KeywordsAb initio, Boron compounds, Crystal structure, Homoaromatics, Three center two electron bond
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract Bishomotriborirane anions with a B-H-B bridge, 7, have been synthesized by a) protonation and b) methylation of bishomodianions, 3, as well as by c) hydride addition to 1,2,4-triboracyclopentanes, 15. Compounds 7 were characterized by 1H, 13C and 11B NMR spectroscopy and X-ray diffraction analyses. The suggested mechanism for the formation of 7 is supported by MP4SDTQ/6-311++G**//MP2(fc)/6-31+G* computations on [C2B3H8]- model compounds. Classical 1,2-dibora-4-borata-cyclopentane intermediates 16 undergo an intramolecular hydrogen shift to the B-B unit in their envelope conformation to give intermediates 17, which easily isomerize to 7. Relative energies for the parent compounds, 16u, 17u, 7u and the transition structures, TS-16/17u and TS-7/17u are predicted to be 30.7, 14.5, 0.0, 32.6 and 23.5 kcal mol-1, respectively. The terms classical and non-classical homobridges are suggested for methylene and hydrogen bridges in 7 and in related compounds on the grounds of common building principles. The strength of homoaromaticity in 7u was estimated to be at least 23.5 kcal mol-1, neglecting the much higher strain in 7u compared to TS-7/17u without a 3c2e bond.
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