COMPARATIVE SOLVOLYTIC REACTIVITY OF BAY-REGION DIOL EPOXIDES DERIVED FROM DIBENZ[A,J]ANTHRACENE AND DIBENZACRIDINES

Rates and products have been measured in 1:9 dioxane-water, ionic strength 0.1 M, at 25 °C for the solvolyses of the two diastereomeric bay-region 3,4-diol 1,2-epoxides derived from trans-3,4-dihydroxy-3,4-dihydrodibenz[c,h]acridine (diol epoxide-1, in which the epoxide and the benzylic hydroxyl groups are cis to each other, and diol epoxide-2, in which these groups are trans). Comparative study of these diol epoxides and their carbocyclic analogues, the dibenz[a,J]anthracene 3,4-diol 1,2-epoxides, showed that the nitrogen atom at position 14 in the ring had only a small influence (2-5-fold deceleration) on the rate constants for either acid-catalyzed (kH) or pH-independent (k0) solvolysis. In these diol epoxides, the nitrogen is not in direct conjugation with the presumed carbocationic intermediate formed at C-1. These aza- and carbocyclic diol epoxides also closely resembled each other in their product distributions upon acid-catalyzed and pH-independent solvolysis. The use of a rapid-mixing technique permitted measurement of solvolysis rates for the dibenz[c,h]acridine diol epoxides at pH values down to ca. 1.2 (t1/2 200-500 ms). At these low pH values, a slight curvature in the plots of log kobsd vs pH was observed, which was consistent with only partial protonation of the nitrogen at the lowest pH values used. From the kinetic data, pka values of 0.8-1.2 were estimated for the protonated N-14 in the dibenz[c,h]acridine diol epoxides. In contrast, p values of 2.5-3.5 were measured for tetrahydro derivatives of dibenz[c,h]acridine that have hydroxyl substituents or are unsubstituted on the tetrahydro benzo ring in the bay region. Dibenz[a,J]acridine 3,4-diol 1,2-epoxide-2, which has a nitrogen at position 7, undergoes hydronium ion catalyzed solvolysis (kH) about 23 times more slowly than does the corresponding carbocyclic dibenzjfljjanthracene diol epoxide. For the dibenz[a,j]acridine 3,4-diol 1,2-epoxide, unlike the diol epoxides derived from dibenz[c,h]acridine, one resonance contributing form of the benzylic C-1 carbocation associated with solvolysis places positive charge on nitrogen. Thus, stability of the transition state for solvolysis of the dibenz [a,j] acridine diol epoxide is decreased. At pH values <2, dibenz[a,j]acridine diol epoxide-2 exhibits reactivity consistent with a mechanism for solvolysis that involves protonation on oxygen of the already N-protonated substrate. This observation constitutes, to our knowledge, the first example of such a dicationic mechanism for solvolysis of a heterocyclic epoxide derivative of this type. The pka for this diol epoxide protonated at N-7 is 4.42 as compared with 5.00 for the corresponding tetrahydro 3,4-diol; thus, the effect of a bay-region epoxide group on the pka of this nitrogen is much less than the analogous effect on the bay-region N-14 in the dibenz[c,h]acridine derivatives. © 1990, American Chemical Society. All rights reserved.