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

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{kappa(2)-C,C-[C(O)CH(2)ImMe]}Cl((PPr3)-Pr-i)(2) (1) undergoes a complex rearrangement promoted by internal alkynes to give Os{k(2)-C,N-[CH(2)ImMe]}Cl(CO)((PPr3)-Pr-i)(2) (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os{kappa(2)-C,C-[CH(2)ImMe]}Cl(CO)((PPr3)-Pr-i)(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{kappa(2)-C,C-[C(O)CH(2)ImMe]}C1(=C=CHPh)((PPr3)-Pr-i)(2) (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF4 center dot OEt2 to the resulting complex Os{kappa(2)-C,C-(=CHPh)C(O)C(2)ImMen]}Cl-(CO)(2)((PPr3)-Pr-i) (5) leads to [Os{kappa(2)-O,C-[O=C(CH=CHPh)CH(2)ImMe]}Cl(CO)(2)((PPr3)-Pr-i)]BF4 (6) containing an NHC-(alpha,beta-unsaturated ketone) ligand.