Free-radical chemistry of organophosphorus derivatives. Nonequivalence of alkoxy groups in (RO)4P

The Role of Structural Effects on the Reactions of Alkoxyl Radicals with Trialkyl and Triaryl Phosphites. A Time-Resolved Kinetic Study

“…The data collected in Table show that for the tetraalkoxphosphoranyl radicals R′OP • (OR) 3 the fragmentation rate constants increase (for a given alkyl group R) in the order R′ = tert -butyl < R′ = benzyl < R′ = cumyl, a behavior that reasonably reflects the relative stabilities of the radicals formed after β-scission . Quite importantly, the observation of the tetraalkoxyphosphoranyl radicals in the reactions of both CumO • and BnO • with trialkyl phosphites is in contrast with previous reports where no intermediate phosphoranyl radical, but only the benzyl radical could be detected by ESR following reaction between BnO • and (EtO) 3 P, even at −140 °C, a result that was interpreted in terms of an extremely fast β-scission reaction in BnOP • (OEt) 3 . , Clearly, the observation of a ≤ 8-fold increase in k β on going from R′ = tert -butyl to R′ = benzyl (and cumyl) is in contrast with this indication.…”

Kinetic applications of electron paramagnetic resonance spectroscopy. VIII. Tetraalkoxyphosphoranyl radicals

“…Thus,15 the (EtO)4P radical is much more stable than the z-BuOP(OEt)3 since it has to eliminate an ethyl, which is much less stable than a tert-butyl radical. In contrast the reaction of benzyloxy radicals (from the photolysis of benzyl hyponitrite) with (EtO)3P yielded only benzyl radicals (see introductory section and ref 19). No phosphoranyl could be detected in cyclopropane at the lowest temperatures at which some hyponitrite remained in solution (ca.…”