Photodecarboxylation study of carboxy-substituted N-alkylphthalimides in aqueous solution:: Time resolved UV-vis spectroscopy and conductometry

The photophysical and photochemical properties of N-phthaloyltranexamic acid (3b) and N-methylsulfanyl-methylphthalimide (4e) as well as several Sulfur-containing carboxylates, N-phthaloylmethionine (1), N-phthaloyl-S-carboxymethylcysteine (2c). phthalimidomethylsulfanyl acetic acid (4a), and its derivatives 4b-4d were studied in aqueous solution by time-resolved UV-vis spectroscopy and conductivity using laser pulses at 248 or 308 nm. From the changes in absorption spectra upon continuous irradiation at 290 nin, the quantum yields of substrate decomposition (Phi(d)) were measured for various cases, The quantum yields of formation of the lowest triplet state (Phi(isc)) are small, when Phi(d) is large and vice versa. The quantum yields of decarboxylation, Phi(-CO2), were determined by conductometry. Phi(-CO2) is largest (0.3) for 3b in argon-saturated aqueous solution at pH 6 - 10 and decreases in the order 4a - c, 2c, 1, and 4d. The increase of Phi(d) vs pH indicates that carboxylic acids exhibit low decarboxylation efficiencies, whereas the corresponding anions effectively eliminate CO, and subsequently cyclize. T e mechanisms of photochemical decomposition, decarboxylation, and cyclization are discussed.