The effect of backbone design on carbene reactivity : from small molecule activation to redox-switchable catalysis
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The ability of N-heterocyclic carbenes (NHCs) to act as organocatalysts as well as versatile ligands in transition metal-mediated processes led us to explore the effect of the NHCs backbone design on the electronic properties and the consequent reactivity of these NHC moieties. Bielawski and others have previously shown that the incorporation of carbonyls into the NHC scaffold enhanced the electrophilicity of the carbenoid center to generate an isolable, ambiphilic N,N' -diamidocarbene (DAC), which was shown to activate various small molecules. For this reason, we explored the ability of DAC to activate compounds containing early p-block elements. At ambient temperature, the DAC activated the Si-H bonds of various silanes to afford the corresponding DAC-silane adducts. The DAC was also found to form a coordination complex with aluminum trichloride and a structurally-rich, tris(aluminum) species was obtained upon exposure of the DAC to trimethylaluminum. Additionally, the DAC promoted the B-H bond activation of various BH₃ complexes and the B-B bond of bis(pinacolato)diboron, constituting the first such examples for an isolable carbene. The resultant DAC-BH₃ adducts contained datively coordinated Lewis bases and facilitated the hydroboration of various olefins under mild conditions and in the absence of exogenous initiators. Furthermore, we have synthesized a series of Ru-based complexes containing a quinone-annulated NHC ligand to provide redox-controlled analogues of the Grubbs' II, III, and Hoveyda-Grubbs II generation catalysts. All of the aforementioned complexes were shown to be active ring-opening metathesis polymerization (ROMP) catalysts. Moreover, in its neutral state, the redox-switchable analogue of Grubbs' III generation catalyst was shown to selectively promote the polymerization of 1,5-cyclooctadiene (COD) while the addition of a reductant inhibited the ROMP of COD. Remarkably, the opposite pattern was observed for the polymerization of norbornene imide derivatives as the ROMP of these monomers was enhanced upon the reduction of the redox-switchable analogue of Grubbs' III. Additionally, the neutral state of the redox-switchable analogue of Hoveyda-Grubbs II generation catalyst was shown to selectively facilitate ring-closing metathesis reactions, which could be reversibly inhibited upon the introduction of a suitable reducing agent.