Excited state carbon acidity of the benzylic position of dibenzannelated cycloheptatrienes

The excited state carbon acidity of the benzylic position of 5H-dibenzo(a,d) cycloheptene (5) and 5H-dibenzo(a,c) cycloheptene (6) and a number of their derivatives has been studied in the presence of a variety of bases by the measurement of kinetic isotope effects and structure-reactivity relationships. The purpose of the study was to determine the factors which control the excited state carbon acidity of 5 and 6. Previous research in this group demonstrated that both 5 and 6 exhibited enhanced carbon acidity in the excited singlet state when irradiated in the presence of a base. It has been proposed that the unprecedented facile ionization of the C-H bond of these systems in S;Primary kinetic isotope effects were measured for fluorescence quenching of 5 by H;Substitution of the 5-position of compound 5 with methyl or phenyl was found to reduce the acidity substantially. A stereoelectronic effect has been proposed to account for this decrease in reactivity. Support for such an effect on excited state carbon acidity was obtained by studying 3H-cyclohepta (2,1-a:3,4-a;Photolysis of 7-deuterio-5H-dibenzo (a,c) cycloheptene (87) revealed that in conjunction with the excited state acidity observed for 6, a competitive base catalyzed (1,3) hydrogen shift also takes place. Substitution of 6 at the 6-position with methyl and phenyl reduced the excited state acidity of this system. An enhanced rate of internal conversion has been proposed to account for this effect.;The results of this study substantially increases our understanding of carbon acid behaviour in the excited state, a process discovered only recently in this laboratory.