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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Stable four-pi-electron, four-membered heterocyclic cations and carbenes
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Stable four-pi-electron, four-membered heterocyclic cations and carbenes

机译：Stable four-pi-electron, four-membered heterocyclic cations and carbenes

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摘要

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-pi-electron four-membered heterocycles HC(N-2,6-diisopropylphenyl)(2)-BR(+) 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 pi 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 degrees 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.

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