Tandem transfer hydrogenation–epoxidation of ketone substrates catalysed by alkene-tethered Ru(ii)–NHC complexes

Abstract

A series of nine cyclopentadienyl Ru(II)–NHC complexes (1–9) have been synthesised by systematically varying the ligand and/or ligand substituents: η⁵-C₅H₄R′ (R′ = H, Me), EPh₃ (E = P, As), NHC (Im, BIm), where NHC = Im(R)(R′) (R, R′ = Me, Bn, 4-NO₂Bn, C₂H₄Ph, C₄H₇). Each of the Ru(II)–NHC complexes features an N-alkenyl tether to attain bidentate NHC ligands. All complexes found application as catalysts in the tandem transfer hydrogenation and epoxidation reactions of carbonyl substrates. The catalytic activity of the complexes was shown to be similar, with efficiencies of up to 69% conversion after 18 hours and varying alcohol : epoxide selectivity for a variety of electronically diverse carbonyl substrates. Complex 3, with a nitro-containing substituent on the NHC ligand, was the only complex that showed preference for the alcohol product over the epoxide after 18 hours of reaction time.