Addition, Substitution, and Ring-Contraction Reactions of Quinones with N-Heterocyclic Carbenes.

Despite the common use of quinones as oxidizing agents in N-heterocyclic carbene (NHC)-based organocatalysis and transition-metal catalysis, the direct reactivity of quinones with NHCs remains underexplored. In this paper, we report the reactivity of NHCs with common p- and o-quinones, uncovering three unreported reactions involving contractions of the quinone ring that lead to push-pull furanolactone chromophores, NHC fulvalenes, and α-acylimidazolium cyclopentenone derivatives. These experiments also provide a rationale for the superior compatibility of tetra- tert-alkylated diphenoquinones in NHC-based oxidative transformations.