1,2-Bis(boronate) Arenes by Borylation Directed Borylation

Aryl boronate esters are ubiquitous in synthesis and are efficiently produced by C-H borylation. However, there are no reported methods to date that convert arenes into vicinal bis-boronate ester derivatives by double C-H borylation. Instead, all current routes require pre-functionalized arene precursors. Vicinal bis-boronate esters are highly desirable, as arenes with this substitution pattern are pervasive in biologically active compounds and organic materials. Herein we show that boron electrophiles derived from tetrahalo-pyrazaboles (X₂B(μ-pyrazole)₂BX₂) and AlX₃ (X = Cl or Br) affect the regioselective vicinal double C-H borylation of arenes and polycyclic aromatic hydrocarbons (PAHs). This afforded diborylated products that were notable in their own right, representing novel cationic PAHs containing an unusual cyclic C₂B₂N₂ unit. A subsequent pinacol installation step then affords the vicinal bis-boronate ester-substituted arene/PAH. A wide range of arenes and PAHs were shown to undergo highly regioselective one-pot conversion to the vicinal bis-boronate ester-substituted products, of which multiple were unprecedented before this work. Overall, this work is the first, to our knowledge, double C-H borylation methodology to form vicinal bis-boronate ester-substituted arenes/PAHs and therefore is a useful addition to the synthetic chemist's toolbox given the considerable utility of organoboranes.