The SET-induced photorearrangement of dimethyl 2-(4-methoxyphenyl)allyl phosphite, 9 (UV light, uranium glass filter, 9,10-dicyanoanthracene (DCA), biphenyl), gives phosphonate 12 in 83% isolated yield. Laser flash irradiation at 355 nm of oxygen saturated solutions of phosphite 9 containing DCA and biphenyl generates the transient UV spectrum of the biphenyl radical cation that is quenched by electron transfer from phosphite 9 (k(q) = 8.9 x 10(9) M(-1) s(-1) at 20 degrees C) to form the 4-methoxystyryl cation 10. The UV spectrum of 10 decays by a measured first-order rate constant of 8.0 x 10(6) s(-1), presumably to generate the cyclic distonic radical cation 11. Intermediate 10 was further characterized by measurement of the second-order rate constants for its reaction with azide, chloride, and bromide ions and with the neutral nucleophile trimethyl phosphite. This study provides the first spectroscopic evidence regarding the proposed mechanism (Schemes 1 and 2) for the SET-induced photorearrangements of dimethyl 2-arylallyl phosphites to the corresponding 2-arylallylphosphonates. Moreover, absolute rate constants for the intramolecular trapping of alkene radical cations have seldom been measured. The removal of the electron from the styryl moiety of phosphite 9, rather than from phosphorus, and the detectability of 10 arise from the stabilizing effect of the 4-methoxy substituent. These results, however, do not allow conclusions to be made concerning the site of removal of an electron in the SET-induced photorearrangement of dimethyl 2-phenylallyl phosphite 1 to phosphonate 6.