Configurational effects in the α-scission of phosphoranyl radicals

Abstract

The rates of α-scission of a series of dialkyldi-t-butoxyphosphoranyl radicals R¹R²Ṗ(OBu^t)₂, in which R¹ and R² may be the same or different, have been measured by kinetic e.s.r. spectroscopy. The results are rationalised in terms of α-scission occurring faster when an apical rather than an equatorial P–C bond undergoes cleavage. It is suggested that the increase in rate of α-scission along the series L₂Ṗ(OBu^t)₂ < L₂Ṗ(OBu^t)OEt < L₂Ṗ(OEt)₂(L = alkyl or R₂N) is due to the increased proportion of the isomer with an apical P–L bond which is present as the number of ethoxy-ligands increases. However, if configurational effects are absent, changing the nature of the alkoxy-substituents in a phosphoranyl radical has only a small effect on the rate of its α-scission. The equatorial P–C bond dissociation enthalpy of MeṖ(OPrⁱ)₃ is found to be ca.+29 kJ mol^–1. The radicals Me₃ṖOR (R = Et, Bu^t, t-pentyl) undergo exchange of apical and equatorial methyl groups on the e.s.r. time scale, and k_exch= 4 × 10⁶s^–1 for Me₃ṖOBu^t at –70°. It is suggested that the low selectivity observed for the overall displacement of alkyl radicals from R¹R²₂P by t-butoxyl radicals may be due to the low selectivity of the phosphoranyl radical formation step, which is product controlling.