EXCITED-STATE CARBON ACIDS - FACILE BENZYLIC C-H BOND HETEROLYSIS OF SUBERENE ON PHOTOLYSIS IN AQUEOUS-SOLUTION - A PHOTOGENERATED CYCLICALLY CONJUGATED 8 PI-ELECTRON CARBANION

The possibility of excited-state carbon acid behavior of the dibenzylic protons of several dibenzannelated systems related to suberene (1) has been investigated in aqueous solution and in several organic solvents. It was found that only suberene (1) displayed observable excited-state carbon acid behavior. Thus photolysis of 1 in D2O-CH3CN solutions resulted in facile exchange of the benzylic protons with deuterium from D2O. Extended photolysis resulted in incorporation of two deuteriums at the benzylic position. Quantum yields for initial deuterium incorporation are in the range 0.02-0.03 for 1 in D2O-CH3CN solutions. Deuterium exchange from the benzylic position with solvent protons was also observed on photolysis of 5,5-dideuterio-5H-dibenzo[a,d]cycloheptene (2), where each deuterium atom was exchanged with solvent H2O in a sequential manner. Fluorescence quenching by water (in CH3CN solution) of the exchanging suberene systems (1 and 2) gave linear Stern-Volmer plots, with k(q) = (1.68 +/- 0.08) x 10(8) M-1 s-1 for 1 and (0.61 +/- 0.06) x 10(8) M-1 s-1 for 2, which corresponds to an isotope effect for quenching by water, (k(H)/k(D))q = 2.8 +/- 0.4. Fluorescence titration of 1 and 11 as a function of medium acidity gave an estimate of pK(S1) = -1 for these carbon acids. This compares to their estimated pK(S0) = 31-38.