Reactivity of Lewis Acid Activated Diaza- and Dithiaboroles in Electrophilic Arene Borylation

Hydride abstraction from <i>N,N</i>′-bis­(adamantyl)-1-hydrido-1,3,2-benzodiazaborole with catalytic [Ph₃C]­[<i>closo</i>-CB₁₁H₆Br₆] resulted in a low yield of arene borylation and a major product derived from migration of both adamantyl groups to the arene backbone. In contrast, the related aryl-substituted diazaborole <i>N,N</i>′-(2,6-diisopropylphenyl)-1-bromo-1,3,2-diazaborole did not borylate benzene or toluene, being resistant to halide abstraction even with strong halide acceptors: e.g., [Et₃Si]­[<i>closo</i>-CB₁₁H₆Br₆]. The reactivity disparity arises from greater steric shielding of the boron p_<i>z</i> orbital in the 2,6-diisopropylphenyl-substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2-benzodithiaborole ((CatS₂)­BCl) are active for arene borylation, displaying reactivity between that of catecholato- and dichloro-boron electrophiles. [(CatS₂)­B­(NEt₃)]­[AlCl₄] is significantly less prone to nucleophile-induced transfer of halide from [AlCl₄]¯ to boron compared to catecholato<i> </i> and dichloro borocations, enabling it to borylate arenes containing nucleophilic −NMe₂ moieties in high conversion (e.g., <i>N,N</i>,4-trimethylaniline and 1,8-bis­(dimethylamino)­naphthalene). Calculations indicate that the magnitude of positive charge at boron is a key factor in determining the propensity of chloride transfer from [AlCl₄]¯ to boron on addition of a nucleophile