Binuclear biscarbene complexes of furan

Abstract

Carbene complexes of chromium and tungsten with a bridging furan substituent were synthesized from lithiated furan precursors and metal hexacarbonyls. The binuclear biscarbene complexes [(CO)₅M{C(OEt)–C₄H₂O–C(OEt)}M′(CO)₅] (M = M′ = Cr (3), W (4)) were obtained as well as the corresponding monocarbene complexes [M{C(OEt)–C₄H₃O}(CO)₅] (M = Cr (1), W (2)). A method of protecting the carbene moiety during the metal acylate stage was used to increase not only the yields of the binuclear Fischer biscarbene complexes 3 and 4 but to establish a method to synthesize analogous mixed heterobinuclear carbene complexes (M = W, M′ = Cr (5)) in high yields. The binuclear biscarbene complexes 3 and 5 were reacted with 3-hexyne and yielded the corresponding benzannulated monocarbene complexes [M{C(OEt)–C₁₄H₁₇O₃}(CO)₅] (M = Cr (6), W(7)). Complex 5 reacted regioselectively with the benzannulation reaction occurring at the chromium–carbene centre. The major products from refluxing 3 in the presence of [Pd(PPh₃)₄] were a monocarbene-ester complex [Cr{C(OEt)–C₄H₂O–C(O)OEt}(CO)₅] (8), the 2,5-diester of furan (9) and a carbene–carbene coupled olefin EtOC(O)–C₄H₂O–C(OEt)C(OEt)–C₄H₂O–C(O)OEt (10). X-Ray structure analysis of 4 and 6 confirmed the molecular structures of the compounds in the solid state and aspects of electron conjugation between the transition metals and the furan substituents in the carbene ligands were investigated.