FLASH-PHOTOLYSIS OF ALPHA-DIAZONAPHTHOQUINONES IN AQUEOUS-SOLUTION - DETERMINATION OF RATES AND EQUILIBRIA FOR KETO-ENOL TAUTOMERIZATION OF 1-INDENE-3-CARBOXYLIC ACID

Flash photolysis of either 1-diazo-2(1H)naphthalenone (1a) or 2-diazo-1(2H)naphthalenone (1b) generates benzofulven-8-one (2). Hydrolysis of ketene 2 forms benzofulvene-8,8-diol (3), the enol tautomer of indene-3-carboxylic acid (4), pH rate profiles for the reactions 2 --> 3 and 3 --> 4 were determined in aqueous solution. Ketonization of 3 is catalyzed by acid and by base. Catalysis by protons saturates in strongly acidic solutions, thereby defining the first ionization constant of the enol, pK(a)(E) = 1.90 +/- 0.05, catalysis by hydroxyl ions saturates in dilute base, defining the second ionization constant, pK(a)'(E) = 8.3 +/- 0.2. The first (OH) and second (CH) ionization constants of 4 were determined by spectrophotometric titration, pK(a)(K) = 4.50 +/- 0.03 and pK(a)'(K) = 15.2 +/- 0.2. Two independent estimates of the enolization constants of 4 and 4-, the first based on thermodynamic cycles, the second on the ratio of enolization and ketonization rates, were combined to give pK(E) = 9.3 +/- 0.3, pK'(E) = 6.6 +/- 0.3. Ketene 2 is formed by irradiation of 1-bromo-2-naphthol at 12 K in an argon matrix, but neither it nor its isomer 2-bromo-1-naphthol were suitable for the generation and observation of 2 and 3 by flash photolysis in aqueous solution.