An N‐Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity

Abstract

AbstractWe report the preparation of a new organic σ‐donor with a C6H4‐linker between an N‐heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N‐heterocyclic quinodimethane (NHQ). The aromatization of the C6H4‐linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N‐heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head‐to‐head dimerization by C−C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open‐shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N‐heterocyclic bis‐quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=−1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet–triplet gap of ΔES→T=4.4 kcal mol−1.