of benzophenone. The catalytic cycle is also characterised by a rather stable pentacoordinated alkoxide [Mn(k1-O-OCHPh2)(P-OP)(CO)2] which determines the energetic span of the reaction.