Reductive Carbonyl Coupling Reactions
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The pinacol reductive coupling of aldehydes and ketones is an old transformation in organic synthesis. Also, the coupling of ketones to give pinacols can be accomplished with a variety of metal reducing agents, particularly alkali metals. The carbonyl coupling reaction involves two steps: reductive dimerization of the starting carbonyl compound to give a 1,2-diolate (pinacolate) with concomitant formation of the carbon-carbon bond, and deoxygenation of the 1,2-diolate intermediate to yield an olefin. Interestingly, pinacol coupling can be carried out using aqueous systems and an ammoniacal solution of titanium(III) chloride is effective for this purpose. Traditionally, many carbonyl coupling reactions have been carried out with magnesium or aluminum systems, although the metals used most commonly are the early electron-rich transition metals.