Excited-state carbon acids. Facile benzylic carbon-hydrogen bond heterolysis of suberene on photolysis in aqueous solution: a photogenerated cyclically conjugated eight .pi. electron carbanion
摘要:
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.
Synthesis of optically active 10,11-dihydro-5h-dibenzo[a,d]cycloheptenes
作者:Johannes Platzek、Günther Snatzke
DOI:10.1016/s0040-4020(01)87675-4
日期:1987.1
dibenzosuberone by dehydrogenation, reduction of the keto group and dicarboxylation. After resolution with cinchonine the (-)-enantiomer served as starting material for various transformation products with trans-configuration of the substituents at the 10,11-positions. Ring closure between these two groups led to the introduction of an additional homo- or heterocyclic ring. Attempted bromination of the dimethyl
Krogh, Erik; Wan, Peter, Journal of the American Chemical Society, 1992, vol. 114, # 2, p. 705 - 712
作者:Krogh, Erik、Wan, Peter
DOI:——
日期:——
Enhanced formation of 8.pi.(4n) conjugated cyclic carbanions in the excited state: first example of photochemical C-H bond heterolysis in photoexcited suberene