J . A m . Chem. SOC.1988, 1I O , 5068-5074
5068
Kinetics and Mechanism of Solvolysis of Precocene I 3,4-0xide: Change in Rate-Determining Step and Trapping of an Intermediate in an Epoxide Solvolysis Reaction J. M. Sayer,* S. J. Grossman, K. S. Adusei-Poku, and D. M. Jerina Contribution from the Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, The National Institutes of Health, Bethesda, Maryland 20892. Received August 31, 1987
Abstract: Solvolysis reactions of most highly reactive benzylic epoxides studied to date exhibit simple biphasic pH-rate profiles consisting of a hydronium ion dependent (kH)and a pH-independent (k,) region. In contrast, the pH-rate profile (extrapolated to zero buffer concentration) for the solvolysis of precocene I 3,4-oxide (3,4-dihydro-3,4-epoxy-7-methoxy-2,2-dimethyl-2~benzo[b]pyran) to diols at 25 O C in 1:9 dioxane-water, at ionic strength 0.1 M (NaCIO,), is more complex. Besides the transition from k , to k , at pH ca. 7, there is a break at pH ca. 9, such that the rate constants observed above this pH fall below those predicted from the pH-independent rate constants determined at lower pH values. Furthermore, the observed rate constants for general acid catalysis of the solvolysis by buffers at pH 8-9.3 show a nonlinear dependence on buffer concentration. Both these observations are consistent with a stepwise mechanism with a change in rate-determining step from formation of a carbocation intermediate (at low pH and low buffer concentrations) to capture of the carbocation by solvent (at higher pH and high buffer concentrations). In the presence of 0 . 2 M acethydrazide, CH3C(0)NHNH2,at pH 8.5, approximately 80% of the reaction products corresponded to acethydrazide adducts of the epoxide, although the rate of reaction was increased by less than 10% by the added nucleophile. This is consistent with the trapping of an intermediate subsequent to the rate-determining step. Products of the reaction at low pH are the 3,4-diols resulting from addition of water trans and cis to the epoxide oxygen, whereas at higher pH a substantial fraction (20-75%) of the products corresponds to a ketone produced presumably by a hydride ion shift. On the basis of kinetic and product data, we propose that this ketone is formed from the epoxide by a process that does not share a common intermediate (or pair of intermediates in rapid protonic equilibrium) with the stepwise formation of diols. The kinetic deuterium isotope effect for the migrating hydrogen is estimated to be ca. 4.0.
Precocene I (7-methoxy-2,2-dimethyl-2H-benzo[b]pyran), a constituent in plants of the Ageratum species,] induces precocious metamorphosis in insects2 and is hepatotoxic in rats.3 The structurally related analogue precocene I1 (6,7-dimethoxy-2,2dimethyl-2H-benzo[b]pyran)has also been shown to be hepatotoxic4 and n e p h r o t ~ x i c . ~Enzymatic activation of precocenes I and I1 to give reactive 3,4-oxides has been proposed as a pathway for the formation of diol metabolites in insects,6 in rat liver microsomes in vitr0~3~ and in rats in vivo.' Through the covalent modification of cellular macromolecules, these oxides are thought to be the agents responsible for the antiallatotropic8 and hepatotoxic9 effects of precocene I. Our interest in the chemistry of the precocene oxides initially arose from their analogy to epoxides that are implicated as ultimate carcinogens metabolically generated from tumorigenic polycyclic aromatic hydrocarbons.I0 A previous study" established the extremely high solvolytic reactivity of precocene I 3,4-oxide, 1,
but contained insufficient kinetic information to permit detailed mechanistic interpretation. Consequently, we have undertaken a more complete kinetic and mechanistic investigation of precocene I 3,4-oxide, under carefully controlled conditions similar to those employed in previous studiesI2 of epoxides derived from the polycyclic aromatic hydrocarbons. By analogy with other epoxides12we expected to observe a simple biphasic pH vs rate profile a t zero buffer concentration, consisting of a hydronium ion dependent and a pH-independent region. However, the p H vs rate profile for the solvolysis of 1 consists of four segments rather than two and exhibits a negative break at p H ca. 9, such that the rate constants observed above this p H fall below those predicted from the pH-independent rate constants determined at p H ca. 7-8.5. In addition, rate constants for catalysis of the solvolysis by buffers show a nonlinear dependence on buffer concentration. We ascribe these observations to a change in rate-limiting step from epoxide ring opening (at low p H and low buffer concentrations) to capture of the resultant carbocation (at higher p H or high buffer concentrations). To our knowledge, this represents the first instance in which such a pH-dependent change in rate-determining step has been observed kinetically in an epoxide solvolysis reaction.
