Revealing the unusual role of bases in activation/deactivation of catalytic systems: O–NHC coupling in M/NHC catalysis

Numerous reactions are catalyzed by complexes of metals (M) with N-heterocyclic carbene (NHC) ligands, typically in the presence of oxygen bases, which significantly shape the performance. It is generally accepted that bases are required for either substrate activation (exemplified by transmetallation in the Suzuki cross-coupling), or HX capture (<i>e.g.</i> in a variety of C-C and C-heteroatom couplings, the Heck reaction, C-H functionalization, heterocyclizations, <i>etc.</i>). This study gives insights into the behavior of M(ii)/NHC (M = Pd, Pt, Ni) complexes in solution under the action of bases conventionally engaged in catalysis (KOH, NaOH, <i>t</i>-BuOK, Cs₂CO₃, K₂CO₃, <i>etc.</i>). A previously unaddressed transformation of M(ii)/NHC complexes under conditions of typical base-mediated M/NHC catalyzed reactions is disclosed. Pd(ii) and Pt(ii) complexes widely used in catalysis react with the bases to give M(0) species and 2(5)-oxo-substituted azoles <i>via</i> an O-NHC coupling mechanism. Ni(NHC)₂X₂ complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O-NHC coupling to give azolones and metallic nickel under the action of <i>t</i>-BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M(ii) into "ligandless" M(0) species. This demonstrates that the disclosed base-mediated O-NHC coupling reaction is integrated into the catalytic M/NHC systems and can define the mechanism of catalysis (molecular M/NHC <i>vs.</i> "NHC-free" cocktail-type catalysis). A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including ¹⁸O labeling experiments.