An electron spin resonance study of the addition of alkoxyl and siloxyl radicals to pyrrolylphosphines

Abstract

E.s.r. spectra are reported for phosphoranyl radicals generated by addition of Bu^tO·, EtO·, or Me₃SiO· to the pyrrolylphosphines (EtO)₂PNC₄H₄ and [graphic omitted]NC₄H₄ in hydrocarbon solution. The pyrrolylphosphoranyl radicals have trigonal–bipyramidal structures, in which the unpaired electron is centred mainly on phosphorus and the apical ligands, rather than the ligand-π electronic structure adopted by the analogous phenylphosphoranyl radicals. For RO(EtO)₂ṖNC₄H₄(4; R = Bu^t, Et, or Me₃Si) at low temperatures, separate signals were detected from isomeric radicals in which the pyrrolyl group is either apical or equatorial, and at higher temperatures rapid interconversion of these isomers gives rise to lineshape effects in their e.s.r. spectra. Above ca. 260 K, the spectra correspond to the concentration-weighted average from the pairs of isomers. Similar effects were observed for the cyclic radicals RO([graphic omitted])ṖNC₄H₄(6), in which the ring bridges apical and equatorial sites, although these species could not be examined at temperatures sufficiently low to allow detection of separate signals from the two isomers. The apicophilicity of the pyrrolyl group is similar to that of RO, and for (6) the proportion of the isomer in which RO is apical decreases in the order Bu^tO > EtO > Me₃SiO. Factors which might cause this trend are discussed and the relevance to previous conclusions regarding site-selectivity of β-scission of alkoxyphosphoranyl radicals is pointed out. The radicals (4; R = Bu^t or Et) undergo β-scission to give R· and (EtO)₂P(O)NC₄H₄ at high temperatures.