Stable four-π-electron, four-membered heterocyclic cations and carbenes

Abstract

Replacement of halide from N-(haloboryl)formamidines for the weakly coordinating anion [B(C₆F₅)₄]⁻, using [Et₃Si(toluene)]⁺[B(C₆F₅)₄]⁻, induces a ring closure leading to the cationic four-π-electron four-membered heterocycles [HC(N-2,6-diisopropylphenyl)₂BR]⁺ [R = N(i-Pr)₂, Ph]. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. According to single-crystal x-ray diffraction studies, both the carbene and its conjugate acid precursor have a planar structure, with no pyramidalization of the ring atoms. The structural parameters indicate that the four π electrons are mostly distributed over the N-C-N fragment with little involvement of the boron center, and therefore both types of heterocycles escape from antiaromaticity. However, considering the ring strain and the presence of the Lewis acid center, the thermal stability of the carbene (mp 98°C without decomposition) is rather surprising. These results clearly suggest that the backbone of N-heterocyclic carbenes can be modified at will, without preventing their isolation.