[Ru(η5-C5H5)(η6-C10H8)]PF6 as a catalyst precursor for the one-pot direct C–H alkenylation of nitrogen heterocycles

Abstract

The ruthenium naphthalene complex [Ru(η⁵-C₅H₅)(η⁶-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(η⁵-C₅H₅)L₃]⁺ (L = nitrogen-based ligand). Reaction of in situ-generated [Ru(η⁵-C₅H₅)(py)₃]⁺ (py = pyridine) with PPh₃ results in the formation of [Ru(η⁵-C₅H₅)(PPh₃)(py)₂]⁺, the active catalyst for direct alkenylation, some [Ru(η⁵-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(η⁵-C₅H₅)(η⁶-C₁₀H₈)]⁺.