Chem. Res. Toxicol. 1988, 1, 19-21
19
Communications ~~~
~
Formation of Reactive Intermediates [ROOOOR] from the Addition of Superoxide Ion (02*-) to CCI,, CF,CCI,, PhCCI,, PhC( O)CI, n-BuBr, and n-BuCI in Acetonitrile Sir: An earlier study (1) has shown that the combination of CC, with excess 02'in aprotic solvents yields HOC(0)O-,C1-, and O2 With primary alkyl halides the initial nucleophilic displacement is rate limiting, but the overall stoichiometry is two 02'-ions per two RX molecules via a three-step mechanism to give ROOR, X-, and O2 as the major products (2). The general belief is that the primary step for the reaction of 02'-with organic halides (RX) yields ROO' plus X- and that ROO' is reduced by a second 02'-to ROO- via electron transfer at near diffusion-controlled rates. However, within biological and environmental matrices limiting amounts of 02'-are expected in the presence of large excesses of organic halides. Such conditions should preclude the reduction of the primary product (ROO') by a second 02'-and increase its lifetime. Because one of the favored toxic species from the ingestion of CCl, in human liver is Cl,COO' (the same species as the primary product from the CCl,/02'- reaction) (3, 4), there is a need to produce this species and characterize its stability and reactivity under controlled laboratory conditions. The present study has been undertaken to ascertain the nature of the primary intermediates from the reaction of 02'-with RX (RX = C C 4 , F3CCC1,, PhCCl,, PhC(O)Cl, BuBr, and BuC1). Here we wish to report that the primary products are Cl,COO', F3CCC1200', PhCC1200', PhC(O)OO', and BuOO' and that for millimolar concentrations these products dimerize (or are reduced by a second 0 2 7a t such a rapid rate that there is not any detectable reactivity by the peroxy radicals (ROO') with l,&cyclohexadiene (1,4-CHD). The latter is a model substrate for the polyunsaturated fatty-acid components of lipids. The combination of 02'-(5) with excess RX results in a rapid stoichiometric (1:l) reaction to give ROOR, ROO-, 02,and C1- as the major products (Table I). With PhCC1, and BuBr there is an immediate flash of color in the solution upon addition of 02'-(Amu for PhCCl,, 456 nm; for BuBr, 520 nm); the half-life of the colored species from PhCC1, is about 1 s and from BuBr is about 3 s. The lifetimes appear to be similar to the reaction times for the reactive intermediates and to have an inverse correlation with the extent of reaction. In the case of CC14a transient yellow color is seen at low temperature (-20 "C). Table I, section A, summarizes (a) the rate constants for the primary reactions of 02*with RX, (b) the conversion efficiencies of 1,4-CHD by the intermediate products of the primary reactions, and (c) the overall products from the combination of 02'with large excesses of RX and 1,CCHD. For 02'-concentrations from 20 to 50 mM there are not any combinations of 02'-/RX/1,4-CHD in MeCN that yield detectable amounts of 1,3-CHD. For each primary reactant, the conversion efficiency for 1,4-CHD to PhH is less than unity and independent of O2 concentration. In contrast the reaction of HOO' with 1,4-CHD in MeCN (6) and Me2S0 (7) has second-order rate constants of 3.5 X lo2and 1.6 X lo2M-ls-', respectively, yields approximately equimolar amounts of 1,3-CHD and PhH, and occurs via the abstraction by HOO' of an allylic hy-
4(Me4N)02(s)
+ CCI4(1)
-
Scheme I COz
+ 4(Me4N)Cl(s) f
302(9)
-
( M ~ ~ N + ) ~ - O C ( ~ ) O O * I (MB,N)02(;j ( S ) '02 (Me~N+)2C-OC(0)C(O)OO-l(s) orange
white
E S R , g , 2.010 (25 OC): IR 1650 (vs), 1321 ct?i'( w
I R 1680,1665,1640.1338. 1250,982, 6 8 6 , 643 cm-'
)
