Discrete versus In Situ‐Generated Aluminum‐Salen Catalysts in Enantioselective Cyanosilylation of Ketones: Role of Achiral Ligands

Abstract

AbstractThe monometallic species {Salen}AlX (X=Me, 2a,b; X=Cl, 4a,b; O‐i‐Pr, 5a,b) and open bimetallic species {Salen}[AlMe2]2 (3a,b) that result from binary combinations between an aluminum precursor [trimethylaluminum, dimethylaluminum chloride, aluminum tris(isopropoxide)] and a diprotio {Salen}H2 pro‐ligand 1a,b (a=1R,2R‐cyclohexyl‐salen; b=1R,2R‐diphenylethylene‐salen) have been isolated. The crystal structures of 3b, 4b and of μ‐oxo species [{diphenylethylene‐salen}Al]2O (6b) are reported. The discrete species 2–5a,b have been individually evaluated in the asymmetric cyanosilylation of acetophenone. It is shown that, in several cases, these discrete catalysts display dramatically different performances than the catalyst systems in situ‐generated from the binary combinations. The influence of the achiral ligand X on both the enantioselectivity and activity of the cyanosilylation reaction has been investigated, resulting in the definition of a highly active and productive hexafluoro‐2‐propoxide‐based catalyst for a variety of aryl alkyl ketones (TON up to 470, TOF up to 140 h−1 at −20 °C for acetophenone).