Imines pinacol coupling reactions

Titanium-mediated pinacol coupling reactions have been reviewed until 2000.80 81 Since then, various intermole-cular pinacol couplings have been reported with aldehydes, - ketones, a-ketoesters, and imines, as well as asymmetric versions thereof.101-104 Scheme 29 shows one example of an asymmetric pinacol coupling of aromatic aldehydes, promoted and catalyzed by the new chiral titanium complex (A)-75, that has been developed by Riant and co-workers.101 Yields for pinacol products 76 are generally high. Under catalytic conditions, ee is moderate (up to 63%), while stoichiometric conditions allow to obtain up to 91% ee. 

Nickel catalysts display significant accelerating effects in reactions mediated by Sml2 [25], such as aza-pinacol coupling reactions of imines [26, 27]. 

A coupling reaction similar to pinacol coupling has been used with imines, which dimerize to give 1,2-diamines. A number of reagents have been used, includ-... 

Additive in Various Reduction Systems. TMSCl has been employed in many different reduction systems to improve the reactivity and/or increase the selectivity. The reagent has been used to activate zinc dust or as additive for Reformatskii reactions. While less reactive than Mg/TMSCl combination, the Zn/TMSCl combination proved to be the preferred reagent system for large scale and high concentrated preparation of diamines through the pinacol coupling of two molecules of imines (eq 76). ... 

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