New Families of Reagents for the Pinacol Coupling Reaction

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. 

As indicated in Section 3.2 Background , a diverse set of metals and metal complexes can effect the pinacol coupling of carbonyl groups. In the present section, newly discovered families of reagents for pinacol couplings will be described (modifications of existing families of reagents will not be discussed). 

Gadolinium. Saussine has reported that gadolinium effects the pinacol coupling of benzophenone [32]. 

Indium. Kim has developed an indium-mediated pinacol coupling of aromatic aldehydes that proceeds in water or in water/t-BuOH (Eq. 3.14) [33]. Sonication greatly enhances the rate of the reaction. The diastereoselectivity of the pinacol coupling is variable, with d,l meso ratios ranging from 6 1 to 1 2. Aliphatic aldehydes, as well as ketones, are inert to these conditions. 

Manganese. Li and Chan [34] and Rieke [35] have independently reported that manganese reagents can accomplish the pinacol coupling of aromatic carbonyl compounds. In the Li/Chan study, reaction of an array of aromatic aldehydes proceeded in good to excellent yield in the presence of Mn/Ac0H/H20, albeit with poor diastereoselectivity (Eq. 3.15). Under these conditions, aliphatic aldehydes are reduced to the corresponding alcohol, and ketones (aromatic or aliphatic) do not react. 

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