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 [Ph3C]­[closo-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 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., [Et3Si]­[closo-CB11H6Br6]. The reactivity
disparity arises from greater steric shielding of the boron pz 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 and dichloro borocations, enabling it to borylate arenes containing
nucleophilic −NMe2 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 [AlCl4]¯ to boron on addition of a nucleophile.