FLPNO Nitroxide Radical Formation by a 1,1-Carboboration Route

The 1-trimethyl­silyl­ethynyl-2-diaryl­phosphanyl­ethynyl­benzene substrates 7a–7c, with the diaryl­phosphanyl groups = PPh2, P­(o-tolyl)2, or P­(mesityl)2, were reacted with the strong boron Lewis acid B­(C6F5)3 at ambient conditions. The starting materials 7a and 7b, featuring rather small nucleo­philic phosphanyl substituents, underwent selective 1,1-carbo­boration at the CC-PAr2 moiety, forming rather unreactive unsaturated vicinal P/B Lewis pairs 8a and 8b that show a relatively strong internal P···B Lewis base/​Lewis acid interaction. In contrast, the PMes2 system 7c reacted with B­(C6F5)3 by a reaction sequence which was initiated by 1,1-carbo­boration and involved tandem C6F5 and mesityl group migration to yield the unsaturated annulated P/B Lewis pair 9, featuring an electron-withdrawing C6F5 group at phosphorus. Compound 9 underwent cooperative P/B addition to the nitrogen atom of nitric oxide to give the persistent FLPNO• nitroxide radical 12. Radical 12 was characterized by electron spin resonance spectroscopy and by X-ray diffraction. It showed typical nitroxide radical reactions.