PHOTOGENERATION OF ANTIAROMATIC 8-PI XANTHENIDE AND THIOXANTHENIDE (PI-EXCESSIVE) CARBANIONS VIA EXCITED-STATE CARBON ACID DEPROTONATION AND PHOTODECARBOXYLATION

The relatively facile generation of formally ground-state antiaromatic 8 pi xanthenide and thioxanthenide carbanions via excited-state C-H b ond deprotonation and photodecarboxylation is reported. The aim of thi s work is to study the effect of heteroatoms (O and S) on the photogen eration of cyclically conjugated 4n pi-electron carbanion intermediate s and comparing it with the corresponding photoreactions which generat e the isoelectronic dibenzosuberenyl carbanion 23 from 22, the first r eported excited-state carbon acid. Quantum yields of proton incorporat ion in 1 and 4 were measured in aqueous ethanol and in CH3CN using NaO H and ethanolamine as deprotonating bases, respectively. Primary kinet ic isotope effect for C-H bond cleavage was measured by fluorescence q uenching of 1 and 3 using ethanolamine as base (in CH3CN) and found to be k(H)/k(D) = 2. The fluorescence quenching rate of 3 by ethanolamin e, k(q) = 2.5 x 10(7) M-1 s(-1), is two orders of magnitude smaller th an that for dibenzosuberene (22) indicating that the 8 pi xanthenide ( and thioxanthenide) carbanion is not as readily photogenerated as the isoelectronic dibenzosuberenyl carbanion (23). Estimated photodecarbox ylation rate constants for xanthene-9-carboxylic acids 11 and 17 are a n order of magnitude less than the corresponding rate constant for sub erene-5-carboxylic acid (24), consistent with the lower rate (and poss ibly reduced driving force) for carbanion photogeneration for these is oelectronic heteroatom systems. (C) 1998 Elsevier Science S.A.