Unsymmetrical E-Alkenes from the Stereoselective Reductive Coupling of Two Aldehydes.

The unprecedented formation of unsymmetrical alkenes from the intermolecular reductive coupling of two different aldehydes is described. In contrast to the McMurry reaction which affords statistical product mixtures, selectivity in the reported procedure is achieved by a sequential ionic mechanism in which a first aldehyde is reacted with a phosphanylphosphonate to afford a phosphaalkene intermediate which, upon activation by hydroxide, reacts with a second aldehyde to the unsymmetrical E-alkenes. The described reaction is free of transition metals and proceeds under ambient temperature within minutes in good to excellent overall yields. It is a new methodology to use feedstock aldehydes for the direct production of C═C double bond-containing products and may impact how chemists think of multistep synthetic sequences in the future.