We herein present a new catalytic system for the palladium‐free Sonogashira coupling reaction. The catalytically active moiety is formed in situ, in a straightforward and user‐friendly manner, by combining a widely available low‐cost copper salt and an N‐heterocyclic carbene precursor. A series of N‐heterocyclic carbene ligand precursors with variable structural features, some of which are novel, were tested and the reaction conditions were optimized. Using the catalytic system with the optimum performance, the scopes of the alkyne and the aryl halide were probed. Aryl iodides readily react with terminal alkynes, providing the coupling products in high to excellent yields. The protocol is highly efficient with alkynes bearing either alkyl or aryl substituents, the latter having either electron‐donating or electron‐withdrawing groups.
An efficient and straightforward organocatalytic method for the direct, multicomponent carboxylation of terminal alkynes with CO2 and organochlorides, towards propargylic esters, is reported for the first time. 1,3-Di-tert-butyl-1H-imidazol-3-ium chloride, a simple, widely-available, stable, and cost-efficient N-heterocyclic carbene (NHC) precursor salt was used as the (pre)catalyst. A wide range of phenylacetylenes, bearing electron-withdrawing or electron-donating substituents, react with allyl-chlorides, benzyl chlorides, or 2chloroacetates, providing the corresponding propargylic esters in low to excellent yields. DFT calculations on the mechanism of this transformation indicate that the reaction is initiated with the formation of an NHC-carboxylate, by addition of the carbene to a molecule of CO2. Then, the nucleophilic addition of this species to the corresponding chlorides has been computed to be the rate limiting step of the process. Scheme 1. Examples of useful organic transformations catalyzed by NHCs.
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