Metallic salts and metallic co-reductant were investigated for the pinacol coupling reaction in water (Scheme 1).⁵ After optimization using benzaldehyde, 100 mol% of VCl₃ as a metallic salt was revealed to promote the reaction in the presence of 300 mol% of Zn as a co-reductant at room temperature to give 1,2-diphenylethane-1,2-diol in 86% yield with 64/36 ratio of the dl and meso isomers. VCl₃ is essential for this reaction because no reaction was observed in the absence of VCl₃. Three metallic Al, Mg, and Mn were employed in the presence of VCl₃ to show that metallic Al was more efficient as a co-reductant (92%, dl/meso : 65/35, Table 1, entry 1). Several co-solvents, such as H₂O-DMF, H₂O-THF, and H₂O-MeOH, can be used in these reaction conditions. As compared with VCl₃, VBr₃ gave a slightly better yield.

Reducing the amount of VCl₃ to 33 mol% to the substrate was not a problem (isolated yield 72%, NMR yield 92%, dl/meso : 56/44, Table 1, entry 2). It should be noted that the catalytic pinacol coupling reaction successfully proceeded in water even in the absence of a chlorosilane. It is in sharp contrast to the reaction in organic solvent, which requires a chlorosilane as an essential additive. These findings provide a synthetically versatile method. Various aromatic aldehydes (R = 4-MePh, 4-MeOPh, 2-ClPh, 3-ClPh, 4-ClPh, 2-BrPh, and 2-furyl) underwent the reductive coupling with the catalytic VCl₃/Al system in water to give the corresponding 1,2-diols in moderate to good yields. Acetophenone was not reduced under the similar conditions.

Although the detailed mechanism of the catalytic reaction requires more investigation, a low-valent vanadium generated by treatment with Al is likely to be involved in a catalytic cycle involving one-electron transfer from the catalyst to the carbonyl group. Another reaction path where vanadium hydroxide formed by in situ hydrolysis acting as a Lewis acid and Al metal serves as a reducing agent might be possible.