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

摘要

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-fert-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, Li_2[6], only one 1,2-phenyl shift has occurred. The doubly ring-contracted product, Li_2[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, Li_2[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 Li_2[1]. In contrast, excess KC_8 reduces 4 to the corresponding dihydro- dibenzo[g,p]chrysene, K_2[5], with a trans-HB-BH core. Hydride abstraction from K_2[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 KC_8. All products have been characterized by multinudear NMR spectroscopy and X-ray crystallography.