Flash photolytic investigation of 4-diazoisothiochroman-3-one in aqueous solution: Observation of a short-lived carboxylic acid enol

4-Diazoisothisochroman-3-one was found to undergo a photo-Wolff reaction up on flash photolysis in aqueous solution, generating 1,3-dihydrobenzo[c]thio phen-1-ylideneketene, whose hydration produced a second, readily observable short-lived species. The latter, through the shape of its decay rate profi le plus solvent isotope effects, was identified as the enol of 1,3-dihdyrob enzo[c]thiophene-1-carboxylic acid. This behavior contrasts with that repor ted previously far the acyclic analogue of the presently studied system: fl ash photolysis of S-methyl phenyldiazothioacetate gives a photo-Wolff react ion as well, but the ketene so produced undergoes hydration 3 orders of mag nitude more slowly than its cyclic counterpart; this makes ketene hydration in the acylic system Blower than enol ketonization, and that renders the a cylic enol a low-concentration, nonobservable intermediate. Analysis of the rate profile for ketonization of the presently examined cyclic enol provid es enol acidity constants as well as rate constants for ketonization of eno l and enolate species.