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

The preparation of an N-heterocyclic quinodimethane (NHQ) is reported. In its closed-shell ground state, the NHQ is a strong organic base whose proton affinity exceeds that of analogous N-heterocyclic olefins. Moreover, the NHQ exhibits radical reactivity and undergoes dehydrogenative head-to-head dimerization via an open-shell singlet pathway.

Abstract

We 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 (E _1/2=−1.71 V vs. Fc/Fc⁺) and exhibits a remarkable small singlet–triplet gap of ΔE _S→T=4.4 kcal mol⁻¹.

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