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

• Published in: Organometallics Vol. 31; no. 5; pp. 1908 - 1916
• Main Authors: Solomon, S. A, Del Grosso, A, Clark, E. R, Bagutski, V, McDouall, J. J. W, Ingleson, M. J
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 12.03.2012; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; BORON; LIGANDS; AMMONIA; PYRIDINE; COMPLEXES; IONS; BORENIUM; BORANE; CATION; H ACTIVATION
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om201228e

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 p z 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.