Aryl-aryl coupling in a polycyclic aromatic hydrocarbon with embedded tetracoordinate boron centre

The addition of 2 eq. of MesLi to the biphenylene-containing 9,10-dihydro-9,10-diboraanthracene (DBA) 2 results in the formation of the corresponding oxaboraphenanthrene 3 after column chromatography under ambient conditions. In the initial step, the anionic B-Mes monoadduct of 2 is generated, which eliminates a formal [Mes₂B:]^- ion with concomitant C-C-bond formation (room temperature, 18 h). The resulting biphenylene-containing borafluorene 4 is still sufficiently Lewis acidic to add the second equivalent of MesLi ([4Mes]^-). Using pristine 9-mesityl-9-borafluorene as a model system, we confirmed that both 4 and [4Mes]^- should be capable of inserting an oxygen atom to furnish the observed oxaboraphenanthrene scaffold. The biphenylene-containing oxaboraphenanthrene 3 is a yellow compound with an absorption maximum at λ_max = 450 nm. Similar to its DBA analogue 2, 3 shows no photoluminescence.