drogen atom from 1,4-CHD to give (1,S-CHD') and HOOH. The absence of significant amounts of 1,3-CHD, ROOH, and ROH in the products from the reactions of Table I indicates that ROO' plus l,CCHD, under the experimental conditions, do not react at a competitive rate (kox < lo2 M-' S-') (6-8). When 02'-is combined with excess CCl, [or an equimolar amount of PhC(O)Cl] in the presence of diphenylisobenzofuran (DPIBF) or rubrene the substrates are dioxygenated by the reaction intermediate (Table I, section B). The time domain for the reaction with DPIBF appears to be consistent with the lifetime (C0.1 s) for the transient intermediate of the 02'-/CCl, reaction, which implicates it as the source of the dioxygen for formation of dibenzoylbenzene (DBB) from DPIBF and of rubrene endoperoxide from rubrene. The short lifetime of this intermediate precludes a net reaction with 1,4-CHD (Table I, section A), which appears to have a much slower rate. These results and the absence of significant reactivity by the intermediate with cis-stilbene parallels the chemistry of '02(9, IO). There is copious emission of white light from the apparent concerted homolytic dissociation of the ROOOOR intermediate to ROOR and lo2[the specific weak emission due to (102)2 cannot be discerned]. Addition of solid (Me4N)02(5) to pure, anhydrous CCl, results in the vigorous evolution of O2 and the transformation of the pale-yellow solid into a bright-orange material. This same product species (A, 450 nm) is formed via a solution-phase reaction between (Me,N)O, and CC14 in anhydrous liquid ammonia. A similar orange material appears as a transient intermediate when 02'-is combined with excess C 0 2 in dry dimethylformamide (11). The reaction stoichiometries and physical properties of the products from the solid-liquid combination of (Me,N)02(s) and CC14(l)are summarized in Scheme I. A similar experiment (12) with KO,(s) and pure C C 4 results in chemiluminescence at 1268 nm from the production of single oxygen (lo2).Hence, the dioxygen that is produced in Scheme I probably is IO2 (via the formation of a C1,COOOOCCl, intermediate). When excess CC14 (300 mM) is combined with (Me4N)02(20 mM) in dry acetonitrile and the solution is purged with argon gas, the reaction stoichiometry is four to one with C 0 2 (collected and titrated in an aqueous Ca(OH), trap), 0 2 and , C1- the only products. The results in Table I and the preceding observations provide the basis for a reaction manifold for the 02*-/excess RX/substrate system (Scheme 11). The limits on the rate constants are consistent with the observed reactivities
0893-228~/88/2701-0019$01.50/0 0 1988 American Chemical Society
20 Chem. Res. Toxicol., Vol. 1, No. 1, 1988
Communications
Table I. Reactivity a n d Products from t h e Addition of 02’-(10 mM) t o Excess RX a n d Acetonitrile apparent 1,4-CHD/ RX/O2’-” PhHb rate constant conversion initial mol ratios RX k, M-’s-’ efficiency, % Section A RX 1,4-CHD 02*1,CCHD 1.25 x 103 1 10 0 CCI, (control) 1.25 x 103 1 10 20 0 0.68 x 103 1 50 20 8 1 50 0 0.09 x 103 42 1 0.09 x 103 50 20 1 18 n-BuBr 0.79 x 103 50 20 1 I x 103 1 1 0 PhC(0)Cl (control) > I x 103 1 1 20 0 PhC(0)Cl
productsC
0 2 , Cl-, (C13COOCC13)d 0 2 , Cl-, (C13COOCC13)d 02, C1-, PhH, (F3CCC1200CC12CF,)‘ 0 2 , C1-, (PhCC1200CC12Ph)‘ PhH, 02, C1-, (PhCC1200CC12Ph)e PhH, BuOOBu, 02,BrPhH, BuOOBU, 0 2 , C1PhC(O)OOC(O)Ph, 02, C1PhC(O)OOC(O)Ph,PhC(0)O-, O,, C1-
Section B RX trap
02.-
‘02-trapping agents CCI,/DPIBF f PhC(O)Cl/DPIBF CCl,/rubrene f PhCCls/cis-PhCH=CHPh PhC(O)Cl/cis-PhCH=CHPh
1,4-Cyclohexadiene (1,4-CHD) in
6000 10 6000 50 10
5 5 5 1 1
1 1 1
20 10
11
15 1 0 0
DBB DBB rubrene endoperoxide (tr) PhCClzOOCCl2Ph PhC(O)OOC(O)Ph
“Decay of absorption band for 02’-(254 nm) monitored in the presence of 10-50-fold excess of RX. Each system obeys a second-order rate law, and the values parallel those from an earlier electrochemical study in dimethylformamide (ref 1). 100% represents one PhH per [ROOOOR] + 2 x 7 . cProducts assayed via GC, NMR, HPLC, and GC-MS. To obtain quantitative results two 02’-ions (202’- + 2RX from GC and NMR the amount of RX was limited to a 10-20-fold excess relative to 02’-.Within these limits the product solution contained a mixture of ROOR and RC(0)OO-. [With larger excesses the dimer RCC1200CC12R may be the major product.] dInferred from other products [Cl,C=O, C02, HOC(0)O-, Cl-] (ref 1). eIndicated by NMR but not proven in the absence of reference compounds. fDecrease of trap monitored by UV-vis spectrometry ([DPIBFIi and [ r ~ b r e n e ]0.1 ~ , mM).
