An ESR and DFT revisitation of phosphonitroxides prompted by an ineffective ESR approach to the hydrophosphination of alkynes

An ESR investigation of the thermal hydrophosphination of 1-hexyne by five different secondary phosphines and phosphine-boranes did not lead to the observation of any radical, but in the presence of the spin trap MNP (2-methyl-2-nitrosopropane) several phosphonitroxides were detected. The unusually low phosphorus splitting exhibited by some of these species could be accounted for on the basis of DFT calculations. Although two phospho-substituted vinyl nitroxides were identified in the course of the hydrophosphination reaction, they could not be considered as evidence of the direct involvement of phosphorus-centred radicals.