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

摘要

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2- benzodiazaborole with catalytic [Ph _3C][closo-CB _(11)H _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 _(11)H _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.