TRAPPING OF A CARBOCATIONIC INTERMEDIATE IN THE SPONTANEOUS HYDROLYSIS REACTION OF 7-BETA,8-ALPHA-DIHYDROXY-9-BETA,10-BETA-EPOXY-7,8,9,10-TETRAHYDROBENZO[A]PYRENE - MECHANISM OF THE SPONTANEOUS AND GENERAL ACID-CATALYZED HYDROLYSIS REACTIONS OF BAY-REGION BENZO[A]PYRENE 7,8-DIOL 9,10-EPOXIDES

The hydrolysis reactions of racemic 7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[o] pyrene (DE-1) and racemic 7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (DE-2) in 1:9 dioxane-water solutions are catalyzed by a series of general acids consisting of C12CHPO3H-, ClCH2PO3H-, H2PO4-, and C2H5PO3H-. For the hydrolysis of DE-1 catalyzed by H3O+, H2O, and the above series of general acids, a plot of log KHA vs pKa gave a Bronsted a of 0.39. A similar Bronsted plot for the hydrolysis of DE-2 catalyzed by H3O+, C12CHPO3H-, ClCH2PO3H-, H2PO4-, and C2H5PO3H- gave an α of 0.40. It is concluded that the mechanism of the hydrolyses of both DE-1 and DE-2catalyzed by the above general acids with pKa's <ca 8, including H3O+, must occur by concerted proton transfer and benzyl C-O bondcleavage to yield carbocation intermediates. Dipolar intermediates are ruled out. An intermediate in the spontaneous reaction of DE-1 was trapped, subsequent to its rate-limiting formation, by azide and TV-acetylcysteine anions. It is proposed that the rate-limiting step for the spontaneous reaction of DE-1 is formation of a benzylic carbocation intermediate, with aneutral water molecule acting as a proton donor. The rate constant for reaction of this carbocation with solvent is estimated to be 1.7 × 107 s-1. Trapping of an intermediate by azide and N-acetylcysteine anions subsequent to a rate-limiting step in the spontaneous hydrolysis of DE-2 was not detected. Possible explanations for the differences in the hydrolysis reactions of DE-1 and DE-2 are given.© 1990, American Chemical Society. All rights reserved.