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.