Discrete versus in situ-generated aluminum-Salen catalysts in enantioselective cyanosilylation of ketones:: Role of achiral ligands

The monometallic species (Salen)AlX (X = Me, 2a,b; X = Cl, 4a,b; O-i-Pr, 5a,b) and open bimetallic species (Salen)[AlMe(2)](2) (3a,b) that result from binary combinations between an aluminum precursor [trimethylaluminum, dimethylaluminum chloride, aluminum tris(isopropoxide)] and a diprotio {Salen}H(2) pro-ligand 1a,b (a=1R,2R-cyclohexylsalen; b=1R,2R-diphenylethylene-salen) have been isolated. The crystal structures of 3b, 4b and of mu-oxo species [{diphenylethylene-salen)Al](2)O (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 degrees C for acetophenone).