Mechanistic studies on the photoaddition reaction of conjugated polialkynes with olefins

Abstract

The photolysis of 3-phenyl-4-phenylethynylcyclobut-3-en-trails-1,2-dicarboxylic acid dimethyl ester (PPCB) in methanol yields 1:1 photoadducts (la-Id, 2a, and 2b) and an acetylene reduction product (3). The major products are 2a and 2b under neutral conditions, whereas la and lb are the major products under acidic conditions and/or in the presence of electron donors, such as 1,4-dimethoxybenzene. The fluorescence of PPCB decreases as the concentration of sulfuric acid is increased. Azulene quenching studies suggest that la-2b are formed from the singlet excited state, while the triplet excited state yields 3. The formation of the photoadducts la-1d can be correlated with the atomic charge of excited PPCB. Photolysis of 1,3-diphenyldiazopropyne (DPP-N2) in methanol and olefins was carried out as model reactions of substituted propargylenes. Major products are OH insertion product (4) in methanol and cyclopropane ring compounds in olefins (5-7). No difference was observed when the reaction is sensitized with benzophenone indicating that the reaction proceeds in the singlet excited state. The energy gap between the higher lying singlet state and the ground triplet state is very small and intersystem crossing between the two states is very fast. No compound containing epoxy ring was detected from the photolysis of DPP-N2 in methyl acrylate, and attempt to trap DPP with acetone is failed. These results show that the singlet state carbene does not react with C=0 double bond but reacts with C=C double bond. To compare the reactivity of methanol and olefins with DPP, DPP-N2 was photolyzed in the mixture of methanol and olefins. The reactivities of methanol and olefins to DPP are in the order of methyl acrylate (MA) acrylonitrile (AN) >> methanol > 2,3-dimethyl-2-butene (DMB) suggesting that the singlet state of DPP has nucleophilic character. Methanol quenching of photoreaction of DPB-DMB yields compound 8. To verify the participation of cyclobutene from the reaction o...