N-Heterocyclic Carbene-Catalyzed Synthesis of Ynones via C–H Alkynylation of Aldehydes with Alkynyliodonium Salts – Evidence for Alkynyl Transfer via Direct Substitution at Acetylenic Carbon

Alkynylation of aldehydes with alkynyl(aryl)iodonium salts catalyzed by an N-heterocyclic carbene (NHC) has been developed. The application of the organocatalyst and the hypervalent iodine group-transfer reagent allowed for metal-free C–H functionalization and C–C bond formation. The reaction proceeds under mild conditions, at –40 ⁰C and in the presence of an amine base, providing access to an array of heteroaryl-propargyl ketones containing various substituents in good to excellent yields. The mechanism of the reaction was investigated by means of both experiments and density functional theory calculations. 13C-labeling and computations determined that the key alkynyl transfer step occurs via an unusual direct substitution at an acetylenic carbon, wherein an iodine-based leaving group is exchanged by a Breslow intermediate nucleophile. Moreover, kinetic studies revealed that the turnover-limiting step of the catalytic cycle is the generation of the Breslow intermediate, whereas the subsequent C–C bond-fo...