Laser flash photolysis evidence for styryl radical cation cyclization in the SET-induced photorearrangement of a p-methoxy-substituted 2-phenylallyl phosphite

The SET-induced photorearrangement of dimethyl 2-(4-methoxyphenyl)allyl pho sphite, 9 (UV light, uranium glass filter, 9,10-dicyanoanthracene (DCA), bi phenyl), gives phosphonate 12 in 83% isolated yield. Laser flash irradiatio n at 355 nm of oxygen saturated solutions of phosphite 9 containing DCA and biphenyl generates the transient UV spectrum of the biphenyl radical catio n 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. Int ermediate 10 was further characterized by measurement of the second-order r ate constants for its reaction with azide, chloride, and bromide ions and w ith the neutral nucleophile trimethyl phosphite. This study provides the fi rst spectroscopic evidence regarding the proposed mechanism (Schemes 1 and 2) for the SET-induced photorearrangements of dimethyl 2-arylallyl phosphit es to the corresponding 2-arylallylphosphonates. Moreover, absolute rate co nstants for the intramolecular trapping of alkene radical cations have seld om been measured. The removal of the electron from the styryl moiety of pho sphite 9, rather than from phosphorus, and the detectability of 10 arise fr om the stabilizing effect of the 4-methoxy substituent. These results, howe ver, 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.