COMPARATIVE CHEMISTRY OF THE BAY-REGION AND NON-BAY-REGION TETRAHYDRO EPOXIDES OF PHENANTHRENE

It has been previously proposed that the great mutagenic and carcinogenic activity of the tetrahydro epoxides of a number of polycyclic aromatic hydrocarbons, in which the epoxide moiety resides at a “bay region“, is due to the greater stability of the bay region benzylic carbonium ion formed upon oxirane ring opening. The rate constants for specific-acid-, generalacid-, and water-catalyzed oxirane ring opening of the bay (I) and nonbay (II) epoxides of 1, 2, 3, 4-tetrahydrophenanthrene are compared. Also compared are the second-order rate constants for nucleophilic attack upon I and II. With few notable exceptions the various second-order rate constants for the various reactions of I and 11 were found to be quite similar with the rate constants for I exceeding those of II by an approximate average of threefold. Similar rate ratios are seen in the reactions of the bay and nonbay phenanthrene arene oxides wherein the bay benzylic carbonium ion stability cannot be a factor. A notable exception is the general-acid-catalyzed oxirane ring opening by H3PO4, where the second-order rate constant for I exceeds that for 11 by 40-fold. Brønsted a (-0.7) and βnuc(+0.25) values obtained with 1 and II are indistinguishable. Also similar are the values of ΔS≠ and kinetic solvent isotope effects (kH/kD2) for H2O-and H3O+-catalyzed oxirane ring opening. The values of ΔS≠ and kH/kD2for H3O+-mediated solvolysis are as expected for a specific acid mechanism. Nevertheless, the values of the log kHfit accurately to the Brønsted plots for general acid catalysis. Product studies show a mixture of cis and trans diols to be formed at low and intermediate pHs, while only trans diols are formed at high pH. These results are discussed. © 1979, American Chemical Society. All rights reserved.