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N-Heterocyclic carbene ligated transition-metal catalysts often show interesting properties and reactivity as compared to conventional ligand systems. In (NHC)Ni and (NHC)Pd hydrides, a dramatic reactivity changed from cross-hydroalkenylation to transfer-dehydroaromatization was observed under optimized conditions. This account summarizes our recent efforts and stories behind this serendipitous discovery. The mechanistic studies revealed that the keys to divert the desired reactivity are the differences in the olefin insertion selectivity and the hydrometallated species reactivity.

Though the properties of N-heterocyclic carbenes (NHCs) are generally dominated by the very strong σ donating character, electronic activation has emerged as an effective method to cooperate with typical carbon-framework steric optimization for highly enantioselective chiral NHC–transition-metal catalysis in recent years. NHC electronic changes associated with structural variations are now better understood by quantitative analysis using various methods. Here we highlighted and correlated some interesting chiral induction improvement methods, which were brought by electronic and steric cooperation on chiral NHC–transition-metal catalysis.1 Introduction2 Hemilabile Sidechains on NHC Ligands3 Electronic and Bond Angle Changes Brought by NHC Core Size Variations4 Electronic Activators on the NHC Core5 Conjugated Systems and Fused Ring Structures6 Remote Electronic Activators on the N-Aryl Ring7 Summary and Outlook

The first tail-to-tail vinyl ether dimerization has been developed by using a newly identified synergistic effect between the NHC-NiH catalyst and organonitrate additive (PhNO 2 or t -BuNO 2 ). The additive enhanced the desired C–C bond-forming reactivity by a proposed competitive coordination with the alkoxyl group of the hydrometallated vinyl ether species to the Ni center. This combination addressed complications related to β-alkoxy (β-OR) elimination and polymerization, and gave unexpected dimerization regioselectivity.

Various NHC―NiH species have been developed as hydroalkenylation and polymerization catalysts and for ionic-liquid synthesis. This article outlines their key features and connections and highlights the opportunities for using NHC―NiH species in the selective preparation of new geminal disubstituted olefins and how they complement traditional P―NiH technology.