Scope, Kinetics, and Mechanism of On Water Cu Catalysis in the C-N Cross-Coupling Reactions of Indole Derivatives

A simple and cost-effective protocol for the C-N cross coupling of indole derivatives with aryl iodides using CuI/phenanthroline catalytic system in aqueous and DME/H2O solvent mixture is described. The reactions were performed in the absence of phase-transfer catalyst, and afforded N-arylated products in moderate to excellent yields under mild reaction conditions. A systematic tuning of reaction conditions using DME as a co-solvent enables to improve product yields of N-arylation reactions. The broad substrate scope, easy performance, and low loading of catalyst as well as ligand render this approach appropriate for large scale processes. The mechanism of on water Cu-catalyzed N-arylation reaction is investigated using kinetic and computational studies, which reveal interesting mechanistic aspects of the reaction. A series of kinetic experiments showed significant rate enhancement for on water Cu-catalyzed N-arylation over the reaction performed in the organic solvent (DME). Computational studies corroborated on water rate acceleration by delineating the role of water in the reaction. The water induces rate acceleration by stabilizing the transition state of oxidative addition through hydrogen bonding interactions, presumably at the oil-water interface, and thus helps to reduce the free energy of activation of oxidative addition of iodobenzene to the Cu complex, which is identified as the rate-limiting step of reaction.