Selective Electrochemical E –Semihydrogenation of Alkyl Alkynes with Main Metal Electrodes and Water

Abstract The semihydrogenation reaction of alkynes to E-alkenes is still a challenge in modern and sustainable organic chemistry, particularly for alkyl alkynes where allyl H-abstraction, alkynyl hydrofunctionalization, and overhydrogenation by reactions easily occur. Here we show that a selective E–electrosemihydrogenation reaction of alkyl alkynes proceeds with Mg and Al as sacrificial electrodes in THF/water mixtures. The reaction is extendable to aromatic alkynes and deuteration processes, proceeds with >98% Faradaic efficiency (FA) in an undivided cell, avoids the hydrogen evolution reaction (HER), and achieves E/Z ratios >3:1, three times higher than any recent electro(catalytic) method reported as far as we know. Mechanistic studies based on reactive, kinetic, isotopic, and computational experiments, among others, reveal that the success of this transition-metal-free electrochemical semihydrogenation reaction relies on the direct one-electron cathodic reduction of the alkyne, circumventing the typical H activation triggered by transition metals and enabling the rapid isomerization of the so-formed vinyl anion radical to the more stable E isomer after final protonation. These results open the way to directly obtain E-alkyl alkenes from the corresponding alkynes with just highly abundant metals, water, and electricity, with broad application in industrial chemistry, herein demonstrated for the gram-scale synthesis of a pheromone precursor.