Conformational Effects in the Hydrolyses of Benzo-Ring Diol Epoxides That Have Bay-Region Diol Groups

Article abstract of DOI:10.1021/ja00313a044

Kinetics of the hydronium ion catalyzed(K_H) and pH-independent (k₀) hydrolyses of several benzo-ring diol epoxides, derived from polycyclic aromatic hydrocarbons, that possess bay-region trans diol groups have been investigated in 1:9 dioxane-water at 25 deg C.These epoxides are 1,2,3,4-tetrahydrobenzanthracene-1,2-diol 3,4-epoxide, 1,2,3,4-tetrahydrotriphenylene-1,2-diol 3,4-epoxide, and 9,10,11,12-tetrahydrobenzopyrene-9,10-diol 11,12-epoxide.Both possible diastereomers of the diol epoxides in which the two hydroxyl groups are trans to each other were investigated: isomer 1, in which the benzylic hydroxyl group is cis to the epoxide oxigen, and isomer 2, in which this hydroxyl group and the epoxide oxygen are trans.The corresponding tetrahydro epoxides that lack a diol group were also investigated.For comparison, isomers 1 and 2 of 1,2,3,4-tetrahydrobenzanthracene-3,4-diol 1,2-epoxide, a bay-region epoxide with a non-bay-region trans diol group, and the corresponding benzanthracene tetrahydro epoxide were also studied.The most striking feature of the reactions of the diol epoxides that possess a bay-region diol group is a reversal of the relative reactivity of isomers 1 and 2 in the k₀ reaction, when compared with diol epoxides whose diol group is not in a bay region.This reversal of reactivity, which causes k₀ to be larger for isomer 2 than for isomer 1 when the diol is in a bay region, is explained by changes in conformational equilibria involving the cyclohexene ring due to steric crowding of the bay-region diol.Preference of this diol group for a pseudodiaxial conformation favors a conformation of isomer 2 which the benzylic C-O bond of the epoxide is more or less aligned with the ? orbitals of the aromatic system and strongly disfavors this aligned conformation of isomer 1.Reaction via the k₀ process is faster for the aligned than for the nonaligned conformer; thus for epoxides with bay-region diol groups, k₀ for isomer 2 is faster than k₀ for isomer 1.The pH-independent reaction of isomer 2 of 1,2,3,4-tetrahydrobenzanthracene-1,2-diol 3,4-epoxide via the aligned conformer gives, in addition to cis and trans tetraol products, substantial quantitites of a keto diol, whereas no keto diol was detected from the corresponding isomer 1.This also represents a reversal of the pattern of product formation generally observed with diol epoxides that lack a bay region in the vicinity of the diol group.Rate constants for hydronium ion catalyzed hydrolysis (k_H) are much less sensitive to conformational factors than k₀.The distribution of cis and trans tetraol products from hydronium ion catalyzed hydrolysis of the 1,2,3,4-tetrahydrobenzanthracene-1,2-diol 3,4-epoxides and the 1,2,3,4-tetrahydrotriphenylene-1,2-diol 3,4-epoxides can be explained by preferential pseudoaxial attack of water upon the benzylic cations formed from these epoxides.On the basis of these observations and previous findings with non-bay-region diol, bay-region epoxides, rules for predicting the effects of conformation on rates...