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

Abstract

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2-benzodiazaborole with catalytic [Ph 3 C][closo-CB 11 H 6 Br 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 arylsubstituted 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 3 Si][closo-CB 11 H 6 Br 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.

Fundamental and Applied Properties of Borocations

“…The Lewis acidity of compounds 4 and 5 can also be assessed by the magnitude of the dip angle (the ring centroid -N -B angle), with 5 having a dip angle of À9. 6 compared to that of À12. 3 for Cp*Fe(C 5 H 4 BBr 2 ), indicating a greater interaction between Fe and boron for the neutral borane [14].…”

“…With borenium cations that have considerable Lewis acidity, B-Y bond heterolysis from the neutral precursor often only proceeds with the strongest MX n Lewis acids, even when L is bulky and electron donating. Furthermore, B-Y bond heterolysis can be reversible, leading to complex dynamic mixtures in the solution phase that can complicate subsequent reactivity [6,10].…”

“…B π donation relative to S ! B π donation would suggest that 6 would be a weaker Lewis acid; however, the significantly greater polarisation of the σ bonding framework in 1 leads to a larger magnitude of positive charge localised at boron in 1 thus contributing to a greater CIA (NBO charges at boron for 1 ¼ +1.338e, for 6 ¼ +0.396e at the MPW1K/6-311++G(d,p) level) [6]. The importance of electrostatics in increasing the binding strength of Lewis bases containing highly electronegative donor atoms towards borocations is not limited to borenium cations; Frenking and co-workers reported analogous trends for the borinium cation [H 2 B] + with EC 5 H 5 (E¼N, P, As, Sb, Bi) [41].…”

“…Finally, we acknowledge that representing the positive charge throughout this review as localised at boron in borenium cations is a simplification and that charge is inherently diffuse. However, we feel that this representation is useful as it emphasises that the locus of electrophilic reactivity is consistently at boron and that boron generally possesses the greatest magnitude of positive charge in these cations (by NBO calculations) [6]. Fig.…”

Haloboration of Internal Alkynes with Boronium and Borenium Cations as a Route to Tetrasubstituted Alkenes

“…[34] structures of 1 and 3 are shown in Figure 1), the boron center in these compounds is four-coordinate, and 2-DMAP chelation to boron is observed. Coordinative saturation at boron suggested that 1-3 may not be viable boron Lewis acids.…”