A laser flash photolysis study of t-butoxyphosphoranyl radicals. Optical spectra and kinetics of formation, fragmentation, and rearrangement

Abstract

Addition of t-butoxyl radicals to a number of phosphorus(III) compounds in solution has been studied using laser flash photolysis techniques. Transient optical spectra ascribed to the phosphoranyl radical adducts have been detected between 300 and 800 nm and the kinetics of their formation and decay examined. Addition of Bu^tO˙ to (EtO)₃P or PhP(OEt)₂ takes place at close to the diffusion controlled rate at ca. 300 K. Comparison of optical and e.s.r. spectra permits correlation of the former with radical structure and hypervalent and ligand-π phosphoranyls give rise to characteristically different types of optical spectra. Activation parameters for β-scission of the t-butoxyl radical adducts of (EtO)₃P, PhP(OEt)₂, Ph₂POMe, and [graphic ommitted]POSiMe₃ agree well with those obtained previously by kinetic e.s.r. spectroscopy. α-Scission of Bu^tOPBuⁿ₃(which is too short-lived to be studied by e.s.r. techniques) to give Buⁿ˙ has been studied and k_α is given by equation (A), in which θ= 2.303RT kJ mol^–1. The, log (k_α/s^–1)= 12.7–24.3/θ(A) optical spectra of the valence isomers Bu^tO(EtO)₂PNCO and Bu^tO(EtO)₂PN–ĊO are readily distinguishable and a kinetic study of the rearrangement of the former to the latter led to equation (B). log(k_rearr./s^–1)= 12.6–28.9/θ(B)