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Reductive alkynylation of aldehydes by the Umpolung approach was developed using a photoredox catalyst under blue LED irradiation. Ketyl radicals, generated by single-electron reduction of aldehydes through proton-coupled electron transfer (PCET), reacted with electrophilic alkynylsulfones. Sterically demanding bulky aldehydes reacted smoothly under the Umpolung reaction conditions. Moreover, the alkynylation proceeded chemoselectively with an aryl aldehyde group in the presence of other carbonyl groups including an aliphatic aldehyde group.
Tanaka et al. (Wed,) studied this question.