Imines pinacol couplings

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. 

Imamoto and Nishimura reported a SmD -induced coupling of imines in direct analogy to the pinacol coupling of aldehydes and ketones (Table 12)186. However, no mechanistic... 

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-... 

There are a few reports on the synthesis of N,N -dimethyl-1,2-diphenyl-ethylenediamine by pinacol couplings or by N-methylation of the primary diamine. Pinacol type couplings of imines have been reported for the synthesis of other C2 symmetrical N-substituted or unsubstituted 1,2-diarylethylenediamines using various reducing metals in different solvents. However, the present synthesis of N,N -dimethyl-1,2-diphenylethylenediamine is the most suitable for large scale preparation. There are no expensive or hazardous starting materials, and the procedure does not require tedious filtrations or chromatographic separation of diastereomers. 

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). ... 

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

More recently the introduction of low-valent transition metal and lanthanoid based reducing systems, especially those based on titanium, has provided dramatic advances in efficiency and selectivity. It is now possible to select appropriate conditions for efficient coupling of all types of carbonyl compounds, often with high chemo-, regio- and stereo-selectivity. Moreover, imino- and thio-carbonyl derivatives are also coupled via pinacolic methodology. The coupling of imines to 1,2-diamines is particularly effective, with excellent control of vicinal stereochemistry. 

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