Catalyst structural effects in titanocene-catalyzed pinacol coupling: activity, stereoselectivity and mechanistic implications

Abstract

The effects of catalyst structural variation on the activity and selectivity of titanocene-catalyzed pinacol coupling of cyclohexane carboxaldehyde by Mn/TMSCl have been evaluated. Complexes which have been tested include: Cp 2TiCl 2 ( 1), Cp 2TiBr 2 ( 2), (C 5Me 5) 2TiCl 2 ( 3), (1,3- t-Bu 2C 5H 3) 2TiCl 2 ( 4), (1,3- t-Bu 2C 5H 3)(Cp)TiCl 2 ( 5), ansa-[(η 5-tetrahydroindenyl)CH 2CH 2(η 5-tetrahydroindenyl)]TiCl 2 ( 6), and ansa-[(η 5-Cp)CH 2CH 2(η 5-fluorenyl)]TiCl 2 ( 7). Cp 2TiCl 2 ( 1) is the most active (pre)catalyst for pinacol silylether ( 8a) formation, but Brintzinger's complex 6 provides the best dl /meso diastereoselectivity (5:1). Complexes 2, 4 and 7 slowly catalyze the predominant formation of the corresponding pinacol acetal 9a as a secondary product. Comparative stoichiometric reactions of benzaldehyde/Me 3SiCl with [Cp 2TiCl·MnCl 2(THF) 2·Cp 2TiCl] ( 10) and [Cp 2TiCl] 2 ( 11) result in highly diastereoselective pinacol silylether formation with binuclear 11 (29:1), but primarily the production of pinacol acetal ( 9b) from trimetallic 10, suggesting a dominant role for the binuclear complex (or derived mononuclear species) in the catalytic systems employing Cp 2TiCl 2/M/TMSCl, contrary to previous suggestions.