Arene extrusion as an approach to reductive elimination at boron: Implication of carbene-ligated haloborylene as a transient reactive intermediate

Herein, we report a boron-centered arene extrusion reaction to afford putative cyclic(alkyl)(amino) carbene (CAAC)-ligated chloroborylene and bromoborylene. The borylene precursors, chloro-boranorbornadiene (ClB(C₆Me₆), 2^Cl) and bromo-boranorbornadiene (BrB(C₆Me₆), 2^Br) were synthesized through the reaction of the corresponding 1-halo-2,3,4,5-tetramethylborole dimer (XBC₄Me₄)₂ (X = Cl, 1^Cl; X = Br, 1^Br) with 2-butyne. Treatment of 2^Cl with CAACs resulted in the release of di-coordinate chloro-borylene (CAAC)BCl from hexamethylbenzene (C₆Me₆) at room temperature. In contrast, the reaction of 2^Br with CAAC led to the formation of a boronium species [(CAAC)BC₆Me₆]⁺Br^– (7) at room temperature. Heating 7 in toluene promoted the release of di-coordinate bromo-borylene (CAAC)BBr as a transient species. Surprisingly, heating 7 in dichloromethane resulted in the C–H activation of hexamethylbenzene. The conversion of a CAAC-stabilized bromo-borepin to a borylene, a boron-centered retro Büchner reaction, was also investigated.