Simple, Highly Active Palladium Catalysts for Ketone and Malonate Arylation:  Dissecting the Importance of Chelation and Steric Hindrance

Abstract

A remarkably active catalyst system for α-arylation of ketones and malonates was developed by proposing that sterically hindered alkylphosphines would accelerate the catalytic reaction rates. We initially tested the bisphosphine ligand D t BPF (1,1‘- bis -(di- tert -butylphosphino)ferrocene) for this palladium-catalyzed chemistry. This catalyst system led to fast reaction rates for reactions of aryl bromides with ketones, including room temperature chemistry in many cases. In some cases turnover numbers were 20 000. The catalyst also gave mild reactions with aryl chlorides with yields that were similar to the chemistry with aryl bromides. Independent synthesis of the arylpalladium enolate complexes with isobutyrophenone enolate showed that only one phosphorus of the bisphosphine ligand D t BPF was coordinated in the enolate complex. Thus, we tested sterically hindered alkylphosphine ligands for the ketone and malonate arylation process and found that P( t -Bu) 3 gave exceptionally fast rates and high turnover numbers for these reactions. These results demonstrate several principles for the catalytic chemistry that we did not anticipate: palladium complexes of monophosphine ligands can activate aryl chlorides under mild conditions, and palladium enolates coordinated by certain monophosphines can undergo C−C bond-forming reductive elimination much faster than β-hydrogen elimination.