Forming B–B Bonds by the Controlled Reduction of a Tetraaryl-diborane(6)

Dimeric aryl­(hydro)­boranes
can provide suitable platforms for the
synthesis of boron-containing graphene flakes through reductive B–B coupling. Two-electron
reduction of 1,2:1,2-bis­(4,4′-di-tert-butyl-2,2′-biphenylylene)­diborane­(6)
(4) with LiNaph/THF establishes a B–B σ
bond but can be accompanied by substituent redistribution. In the
singly rearranged product, Li2[6], only one
1,2-phenyl shift has occurred. The doubly ring-contracted product,
Li2[7], consists of two 9H-9-borafluorenyl moieties that are linked via their boron atoms.
When the amount of LiNaph/THF is increased to 4 equiv, Li2[6] is subsequently observed as the dominant species.
Addition of 11 equiv of LiNaph/THF results in over-reduction with
hydride elimination to afford the doubly boron-doped dibenzo­[g,p]­chrysene Li2[1]. In contrast, excess KC8 reduces 4 to the
corresponding dihydro-dibenzo­[g,p]­chrysene, K2[5], with a trans-HB–BH core. Hydride abstraction from K2[5] with 1 equiv of 4 leads to K­[8], in which the central B–B bond is bridged by a single hydrogen
atom. K­[8] is also obtained upon treatment of 4 with 1 equiv of KC8. All products have been characterized
by multinuclear NMR spectroscopy and X-ray crystallography.