New zwitterionic heterocycles derived from 1-chloro(dimethyl)silyl-1-borylalkenes and 2-lithio-1-methylimidazole and 1-lithio-indazole

The 1-alkynyl(chloro)dimethylsilane tBuCCSiMe2Cl (1) reacts with tetraethyl-diborane(6) or 9-borabicyclo[3.3.1]nonane dimer, (9-BBN)2, to give the (Z)-1-chloro-dimethylsilyl-1-diethylboryl-alkenes 2 and 3. These alkenes react with 2-lithio-1-methylimidazole or 1-lithio-indazole by formation of LiCl and of 4, 5 and 9 together with small amounts of 6, 7 or 8 as minor products. In 4 and 5, a zwitterionic structure is present which can be alternatively described as a borane adduct of a carbene. All new compounds were characterised by 1H-, 11B-, 13C-, 15N- and 29Si-NMR spectroscopy, and the molecular structure of 5 was determined by X-ray structural analysis.