In pinacol coupling

Another reagent that has found use in pinacolic coupling is prepared from VC13 and zinc dust.264 This reagent is selective for aldehydes that can form chelated intermediates, such as (3-formylamides, a-amidoaldehydes, a-phosphinoylaldehydes,265 and 8-ketoaldehydes.266 The vanadium reagent can be used for both homodimerization and heterodimerization. In the latter case, the reactive aldehyde is added to an excess of the second aldehyde. Under these conditions, the ketyl intermediate formed from the chelated aldehyde reacts with the second aldehyde. 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

In some instances the carbonyls in pinacolic coupling reactions exhibit chirality (enantiomer) recognition, that is the ability of an enantiomer to recognize a molecule of like chirali and react exclusively... 

In certain instances, excellent yields and stereoselectivities are observed in pinacol coupling reactions. However, it is clear from the reports described above that, in many instances, only modest yields and stereoselectivities are obtained. Indeed, it is a testament to the power of this transformation that the pinacol coupling is used in the. synthesis of complex natural products (e.g., taxol) de,spite these limitations. The current deficiencies point to a number of opportunities for future work, including the development of both more efficient reagents and reagents that effectively control product stereochemistry in a predictable manner. 

The relative and absolute stereochemistry in pinacol coupling reactions, for instance in the natural product syntheses mentioned earlier,... 

Lithium bromide and lithium chloride can also enhance the reactivity of Sml2 in THF (Fuchs et ah, 1997 Miller et ah, 2000). Addition of anhydrous LiCl and LiBr to Sml2 resulted in rate enhancement and better yields in pinacol coupling of cyclohexanone. In the Barbier type reaction of 1-iodobutane with 2-octanone, the presence of LiBr and LiCl resulted in the formation of pinacol coupling product (diol from 2 moles of 2-octanone) as the major product rather than the nucleophilic addition of halide to ketone (eq. (52), table 8). 

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