Oxidative Coupling of Terminal Alkynes with Aldehydes Leading to Alkynyl Ketones by Using Indium(III) Bromide

An indium(III)‐promoted direct acylation of terminal alkynes using aldehydes leading to ynones was developed. In contrast to the previous addition reactions of alkynes to aldehydes, which provide propargylic alcohols, the oxidative coupling proceeded exclusively to afford alkynyl ketones. The products were likely generated through an Oppenauer oxidation of the indium propargylic alkoxide species by excess amounts of aldehydes. Addition/oxidation strategy: Direct access to alkynyl ketones from terminal alkynes with aldehydes was achieved. Observation of the alcohol originated from the aldehyde, and an isotope labeling study suggested that this transformation proceeds through the addition of an organoindium to an aldehyde followed by an Oppenauer oxidation with the next aldehyde. Basically, an excess of aldehyde plays the dual roles of substrate and hydride acceptor.