NANOSECOND AND PICOSECOND DYNAMICS OF THE RADICAL-CATION MEDIATED DIMERIZATION OF 4-METHOXYSTYRENE

The addition of the 4-methoxystyrene radical cation to neutral 4-metho xystyrene and the cleavage reactions of the 1,2-bis(4-methoxyphenyl)cy clobutane radical cation in acetonitrile have been studied by product analysis and by nanosecond and picosecond transient absorption spectro scopy. The main product upon radical cation mediated dimerization of 4 -methoxystyrene using chloranil as the electron transfer sensitizer is a substituted dihydronaphthalene, dihydro-7-methoxy-1-(4'-methoxyphen yl)naphthalene, at low concentrations of 4-methoxystyrene. At higher c oncentrations, the main product is 1,2-bis(4-methoxyphenyl)cyclobutane . Cleavage of the cyclobutane radical cation is found to give 4-methox ystyrene and the dihydronaphthalene in a 1:3 ratio. In the time-resolv ed experiments, the radical cations are generated from 4-methoxystyren e and 1,2-bis(4-methoxyphenyl)cyclobutane by 266-nm photoionization or by 355-nm photoinduced electron transfer using triplet chloranil as t he sensitizer. A transient with an absorption maximum at 500 nm is obs erved as a short-lived intermediate in both the radical cation mediate d 4-methoxystyrene dimerization and the cyclobutane radical cation cle avage experiments. Spectroscopic and kinetic considerations lead to th e conclusion that this transient is a substituted hexatriene radical c ation produced as an intermediate in the conversion of the 1,2-bis(4-m ethoxyphenyl)cyclobutane radical cation to the dihydronaphthalene. Obs erved rate constants for the decay of the 4-methoxystyrene radical cat ion measured at 4-methoxystyrene concentrations of 0.0001 to 0.015 M i ncrease in a linear fashion with concentration, while at concentration s >0.2 M, the observed rate constant is found to be independent of con centration. Analysis of the kinetic data according to a rate law deriv ed from a concerted [2 + 1] mechanism leads to the following rate cons tants: k(1) = 1.4 X 10(9) M(-1) s(-1) for the addition reaction, k(-1) = 8 x 10(7) s(-1) for the cycloreversion reaction, k(2) = 2.5 X 10(8) s(-1) for the rearrangement of the cyclobutane radical cation, and k( 3) = 1.5 X 10(10) M(-1) s(-1) for the reduction of the cyclobutane rad ical cation by neutral 4-methoxystyrene. Monitoring the kinetics of th e reactions of the cyclobutane radical cation gives the same values fo r the rate constants k(2) and k(-1).