A Novel Boron Heterocycle Comprised of an Inner Boronium/Borate Adduct

The reaction of [MeSCH2Li·(tmeda)] with 9-Cl-9-BBN led to the formation of the unprecedented boron heterocycle 3, which is an inner salt formally composed of boronium and borate subunits. This product rearranges upon heating to the thermodynamically more stable Ci-symmetric dimer 4, which can also be directly prepared if 9-MeO-9-BBN is chosen as the starting material instead of 9-Cl-9-BBN. The reasons for the different outcomes are discussed, and the solid-state structures of the dimeric products 3 and 4, as well as of the derived α-functionalized organoboron species 6, 9, and 12, are presented.