Phosphine–Amido Complexes of Ruthenium and Mechanistic Implications for Ketone Transfer Hydrogenation Catalysis

Transfer Hydrogenation from 2-propanol to Acetophenone Catalyzed by [RuCl2(η6-arene)P] (P = monophosphine) and [Rh(PP)2]X (PP = diphosphine, X = Cl−, BF4−) Complexes

The reduction of ketones through homogeneous transfer hydrogenation catalyzed by transition metals is one of the most important routes for obtaining alcohols from carbonyl compounds. The interest of this method increases when opportune catalytic precursors are able to perform the transformation in an asymmetric fashion, generating enantiomerically enriched chiral alcohols. This reaction has been extensively studied in terms of catalysts and variety of substrates. A large amount of information about the possible mechanisms is available nowadays, which has been of high importance for the development of systems with excellent outcomes in terms of conversion, enantioselectivity and Turn Over Frequency. On the other side, many mechanistic aspects are still unclear, especially for those catalytic precursors which have shown only moderate performances in transfer hydeogenation. This is the case of neutral [RuCl2(η6-arene)(P)] and cationic [Rh(PP)2]X (X = anion; P and PP = mono- and bidentate phosphine, respectively) complexes. Herein, a summary of the known information about the Transfer Hydrogenation catalyzed by these complexes is provided with a continuous focus on the more relevant mechanistic features.

Mechanistic diversity in acetophenone transfer hydrogenation catalyzed by ruthenium iminophosphonamide complexes

A series of arene ruthenium iminophosphonamide complexes, [(Arene)RuCl{R₂P(NR')₂}] (1), bearing various arenes and R,R' substituents on the NPN ligand have been investigated as precatalysts in acetophenone transfer hydrogenation in basic and base-free isopropanol. The results clearly demonstrate the presence of two distinct reaction mechanisms, which are controlled by the basicity of the N-atoms. Complexes 1 in which both R' substituents are aryl groups are only active once the neutral hydride complex [(Arene)RuH{R₂P(NR')₂}] (2) is generated in basic isopropanol, the latter being able to reduce a ketone via a stepwise hydride and proton transfer. On the other hand, complexes in which at least one R' group is Me readily catalyze the reaction in the absence of base. In the latter case, the results of kinetic studies and DFT calculations support an outer-sphere concerted asynchronous hydride and proton transfer assisted by the basic N-atom of the NPN ligand, which promotes catalysis via precoordination of an alcohol molecule by hydrogen bonding.

Highly productive CO2 hydrogenation to methanol – a tandem catalytic approach via amide intermediates

Amines to an end – homogeneous ruthenium catalysts based on mixed donor P–N ligands with a range of amine auxiliaries are highly productive for CO₂ reduction to methanol, the nature of this amine having a profound effect on performance.

Substitution of alcohols by N-nucleophiles via transition metal-catalyzed dehydrogenation

This review summarizes the recent advances in transition metal-catalyzed dehydrogenative substitution of alcohols by N-nucleophiles since 2009.