An electron spin resonance study of radical addition to vinylphosphines

Abstract

E.s.r. spectroscopy has been used to show that alkoxyl and methyl radicals add to phosphorus in the vinylic phosphines R₂CC(H)PX₂(X = MeO, EtO, or Me₂N) to form ‘phosphoranyl’ radicals in which the unpaired electron is centred mainly on the remote carbon. These adducts are π-radicals and can be regarded as alkene radical anions which carry a phosphonium substituent, although there is significant spin density on phosphorus. The adducts R₂Ċ–C(H)P(OR′)₂OBu^t(R = H or Me) undergo fragmentation to give t-butyl radicals and the phosphonate ester R₂CC(H)P(O)(OR′)₂.

t-Butyl radicals add to the unsubstituted carbon of the vinyl group in H₂CC(H)PX₂(X = RO, Me₂N, or Et) to give α-phosphinoalkyl radicals Bu^tCH₂Ċ(H)PX₂, the e.s.r. spectra of which exhibit relatively large splitting from ³¹P. There appears to be appreciable delocalisation of the unpaired electron onto phosphorus, a result which was not anticipated on the basis of previous reports of e.s.r. spectra ascribed to α-phosphinoalkyl radicals.