SOLVOLYSIS OF K-REGION ARENE OXIDES - SUBSTITUENT EFFECTS ON REACTIONS OF BENZ[A]ANTHRACENE 5,6-OXIDE

The solvolytic reactivity and products formed from benz[a]anthracene 5,6-oxide (BA-O) on substitution of a methyl group at positions 1 (1-MBA-O), 4 (4-MBA-O), 7 (7-MBA-O), 11 (11-MBA-O), and 12 (12-MBA-O), on 7,12-dimethyl substitution (7,12-DMBA-O), and on 7-bromo substitution in 1:9 dioxane-water and in methanol at 25-degrees-C are reported. These substitutions result in > 150-fold differences in their rates of acid-catalyzed solvolysis and cause marked changes in the distribution of solvent adducts and phenols resulting from isomerization. Optically pure BA-O, 7-MBA-O, 12-MBA-O, and 7,12-DMBA-O as well as their optically pure trans dihydrodiols were utilized to determine the point of attack by water in the hydrolysis reactions. In general, the reactions in aqueous dioxane (0.1 M NaClO4) obeyed the rate equation k(obsd) = k(H)[H+] + k0, where k(H) is the second-order rate constant for acid-catalyzed reaction and k0 is the first-order rate constant for spontaneous reaction, to provide biphasic pH-rate profiles. When ionic strength was maintained with 0.5 M KCl, however, more complex pH-rate profiles were observed for some of the arene oxides due to attack of chloride on the neutral epoxide to produce steady-state concentrations of chlorohydrins. Rate enhancement on methyl substitution is largest (k(H), ca. 5-fold) when the methyl group is present in the hindered bay region (C1 or C-12) or adjacent to the epoxide at C-7. The combined effect of two methyl groups (7,12-DMBA-O) is additive (ca. 25-fold). Theoretical calculations (molecular mechanics by PCMODEL-PI and ab initio by GAUSSIAN 86 and 88 programs) of carbocation stability indicate the importance of steric factors in determining relative reactivity and types of products formed from substituted benz[a]anthracene 5,6-oxides.