PHOTODECARBOXYLATION OF DIARYLACETIC ACIDS IN AQUEOUS-SOLUTION - ENHANCED PHOTOGENERATION OF CYCLICALLY CONJUGATED 8 PI-ELECTRON CARBANIONS

The photodecarboxylation of a series of diarylacetic acids 1-6 has been studied in aqueous solution. The mechanism of these photodecarboxylations is shown to involve C-C bond heterolysis from the singlet excited state, giving rise to carbanion intermediates. Quantum yields for photodecarboxylation (PHI(d)) are reported at several pH's. Fluorescence quantum yields and lifetimes are reported as a function of pH, and their behavior is consistent with reaction via the carboxylate ions. Rate constants of photodecarboxylation (k(dc)) are estimated and show a marked dependence on the number of pi-electrons in the internal cyclic array (ICA). The most reactive compound was suberene-5-carboxylic acid (1) (PHI(d) = 0.60 +/- 0.05; k(dc) almost-equal-to 6 x 10(9) s-1), which gives a carbanion intermediate of eight pi (4n) electrons in the ICA. The least reactive system was fluorene-9-carboxylic acid (4) (PHI(d) = 0.042 +/- 0.004; k(dc) = 8.8 x 10(6) s-1), which on decarboxylation gives rise to a carbanion with an ICA of six pi (4n + 2) electrons. The observed reactivity trend in the excited state is the reverse of that observed in the ground state and appears to demonstrate further the utility of the ''4n rule'' for predicting the relative reactivity of photochemical reactions giving rise to cyclically conjugated, charged intermediates. The possible theoretical significance of these observations is discussed.