Synthesis and reactivity of metal carbene complexes with heterobiaryl spacer substituents

Abstract

Mono- and binuclear Fischer carbene complexes, [M(CO)₅{C(OR)Ar-ArX}], X = H, {C(OR)M′(CO)₅}; M, M′ = W or Cr; R = Me, Et or (CH₂)₄OMe; Ar = thiophene, N-methylpyrrole or furan units 1–20, were synthesized. For this purpose, mono-, bi- or stepwise lithiated bithiophene, N,N′-dimethylbipyrrole, thienylfuran and N-methyl(thienyl)pyrrole were reacted with chromium and tungsten hexacarbonyl precursors. Dilithiation in the 2- and 9-positions of N-methyl(thienyl)pyrrole could not be achieved. Alkylation of acyl metallates with triethyloxonium tetrafluoroborate or methyl trifluoromethanesulfonate in THF afforded not only the expected carbene complexes with ethoxy or methoxy substituents, but in the case of bithiophene with methyl trifluoromethanesulfonate, carbene complexes with alkoxy substituents incorporating a ring-opened tetrahydrofuran moiety. X-Ray crystallographic structure determinations were performed on [W(CO)₅{C(OMe)(thienylfuran)}] (14), [W(CO)₅{C(OMe)(N-methylthienylpyrrole)}] (20) and [{W(CO)₅}₂{µ-C(OEt)(N,N′-dimethylbipyrrolylC(OEt)}] (12) to assess the role of the heterobiaryl substituent on the structural features of the carbene ligand in the complexes. Complexes [{Cr(CO)₅}₂{µ-C(OMe)bithienylC(OEt)}] (3), [(CO)₅Cr{µ-C(OMe)bithienylC(OMe)}W(CO)₅] (5) and [{Cr(CO)₅}₂{µ-C(OMe)thienylfuranC(OMe)}] (15) were reacted with 3-hexyne to study their behaviour in benzannulation reactions. The major products generated by the biscarbene complexes were regio-selectively determined by the nature of the metal site and that of the heteroatom in the arene rings. The monocarbene complexes [Cr(CO)₅{C(OMe)thienylfuran] (13) and [Cr(CO)₅{C(OEt)(N-methylthienylpyrrole)}] (19) were refluxed in THF for 2 hours in the presence of [Pd(PPh₄)₄] to afforded the carbene–carbene coupled olefinic products and small amounts of the corresponding 2-ethyl(biheteroaryl)acetate. By contrast, the biscarbene complex of thienylfuran (15), afforded only the 2,9-diester of thienylfuran.