Nanosecond and Picosecond Dynamics of the Radical Cation Mediated Dimerization of 4-Methoxystyrene
摘要:
The addition of the 4-methoxystyrene radical cation to neutral 4-methoxystyrene and the cleavage reactions of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation in acetonitrile have been studied by product analysis and by nanosecond and picosecond transient absorption spectroscopy. The main product upon radical cation mediated dimerization of 4-methoxystyrene using chloranil as the electron transfer sensitizer is a substituted dihydronaphthalene, 1,2-dihydro-7-methoxy-1-(4'-methoxyphenyl)naphthalene, at low concentrations of 4-methoxystyrene. At higher concentrations, the main product is 1,2-bis(4-methoxyphenyl)cyclobutane. Cleavage of the cyclobutane radical cation is found to give 4-methoxystyrene and the dihydronaphthalene in a 1:3 ratio. In the time-resolved experiments, the radical cations are generated from 4-methoxystyrene and 1,2-bis(4-methoxyphenyl)cyclobutane by 266-nm photoionization or by 355-nm photoinduced electron transfer using triplet chloranil as the sensitizer. A transient with an absorption maximum at 500 nm is observed as a short-lived intermediate in both the radical cation mediated 4-methoxystyrene dimerization and the cyclobutane radical cation cleavage experiments. Spectroscopic and kinetic considerations lead to the conclusion that this transient is a substituted hexatriene radical cation produced as an intermediate in the conversion of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation to the dihydronaphthalene. Observed rate constants for the decay of the 4-methoxystyrene radical cation measured at 4-methoxystyrene concentrations of 0.0001 to 0.015 M increase in a linear fashion with concentration, while at concentrations >0.2 M, the observed rate constant is found to be independent of concentration. Analysis of the kinetic data according to a rate law derived from a concerted [2 + 1] mechanism leads to the following rate constants: 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 radical cation by neutral 4-methoxystyrene. Monitoring the kinetics of the reactions of the cyclobutane radical cation gives the same values for the rate constants k(2) and k(-1).
Photocatalytic hydrogen-evolution dimerization of styrenes to synthesize 1,2-dihydro-1-arylnaphthalene derivatives using Acr+-Mes and cobaloxime catalysts
hydrogen-evolution dimerization of styrenes via the synergistic merger of Acr+-Mes photocatalyst and cobaloxime proton reduction catalysts. By utilizing this dual catalyst system, 1,2-dihydro-1-arylnaphthalene derivatives can be directly constructed from commercially available styrenes. Our reaction proceeds smoothly under mild conditions without the need for oxidants or hydrogen atom transfer reagents, and
Novel Cerium(IV) ammonium nitrate induced dimerization of methoxystyrenes
作者:Vijay Nair、Jessy Mathew、Puthuparampil P. Kanakamma、Sreeletha B. Panicker、V. Sheeba、S. Zeena、Guenter K. Eigendorf
DOI:10.1016/s0040-4039(97)00279-7
日期:1997.3
Methoxystyrenes undergo facile dimerization when heated with Cerium (IV) ammonium nitrate in methanol; cyclized products are obtained in acetonitrile. (C) 1997 Elsevier Science Ltd.