-
*
Scheme I P 1.4-CHD
pE--
1%
kox
io7 M-’s-’
I
k D > lo8
[ROOOORI
M-l S-’ ROOR t 0 2 PhH
+ ROOR
DEB t ROOR kdiox
kd,100-103S-r HOOH kdehydmt 101-io3 M-’s-~
+ 1/202 kdiox/104-106M-’s-’
” RX: CCl,, F&CC13, PhCC13,PhC(O)Cl, BuBr, BuC1. 1,4-CHD: l,4-cyclohexadiene. DPIBF: diphenylisobenzofuran. DBB: dibenzoylbenzene. and products and are reasonable estimates relative to similar processes. Thus, the dimerization of ROO’ to give ROOOOR is a radical-radical coupling and should be close to a diffusion-controlled process (13). The lifetime of [ROOOOR] should be related to its 0-0bond energies (14, 15), which will be smaller when R is an electron-withdrawing group (C13C). Hence, the short lifetime for [Cl3COO0OCCl3]and a moderate rate for its reaction with 1,4-CHD accounts for the absence of a net reaction (Table I). The formation of substantial amounts of DBB indicate that the reactivity of DPIBF is at least three orders of magnitude faster. The present results demonstrate that the lifetime of the ROO’ formed from the reaction of 02’with excess RX is too short in MeCN (at millimolar concentrations) for it to react with the allylic hydrogens of 1,4-CHD. Because the
latter is believed to be the mode of toxicity for C C 4 in mammalian liver (3, 4),other reaction paths need to be considered. The rapid formation of ROOOOR species from 02*in the presence of excess RX, and their ability to dehydrogenate 1,4-CHD and to dioxygenate highly conjugated aromatic systems with reactivities analogous to IO2 represents a serious biological hazard. Because HOOH and ROOR are coproducts, their toxicity needs to be considered a part of the hazard from ingestion of halogenated hydrocarbons. Acknowledgment. This work was supported by the Welch Foundation under Grant A-1042. Registry No. CCl,, 56-23-5; C13CCF3,354-58-5; Cl,CPh, 9807-7; PhCOCl, 98-88-4; BuBr, 109-65-9; BuCl, 109-69-3; CI,COOCCI,, 111005-89-1; F3CCC1200CClzCF3, 111005-90-4; PhCClZOOCC12Ph, 111005-91-5; BuOOBU, 3849-34-1; PhC(0)OOC(O)Ph, 94-36-0; superoxide, 11062-77-4.
References (1) Roberts, J. L., Jr., Calderwood, T. S., and Sawyer, D. T. (1983) “Oxygenation by superoxide ion of CCl,, FCCl,, HCCl,, p,p’-DDT,
and related trichloromethyl substrates (RCCI,) in aprotic solvents”. J. Am. Chem. SOC.105, 7691-7696. (2) Sawyer, D. T., and Valentine, J. C. (1981) “How super is superoxide?”. Acc. Chem. Res. 14, 393-400 and references therein. (3) Slater, T. F. (1982) “Activation of carbon tetrachloride: chemical principles and biological significance”. In Free Radicals Lipid Peroridation, and Cancer (McBrien, D. C. H., and Slater, T. F., Eds.) pp 243-274, Academic: New York. (4) Slater, T. F., Cheeseman, K. H., and Ingold, K. 0. (1985) “Carbon tetrachloride toxicity as a model for studying free-radical mediated liver injury”. Philos. Trans. R. SOC.Lond., B B311, 633-645. (5) Yamaguchi, K., Calderwood, T. S., and Sawyer, D. T. (1986) “Corrections and additional insights to the synthesis and characterization of tetramethylammonium superoxide [(Me,N)O,)”. Inorg. Chem. 25, 1289-1290. (6) Howard, J. A,, and Ingold, K. U. (1967) “Absolute rate constants for hydrocarbon autoxidation. V. The hydroperoxy radical in
Communications chain propagation and termination”. Can. J. Chem. 45,785-792. (7) Sawyer, D. T., McDowell, M. S., and Yamaguchi, K. S. (1988) in aprotic media”. Chem. Res. “Reactivity of perhydroxy (HOO’) Toxicol., in press. (8) Burton, G. W., Foster, D. O., Perly, B., Slater, T. F., Smith, I. C. P., and Ingold, K. U. (1985) “Biological antioxidants”. Philos. Trans. R. SOC.Lond., B B311,565-578 and references therein. (9) Foote, C. S. (1982) “Light, oxygen, and toxicity”. In Pathology of Oxygen (Autor, A. P., Ed.) pp 21-44, Academic: New York. (10) Foote, C. S. (1984) Department of Chemistry, University of California, Los Angeles, CA 90024, private communication. (11) Roberta, J. L., Jr., Calderwood, T. S., and Sawyer, D. T. (1984) “Nucleophilic oxygenation of carbon dioxide by superoxide ion in aprotic media to form the C206%species”. J.Am. Chem. SOC.106, 4667-4670. (12) Kanofsky, J. R. (1986) “Singlet oxygen production in superoxide ion-halocarbon systems”. J . Am. Chem. SOC.108, 2977-2979.
Chem. Res. Toxicol., Vol. 1, No. 1, 1988 21 (13) Howard, J. A. (1978) “Self-reactions of alkylperoxy radicals in solution”. In Organic Free Radicals (Pryor, W. A,, Ed.) pp 413-432, ACS Symposium Series 69, American Chemical Society: Washington, DC, and references therein. (14) Bartlett, P. D., and Gunther, P. (1966) “Oxygen-rich intermediates in the low-temperature oxidation of t-butyl and cumyl hydroperoxides”. J . Am. Chem. SOC.88, 3288-3294. (15) Benson, S. W. (1964) “On the thermochemistry of alkyl polyoxides and their radicals”. J . Am. Chem. SOC.86, 3922-3924.
Shigenobu Matsumoto, Hiroshi Sugimoto Donald T. Sawyer* Department of Chemistry Texas A&M University College Station, Texas 77843 Received June 30, 1987
