New Insights on the Mechanisms of the pH-Independent Reactions of Benzo[a]pyrene 7,8-Diol 9,10-Epoxides

• Published in: Journal of the American Chemical Society Vol. 123; no. 28; pp. 6785 - 6791
• Main Authors: Doan, Lanxuan, Lin, Bin, Yagi, Haruhiko, Jerina, Donald M, Whalen, Dale L
• Format: Journal Article
• Language: English
• Published: American Chemical Society 18.07.2001
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja004359z

The rates and products of the reactions of (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (1) and (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (2) in water and dioxane−water mixtures have been determined over a pH range wider than that of earlier studies. This study provides additional insight on the mechanisms of the pH-independent reactions of 1 and 2. The rate profile for reaction of 1 shows acid-catalyzed hydrolysis at pH <5, a rate plateau at pH 5−9.5, a negative inflection at pH 10−11.5, and a rate increase at pH >11.5. The rate decrease between pH 10 and pH 11.5 is accompanied by a decrease in the yield of tetrols from 60% (pH 8) to 29% (pH 11.2) and is interpreted to be the result of a partial change in mechanism brought about by attack of hydroxide ion acting as a base to deprotonate a carbocation intermediate and regenerate 1 at pH >10, thus reducing the contribution of the pathway for tetrol formation in which water attacks the carbocation. The rate profile for the reaction of 2 exhibits only a single rate plateau at intermediate pH, along with increases in rate at low and high pH because of second-order reactions of 2 with H+ and HO-, respectively. The lack of a rate depression at pH >10 and the product studies for the reaction of 2 in dilute sodium azide solutions suggest that the tetrol-forming reactions of the pH-independent reaction of 2 are concerted or near-concerted.