Synthesis, reduction and C-H activation chemistry of azaborinines with redox-active organoboryl substituents.

The 2,3,4,5,6-pentaphenyl-1,2-azaborinin-1-yl (PPAB) potassium complex 1 undergoes facile salt metathesis with 9,10-dibromo-9,10-dihydroboraanthracene (DBA), 5-bromodibenzo[,]borole (DBB), 1-chlorotetraphenylborole (TPB) and dibromo(phenyl)borane (BBrPh) to yield the corresponding -borylated azaborinines -DBA-PPAB (2, which hydrolyses and dimerises to the oxo-bridged ,'-O(DBA)-(PPAB), 3), ,'-DBA-(PPAB) (4), -DBB-PPAB (5), -PPB-PPAB (7) and -BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙- and dianion 42-. One-electron reduction of 5 with KC yields the purple radical anion 5˙-, which forms a highly insoluble coordination polymer. 5˙- undergoes very slow radical intramolecular -C-H activation at the C4-phenyl substituent of the PPAB moiety, yielding a BN-analogue of the 5,5'-spiro-bi[dibenzoborole] anion, [6]K. Compound 7 cannot be isolated and undergoes spontaneous and diastereoselective 2,5--addition of the -C-H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the -C-H activation of the PPAB C4-phenyl substituent at an -generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9-fluorene, the borate 11-. DFT calculations provide a rationale for the diverse C-H activations observed in these reactions.