Ru(η5-C5H5)(η6-C10H8)]PF6as a catalyst precursor for the one-pot direct C–H alkenylation of nitrogen heterocycles

Abstract

The ruthenium naphthalene complex [Ru(η(5)-C5H5)(η(6)-C10H8)](+) is a catalyst precursor for the direct C-H alkenylation of pyridine and related nitrogen heterocycles by terminal alkynes. Stoichiometric studies have demonstrated that the naphthalene ligand may be displaced by either pyridine, 4-methylpyridine or dimethylaminopyridine (DMAP) to give species [Ru(η(5)-C5H5)L3](+) (L = nitrogen-based ligand). Reaction of in situ-generated [Ru(η(5)-C5H5)(py)3](+) (py = pyridine) with PPh3 results in the formation of [Ru(η(5)-C5H5)(PPh3)(py)2](+), the active catalyst for direct alkenylation, some [Ru(η(5)-C5H5)(PPh3)2(py)](+) is also formed in this reaction. A one-pot procedure is reported which has allowed for the nature of the nitrogen heterocycle and phosphine ligand to be evaluated. The sterically demanding phosphine PCy3 inhibits catalysis, and only trace amounts of product are formed when precursors containing a pentamethylcyclopentadienyl group were used. The greatest conversion was observed with PMe3 when used as co-ligand with [Ru(η(5)-C5H5)(η(6)-C10H8)](+).