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

Abstract

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^Br2), 5-bromodibenzo[b,d]borole (DBB^Br), 1-chlorotetraphenylborole (TPB^Cl) and dibromo(phenyl)borane (BBr₂Ph) to yield the corresponding N-borylated azaborinines N-DBA^Br-PPAB (2, which hydrolyses and dimerises to the oxo-bridged N,N′-O(DBA)₂-(PPAB)₂, 3), N,N′-DBA-(PPAB)₂ (4), N-DBB-PPAB (5), N-PPB-PPAB (7) and N-BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙⁻ and dianion 4²⁻. One-electron reduction of 5 with KC₈ yields the purple radical anion 5˙⁻, which forms a highly insoluble coordination polymer. 5˙⁻ undergoes very slow radical intramolecular ortho-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-anti-addition of the ortho-C–H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[l]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the ortho-C–H activation of the PPAB C4-phenyl substituent at an in situ-generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9H-fluorene, the borate 11⁻. DFT calculations provide a rationale for the diverse C–H activations observed in these reactions.