Mono- and Bis-Phosphine Promoted Incorporation of Boron, Nitrogen, and Phosphorus into Heterocycles via Staudinger Reactions of Borafluorene Azides.

We report the synthesis and characterization of a series of BNP-incorporated borafluorenate heterocycles formed via thermolysis reactions of pyridylphosphine and bis(phosphine)-coordinated borafluorene azides. The use of diphenyl-2-pyridylphosphine (PyPh 2 P), trans-1,2-bis(diphenylphosphino)ethylene (Ph 2 P(H)C═C(H)PPh 2 ), and bis(diphenylphosphino)methane (Ph 2 PC(H 2 )PPh 2 ) as stabilizing ligands resulted in Staudinger reactions to form complex heterocycles with four- (BN 2 P, BNPC, P 2 N 2 ) and five-membered (BNP 2 C and BN 2 PC) rings, which were successfully isolated and fully characterized by multinuclear NMR and X-ray crystallography. However, when bis(diphenylphosphino)benzene (Ph 2 P-Ph-PPh 2 ) was used as the ligand in a reaction with 9-bromo-9-borafluorene (BF-Br), due to the close proximity of the donor P atoms, the diphosphine-stabilized borafluoronium ion with an unusual borafluorene dibromide anion was formed. Reaction of the borafluoronium ion with trimethylsilyl azide left the cation intact, and the dibromide anion was substituted by a diazide. Density functional theory calculations were used to provide mechanistic insight into the formation of these new boracyclic compounds. This work highlights a new method in which donor phosphine ligands may be used to promote dimerization, cyclization, and ring contraction reactions to produce boracycles via Staudinger reactions.