Photolysis of 3-Phenyl-4-phenylethynylcyclobut-3-en-$trans$-1,2-dicarboxylic Acid Dimethyl Ester in Methanol

Abstract

The photolysis of 3-phenyl-4-phenylethynylcyclobut-3-en-trans-1,2-dicarboxylic acid dimethyl ester (PPCB) in methanol yields 1:I adducts (la-ld, 2a and 2b) and an acetylene reduction product (3). The major products are 2a and 2b under neutral conditions, whereas la and Ib are the major products under acidic conditions and/or in the presence of electron donors, such as I,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 la-ld can be correlated with the atomic charge of excited PPCB, suggesting an exciplex and a carbocation, rather than carbenoid, intermediate in the reaction.