Cross-pinacol-type coupling reactions between α,β-unsaturated ketones and aldehydes with low-valent metals

Treatment of an α,β-unsaturated ketone and an aldehyde with chromium (II) chloride and R₃SiCl in DMF gives cross-pinacol-type coupling products, 1,2-diols selectively. The anti/syn ratios of the produced 1,2-diols are shown to depend on the reaction temperature: At lower temperatures, the anti adduct is produced selectively, but at higher temperatures, the anti/syn ratios gradually decrease. When a combination of manganese and a catalytic amount of lead are used instead of chromium(II), 1,6-diketone, a dimer of the α,β-unsaturated ketone, is produced selectively.