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

Herein, we report boron-centered arene extrusion reactions to afford putative cyclic(alkyl)(amino) carbene (CAAC)-ligated chloroborylene and bromoborylene intermediates. The borylene precursors, chloro-boranorbornadiene (ClB(C 6 Me 6 ), 2 Cl ) and bromo-boranorbornadiene (BrB(C 6 Me 6 ), 2 Br ) were synthesized through the reaction of the corresponding 1-halo-2,3,4,5-tetramethylborole dimer (XBC 4 Me 4 ) 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 (C 6 Me 6 ) at room temperature. In contrast, the reaction of 2 Br with CAAC led to the formation of a boronium species [(CAAC)BC 6 Me 6 ] + 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. Herein, we report boron-centered reductive elimination reactions to afford cyclic(alkyl)(amino) carbene (CAAC)-ligated chloroborylene and bromoborylene.