Robert W. Dennis

Homolytic organometallic reactions. Part XII. An electron spin resonance study of the α-scission of alkoxyalkylphosphoranyl radicals in solution

The structures and the kinetics of α-scission of a series of alkoxyalkylphosphoranyl radicals have been studied by e.s.r. spectroscopy in solution. These radicals were generated by the addition of photolytically produced alkoxyl radicals to alkoxyalkylphosphines. The stability towards α-scission of the radicals EtnṖ(OR)4 –n exhibits a maximum for n= 2. Phosphoranyl radicals R2Ṗ(OBut)2 fragment by loss of an alkyl radical, R·, with increasing rate along the series R = Me < But < Et ∼ Prn < Pri∼ Bus < allyl. These results are interpreted in terms of a preference for departure of an alkyl radical from an apical rather than an equatorial site, in α-scission of the trigonal bipyramidal phosphoranyl radical. Factors which influence the relative rates of α- and β-scission of alkoxyalkylphosphoranyl radicals are discussed.

An electron spin resonance study of the structure and reactivity of aminophosphoranyl radicals in solution

The structure and reactivity of a series of aminophosphoranyl radicals, generated by addition of photochemically produced alkoxyl radicals to aminophosphines, have been investigated by e.s.r. spectroscopy in solution. These phosphoranyl radicals possess a trigonal bipyramidal structure and some are stereochemically non-rigid, showing temperature-dependent line-shape effects in their spectra. In many cases, two isomers may be detected differing in the apical or equatorial placement of a dialkylamino-group.The apicophilicities of alkoxy- and dialkylamino-groups in phosphoranyl radicals are very similar, although the latter generally have a greater preference for equatorial placement. In monocyclic aminophosphoranyl radicals in which two ligands form part of a five-membered ring, the two acyclic ligands appear to exchange position more rapidly than the endocyclic ligands, precluding pseudoratation, with the unpaired electron as pivot, as the mechanism of ligand permutation in such species. Both five- and six-membered ring systems incorporating two P–N bonds preferentially bridge apical and equatorial positions in aminophosphoranyl radicals.Alkoxy(dialkylamino)phosphoranyl radicals fragment by α- or β-scission, depending upon the structure of the radical, but preliminary kinetic studies indicate that interconversion of isomers differing in the apical or equatorial placement of a dimethylamino-group is rapid in comparison with α-scission with P–N bond cleavage.

The pronounced differences in reactivity of perfluoro-t-butoxyl and t-butoxyl radicals

Perfluoro-t-butoxyl radicals differ from t-butoxyl radicals in that they add to double bonds rather than abstract allylic hydrogens, and abstract hydrogen from the alkyl groups of trialkyl phosphites or trialkyl-boranes in preference to exclusive attack at the metal centre.

An electron spin resonance study of fluoroalkoxy- and fluoroalkylphosphoranyl radicals

Two series of phosphoranyl radicals, containing fluoroalkoxy or trifluoromethyl ligands, respectively, have been generated in solution by radical addition to trivalent phosphorus compounds, and their e.s.r. spectra have been studied. The spectra of the radicals RFOṖ(OR)3 and RFOṖMe(OR)2exhibit large isotropic phosphorus hyperfine splittings and long-range fluorine coupling with (in many cases) fewer than the maximum number of chemically equivalent 19F nuclei. Temperature-dependent line-shape changes were observed and interpreted in terms of the presence of rapid conformational equilibria. The e.s.r. spectra were consistent with trigonal bipyramidal structures for the phosphoranyl radicals, in which the RFO ligands are apical and the CH3 groups equatorial.The structures of the trifluoromethylphosphoranyl radicals, assigned on the basis of their e.s.r. spectra, show that the apicophilicity of the CF3, group is less than that of Cl and usually greater than that of RO. Phosphoranyl radicals containing an apical CF3 ligand exhibit lower values of a(P) than their methylphosphoranyl analogues, although the group electronegativity of CF3 is much greater than that of CH3. Temperature-dependent line-shape effects demonstrate that rotation about the P–CF3 bond is restricted to a degree dependent upon the nature of the other substituents. Trifluoromethyl radicals add reversibly to trialkyl phosphites.