'""9 (1) Kasturi, T. R.; Manithomas,
T.Indian J . Chem. 1967, 11, 91-95. (2) Bowers, W. S.; Ohta, T.; Cleere, J. S.; Marsella, P. A Science (Washington, D.C.)1976, 193, 542-547. (3) Halpin, R. A.; Vyas, K. P.; El-Naggar, S. F.; Jerina, D.M. Chem.-Biol. Interact. 1984, 48, 297-31 5. (4) Hsia, M. T. S.; Grossman, S.; Schrankel, K. R. Chem.-Biol. Inreracr. 1981, 37, 265-277. (5) Schrankel, K. R.; Grossman, S. J.; Hsia, M. T. S. Toxic01 Lett. 1982, 12. 95-100 (6) Burt, M. E.; Kuhr, R. J.; Bowers, W. S. Pestic. Biochem. Physiol. 1978, 9, 300-303. (7) Grossman, S. J.; Hsia, M. T. S. Toxicology 1982, 25. 293-298. (8) Pratt, G . E.; Jennings, R. C.; Hamnett, A. F.; Brooks. G. T. Narure (London) 1980, 284, 320-323. (9) Ravindranath, V.; Boyd, M . R.; Jerina, D.M. Biochem. Pharmacol. 1987, 36, 441-446. (IO) Jerina, D. M.; Sayer, J. M.; Agarwal, S. K.; Yagi, H.; Levin, W.; Wood, A. W.; Conney, A. H.; Pruess-Schwartz, D.:Baird, W. M.; Pigott, M. A.; Dipple, A. In Biological Reactiue Intermediates III; Kocsis, J. J., Jollow. D. J., Witmer, C. M.,Nelson, J. O., Snyder, R., Eds.; Plenum: New York, 1986; pp 11-30, ( 1 I ) Hamnett, A. F.; Ottridge, A. P.; Pratt. G. E.; Jennings. R. C.; Stott. K. M . Pestic. Sri. 1981, 12, 245-254. ~
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Results and Discussion Effect of pH and Buffer Concentration on Rate. Figure 1 (lower line and solid symbols) shows the p H dependence of the pseudo-first-order rate constants (kobsdo).in the absence of buffer catalysis or extrapolated to zero buffer concentration, for solvolysis of 1 measured a t 25 O C and ionic strength 0.1 M (NaC10,) in I :9 (v/v) dioxane-water. Unlike many epoxide solvolysis reactions,12 which exhibit simple pH-rate profiles corresponding to a rate law, kobsdo= kHaH++ k,, the solvolysis of 1 exhibits a pH-rate profile with a negative break at p H 9-10, such that the observed rate constants at p H values >9 are smaller than those predicted by extrapolation of the line corresponding to the pHindependent rate constants observed at p H 7-8.5. The difference (12) Whalen, D. L.; Montemarano, J. A.; Thakker, D. R.; Yagi, H.; Jerina, D. M. J . Am. Chem. Sot. 1977, 99, 5522-5524. Sayer, J. M.; Whalen, D. L.; Friedman, S. L.; Paik, A.; Yagi, H.; Vyas, K. P.; Jerina, D. M. Ibid. 1984, 106, 226-233. Yagi, H.; Sayer, J. M,;Thakker. D. R.; Levin, W.; Jerina, D. 41.Ibid. 1987, 109, 838-846.
This article not subject to U S . Copyright. Published 1988 by the American Chemical Society
J . Am. Chem. SOC.,Vol. 110, No. 15, 1988 5069
Solvolysis of Precocene I Oxide I
\
0.075
0.000
I
t
'
0.000
0.050
PH
0,100
M
[Buffer],
Figure 1. Dependence on pH of the observed pseudo-first-order rate constants (kowo,solid symbols) for solvolysis of 1, measured in 1 m M buffers (pH 210.5). or obtained by extrapolation to zero buffer of data obtained at several low buffer concentrations (pH
