| Popis: |
The photolysis of 3-phenyl-4-phenylethynylcyclobut-3-en- trans -1,2-dicarboxylic acid dimethylester (PPCB) in methanol yields 1:1 adducts ( 1a–1d, 2a and 2b ) and an acetylene reduction product ( 3 ). The major products are 2a and 2b under neutral conditions, whereas 1a and 1b 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 sulphuric acid is increased. Azulene quenching studies suggest that 1a–2b are formed from the singlet excited state, whereas the triplet excited state yields 3 . The formation of the photoadducts 1a–1d can be correlated with the atomic charge of excited PPCB, suggesting an exciplex and a carbocation, rather than carbenoid, intermediate in the reaction. |
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