Samarium reagents pinacolic coupling reactions

Diastereomeric ratios of pinacol dimers obtained by coupling of benzaldehyde and acetophenone with samarium reagents are summarized in Tables 3 and 4, respectively. In general, the stereochemical control of pinacol coupling reactions with a stoichiometric or catalytic samarium(ll) reagent is poor, giving nearly a 1 1 dljmeso mixture. 

The two reactions discussed here are new to you—they are reductive processes where an alkali or alkaline earth metal supplies electrons, to convert carbonyl compounds into radical anions, which then couple together on the metal surface. The first is the pinacol coupling reaction of ketones, which, classically, uses magnesium metal as an electron source. The reaction mechanism is shown in Figure 20.8. In recent years, many other metals have been used similarly titanium and samarium reagents have been particularly popular, and some applications are shown in Figure 20.9. 

Lebrun, A., Namy, J.-L., and Kagan, H. B., Samarium dibromide, an efficient reagent for pinacol coupling reactions. Tetrahedron Lett., 34, 2311,1993. 

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