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

摘要

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-[(#eta#~5-tetrahydroindenyl)CH_2CH_2(#eta#~5-tetrahydroindenyl)]TiCl_2 (6), and ansa-[(#eta#~5-Cp)CH_2CH_2(#eta#~5-fluorenyl)]TiCl_2 (7). Cp_2TiCl_2 (1) is the most active (pre)catalyst for pinacol silylether (8a) formation, but Brintizinger'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.