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.