Optimizing ligand structure for low-loading and fast catalysis for alkynyl-alcohol and -amine cyclization

Abstract

A series of [Ru(Cp/Cp*)(P^R₂N^R′₂)(MeCN)]PF₆ complexes were prepared, in which the steric and electronic properties of the primary coordination sphere were varied (R = Ph, t-Bu, Bn; and Cp vs. Cp*). These complexes were catalytically active in the cyclization of alkyne substrates with an intramolecular nucleophile (amine or alcohol) to produce 5- and 6-membered heterocycles. The effect of the 1° coordination sphere structure on catalyst performance was evaluated. Steric bulk around the metal centre was a key feature to achieve rapid catalysis at low temperatures. The catalyst [Ru(Cp)(P^t-Bu₂N^Ph₂)(MeCN)]PF₆ gave a turnover number that was >1 order of magnitude more active than previous catalysts in the cyclization of the benchmark substrate 2-ethynylaniline. This catalyst is tolerant of a diversity of functional groups and is competent at the formation of various substituted indoles.