Engineering a Small HOMO–LUMO Gap and Intramolecular C−H Borylation by Diborene/Anthracene Orbital Intercalation

The diborene 1 was synthesized by reduction of a mixture of 1,2‐di‐9‐anthryl‐1,2‐dibromodiborane(4) ( 6 ) and trimethylphosphine with potassium graphite. The X‐ray structure of 1 shows the two anthryl rings to be parallel and their π(C 14 ) systems perpendicular to the diborene π(B=B) system. This twisted conformation allows for intercalation of the relatively high‐lying π(B=B) orbital and the low‐lying π* orbital of the anthryl moiety with no significant conjugation, resulting in a small HOMO–LUMO gap (HLG) and ultimately a C−H borylation of the anthryl unit. The HLG of 1 was estimated to be 1.57 eV from the onset of the long wavelength band in its UV/Vis absorption spectrum (THF, λ onset =788 nm). The oxidation of 1 with elemental selenium afforded diboraselenirane 8 in quantitative yield. By oxidative abstraction of one phosphine ligand by another equivalent of elemental selenium, the B−B and C 1 −H bonds of 8 were cleaved to give the cyclic 1,9‐diborylanthracene 9 .