Investigating the chemistry of the 9-borataphenanthrene anion.

Boron-containing analogues of unsaturated hydrocarbons exhibit a diverse array of reactivity modes that can be exploited for the synthesis of other more complex boron-containing compounds. Examples of such analogues include boroles, analogous with the cyclopentadienyl cation; boratabenzenes, analogous with benzene; and borataalkenes, analogous with olefins. A dibenzo-fused borole, 9-phenyl-9-borafluorene, was shown to undergo insertion of one or two carbene units upon reaction with trimethylsilyldiazomethane to produce six- or seven-membered nonaromatic boracycles. The six-membered product was found to undergo deprotonation to form the first example of a 9-borataphenanthrene anion, which exhibits structural features characteristic of boratabenzenes and borataalkenes. Its boratabenzene-like character was demonstrated through η6 complexation with chromium(0), while its borataalkene-like character was demonstrated through protonation, C-methylation with iodomethane, B=C hydroboration with pinacolborane, and η2 complexation with gold(I). The B=C bond at the 9- and 10-positions of the 9-borataphenanthrene anion was also shown to undergo a broad range of hydrofunctionalization reactivity, including hydroalkylation, hydroarylation, hydroalkynylation, hydroamination, hydroalkoxylation, and hydration. The central BC5 ring was demonstrated to form η6 complexes with rhodium(I), iridium(I), and iron(II) centers. The wide array of boratabenzene- and borataalkene-like reactivity modes indicates the promising potential of 9-borataphenanthrene anions as reagents for inorganic and organic synthesis.