Electron spin resonance studies of the structure and reactivity of some spirophosphoranyl radicals in solution
Spirophosphoranyl radicals have been generated by abstraction of hydrogen from 1,4,6,9-tetraoxa-5-phosphaspiro[4,4]nonane (IV; R = H) and related compounds. The e.s.r. spectra of these radicals have been observed and correlated with their structures. The radical (VII) from 2,2,7,7-tetramethyl-1,6-diaza-4,9-dioxa-5-phosphaspiro[4,4]nonane was also detected. These spirophosphoranyl radicals possess trigonal bipyramidal structures in which the unpaired electron is stereochemically active and occupies an equatorial ligand site. There is no evidence that any rapid exchange of apical and equatorial ligands (e.g. by pseudo-rotation) occurs on the e.s.r. time scale. A tertiary alkoxy-group occupies an equatorial position in preference to a primary alkoxy-group in these radicals The spirophosphoranyl radicals are remarkably stable towards unimolecular α- or β-scission, but they will add to terminal olefins and to 2-methyl-2-nitrosopropane.