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.
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. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. Replacement of halide from N-(haloboryl)formamidines for the weakly coordinating anion −, using + −, induces a ring closure leading to the cationic four-π-electron four-membered heterocycles +.
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