Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me 3 P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX 2 Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (Me 3 P)BX 2 Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL′BH + 2 BX 2 Y → LHB═BHL + 2 L′BX 2 Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe 3 abstraction from the borylene precursor by BX 2 Y, multiple halide transfers between (di)boron intermediates and BX 2 Y/[BX 3 Y] −, and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me 3 P)BH] •+ . The findings suggest that [LBXR] • boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX 2 R boranes to LRB═BRL diborenes.