Reactivity of Lewis acid activated diaza- and dithiaboroles in electrophilic arene borylation

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2- benzodiazaborole with catalytic [Ph ₃C][closo-CB ₁₁H ₆Br ₆] 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 N,N′-(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., [Et ₃Si][closo-CB ₁₁H ₆Br ₆]. The reactivity disparity arises from greater steric shielding of the boron p _z orbital in the 2,6-diisopropylphenyl- substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2- benzodithiaborole ((CatS ₂)BCl) are active for arene borylation, displaying reactivity between that of catecholato- and dichloro-boron electrophiles. [(CatS ₂)B(NEt ₃)][AlCl ₄] is significantly less prone to nucleophile-induced transfer of halide from [AlCl ₄] to boron compared to catecholato and dichloro borocations, enabling it to borylate arenes containing nucleophilic -NMe ₂ moieties in high conversion (e.g., N,N,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 [AlCl ₄] to boron on addition of a nucleophile.