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{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·OEt2 to the resulting complex Os­{κ2-C,C-[C­(CHPh)­C­(O)­CH2ImMe]}­Cl­(CO)2(PiPr3) (5) leads to [Os­{κ2-O,C-[OC­(CHCHPh)­CH2ImMe]}­Cl­(CO)2(PiPr3)]­BF4 (6) containing an NHC-(α,β-unsaturated
ketone) ligand.