A primary focus of our group involves the development of a general catalytic approach for preparing heterocycles from readily available precursors. For example, cycloadditions of unsaturated hydrocarbons with heterocumulenes (e.g., CO₂ and isocyanates) is particularly attractive because the starting materials are abundant and the reaction lacks waste by-products. We have discovered that Ni based catalysts coordinated with highly donating and sterically hindered imidazolyidene ligands (e.g., 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene, IPr) are effective for mediating annulations between diynes and heterocumulenes (Equation 1). Thus far, a variety of 2-pyrones and 2-pyridones have been synthesized with diverse sets of functionality. We have also been successful at developing a regioselective variant of this reaction. Our investigations have led to a deeper understanding of the key mechanistic features of this reaction which includes pertinent intermediates as well as the requisite order of reactivity of the substrates. Efforts toward developing a more general cycloaddition catalyst by expanding the substrate scope to include enynes, dienes, and carbonyls as well as other heterocumulene partners are currently underway.
     As a collorary to our investigation, we found that the imidazolylidenes react with heterocumulenes. The reaction is highly dependent on the nature of the heterocumulene as well as imidazolylidene ligand: reaction between CO₂ and IPr afforded zwitterionic imidazolylidene carboxylates whereas 1,3-bis-(1,3,5-trimethyl)-imidazol-2-ylidene (IMes) oligomerized isocyanates to their dimers (uretinones) or trimers (isocyanurates, Scheme 1). The products of these reactions should find application in various aspects of polymer science. For example, development of polymeric imidazolylidene carboxylates would provide carbon/CO₂ sequestering and/or delivery agents and the efficient formation of uretinones/isocyanurates is a critical process in industrial syntheses of polyurethanes.

      A potentially attractive method for preparing cyclopentenes, a common carbocyclic building block and structural motif in many biological systems, is the rearrangement of vinylcyclopropanes (VCPs). This atom-efficient protocol has attracted considerable attention but, unfortunately, harsh reaction conditions and activated VCP precursors (which are often challenging to prepare) has severely curtailed its synthetic utility. We recently reported that the combination of Ni0 with a sterically hindered imidazolylidene ligand (IPr) catalyzes the isomerization of a variety of activated and unactivated VCPs under mild conditions to afford the respective cyclopentenes in good yields (Equation 2). We have also discovered that this methodology can be extended to cyclopropylen-ynes (Equation 3) and that the two different carbocycles can be generated exclusively depending on reaction conditions. Studies toward understanding the mechanism, broadening the substrate scope, and applying this method for the synthesis of pharmacophores are in progress.