Preparation, characterization, and reactions of [(EDBP)Al(μ-OiPr)]2, a novel catalyst for MPV hydrogen transfer reactions

The reaction of [(mu-EDBP)AlMe](2) (EDBP-H-2 = 2,2'-ethylidenebis(4,6-di-tert-butylphenol) with 2 mol equiv of 2-propanol at ambient temperature yields [(EDBP)Al(mu-(OPr)-Pr-i)](2) (1), which has shown excellent catalytic activities toward hydrogen transfer reactions between aldehydes and 2-propanol (Meerwein-Ponndorf-Verley (MPV) reactions). The addition of 4 mol equiv of benzaldehyde, 4-methoxybenzaldehyde, or 4-chlorobenzaldehyde to [(EDBP)Al(mu-(OPr)-Pr-i)](2) yields [(EDBP)Al(mu-OBz-p-X)(O=CHC6H4-p-X)](2) (2, X = H; 3, X = OMe; 4, X = Cl). However, in the reaction of 1 with excess 4-nitrobenzaldehyde, only the four-coordinated complex [(EDBP)Al(mu-OBz-p-NO2)](2) (5) was obtained. Complex 1 further reacts with 2 mol equiv of OPPh3 or hexamethylphosphoramide (HMPA), yielding the four-coordinated monomeric complex [(EDBP)Al((OPr)-Pr-i)(S)] (6, S = OPPh3; 7, S = HMPA). Compounds 1, 3, 6, and 7 have been characterized by X-ray crystal structure determinations, enabling us to describe the intermediate of the MPV reactions.