The photooxygenation of electron-rich 1,2-diarylcyclopropanes such as 1,2-bis(4-methoxyphenyl)cyclopropane and 1-(4-methoxyphenyl)-2-phenylcyclopropane in the presence of 9,10-dicyanoanthracene(DCA) as a sensitizer in acetonitrile affords trans- and cis-3,5-diaryl-1,2-dioxolanes in high yields. This photooxygenation is initiated by the electrontransfer from 1,2-diarylcyclopropanes to the excited singlet
The efficiencies of the 9,10-dicyanoanthracene (DCA)-sensitized cis-transphotoisomerization and photooxygenation of cis- and trans-1,2-bis(4-methoxyphenyl)cyclopropanes (c-CP and t-CP) in CH3CN were remarkably enhanced by the addition of inorganic salts such as LiBF4 and Mg(ClO4)2. The mechanistic implication of the salt effects on these photoreactions was discussed.
Generation of Polyphenylene Radical Cations and Their Cosensitization Ability in the 9,10-Dicyanoanthracene-Sensitized Photochemical Chain Reactions of 1,2-Bis(4-methoxyphenyl)cyclopropane
Cosensitization effects of polyphenylene compounds (PP) such as biphenyl (BP) terphenyls (o-, m-, p-TP), and phenanthrene (Phen) in photoinduced electron-transfer reactions were examined. The 9,10-dicyanoanthracene (DCA)-sensitized cis-trans photoisomerization of 1,2-bis(4-methoxyphenyl)-cyclopropane (CP), which proceeds in a chain reaction via free radical cation of CP (CP.+) as a chain carrier, was accelerated by adding PP, particularly by TP. A similar accelerating effect was observed in the DCA-sensitized photooxygenation of CP as another example. BP and TP were more stable under the oxygenation condition than phenanthrene and naphthalene, which also accelerate the photooxygenation CP. CP.+ is generated by the direct electron transfer from CP to the excited singlet state of DCA ((1)DCA*) and also by the secondary electron transfer from CP to PP.+, which is generated by the primary electron transfer from PP to (1)DCA* The laser flash photolysis study revealed that the quantum yield for the formation of free CP.+ in the direct electron transfer from CP to (1)DCA* (Phi(CP.+) approximate to 0.1) was smaller than that in the presence of PP. This is due to the high yield of free PP.+ generation by the primary electron transfer and the efficient secondary electron transfer from CP to PP.+. The secondary electron transfers were found to take place in nearly diffusion-controlled rates (0.9-1.5 x 10(10) M-1 s(-1)). The high yield of PP.+ as free radical ions does not seem to be the sole factor of the cosensitization of PP for the DCA-sensitized photoreactions of CP. The ratio of the quantum yields of the photoreactions to that of the initial CP.+ formation (turnover) also increased by the addition of PP from 3 (isomerization) and 15 (oxygenation) to 32-90 for both reactions. The second-order rate constant for the decay of CP.+ in aerated acetonitrile was decreased by a factor of 0.5-0.8 by the addition of PP. We concluded that the cosensitization effect in the photoreaction involves pi-complex formation between CP.+ and PP assisting the chain reaction as well as initial CP.+ formation.