Conceptual Extension of the Degradation–Transformation of N-Heterocyclic Carbenes: Unusual Rearrangements on Osmium

Abstract

The range of processes of degradation–transformation of NHC ligands in the coordination sphere of a transition metal has been enlarged. The NHC-acyl ligand of the complex Os{κ2-C,C-[C(O)CH2ImMe]}Cl(PiPr3)2 (1) undergoes a complex rearrangement promoted by internal alkynes to give Os{κ2-C,N-[CH2ImMe]}Cl(CO)(PiPr3)2 (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os{κ2-C,C-[CH2ImMe]}Cl(CO)(PiPr3)2 (3), followed by a thermally activated stoichiometric 1,2-methylene shift from N to C. The catalytic activity of the alkynes depends upon their substituents, decreasing in the sequence diphenylacetylene > 1-phenyl-1-propyne > 3-hexyne > 2-butyne. Phenylacetylene tautomerizes in the metal coordination sphere to afford the stable vinylidene Os{κ2-C,C-[C(O)CH2ImMe]}Cl(═C═CHPh)(PiPr3)2 (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF4·...