Environ. Sci. Technol. 1996, 30, 1192-1197
Borohydride Photoreduction of Nitroaromatic Compounds Related to Military Ordnance Constituents RICHARD A. LARSON,* PENNEY L. MILLER, AND THOMAS O. CROWLEY Institute for Environmental Studies, University of Illinois at Urbana-Champaign, 1101 West Peabody Drive, Urbana, Illinois 61801
Nitroaromatic compounds are pollutants of considerable concern due to their toxicity and the extent of their use as synthetic intermediates, propellants, and explosives. Oxidative methods for treating wastes containing these compounds are not entirely satisfactory because of the recalcitrant nature of the compounds toward aerobic microbial decomposition and common chemical oxidizing agents. Recently, several publications have appeared indicating that these substances are susceptible to microbial reduction under anaerobic conditions. We have evaluated the chemical reduction of these compounds using alkaline sodium borohydride. In the presence of excess borohydride, exposure of the compounds to ultraviolet wavelengths >280 nm strongly promoted their decomposition. Rates of disappearance for four tested nitroaromatic compounds were increased even more if the illumination was conducted in the absence of oxygen. Numerous denitration, desulfonation, reduction, and condensation products were tentatively identified from the reaction mixture. The photoreaction appeared to occur by a mechanism including electron transfer from borohydride to an excited state of the nitro compound.
Introduction Nitroaromatic compounds are pollutants of great concern because of their toxicity, stability, and role as precursors in the formation of compounds of possibly greater toxicity (i.e., nitroso compounds). Commonly found substances include mono-, di-, and trinitro-substituted benzenes. Other occasionally detected water pollutants, dinitrotoluenes (DNTs: 2,4- and 2,6-isomers), are released from the manufacturing of 2,4,6-trinitrotoluene (TNT) (1) as well as the manufacturing of polyurethane foams, which uses DNT as a starting material for the synthesis (2). Other nitroaromatic contaminants include dinitrobenzene sulfonic acids * Address correspondence to this author; telephone: 217-333-7269; fax: 217-333-8046; e-mail address:
[email protected].
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and partially reduced nitro compounds formed from munitions manufacturing. Nitroaromatic compounds are relatively stable in an aerobic environment because of their highly oxidized state; however, they are more reactive in an anaerobic environment where reduction can occur. Redwater, a wastewater formed in TNT manufacturing, contains nitrosulfonic acids and many other impurities and presents a special problem in disposal. These acids are exceptionally electron-poor and accordingly are very difficult to remove by conventional oxidation processes. The most common method currently used is to incinerate the waste and to landfill the resultant ash. The excited states of nitro compounds undergo many reactions (3, 4). Nitroaromatic compounds are particularly susceptible to nucleophilic reactions such as reductions, and some of these reactions are enhanced photochemically (5), for example, reactions with methoxide (6), cyanide (7, 8), and borohydride (9). Thus, nitroaromatics demonstrate a wide range of reactivity when illuminated, especially in the presence of added electron donors and/or nucleophiles. An external reductant for the photochemical degradation of electron-deficient aromatic pollutants was used by Epling et al. (10), who noted greatly improved reaction rates for the photoreduction of some polychlorinated biphenyls (PCBs) upon the addition of NaBH4 to the reaction mixture. The reaction was initiated by the excitation of the PCBs with 254-nm (shortwave UV) light, which is absorbed by these compounds. Since the colored constituents of redwater obviously absorb some longer (visible) wavelengths, we thought it might be possible to degrade some redwater constituents and related nitro compounds with NaBH4 by illuminating the solution with ordinary or simulated sunlight. (Although shorter wavelength UV exposure could also have been used, we selected solar UV because of the ability to illuminate the reaction mixtures in Pyrex rather than quartz vessels.) Alkaline borohydride formulations are available commercially and could represent a viable treatment option for some wastes.
Experimental Section Reagents. High-performance liquid chromatograpy (HPLC)grade methanol and acetonitrile were purchased from Baxter Healthcare (Muskegon, MI) and Fisher Chemical (Fair Lawn, NJ), respectively. 1,3-Dinitrobenzene (DNB), 2,4-dinitrotoluene (DNT), 2,4-dinitrobenzenesulfonic acid (DNBSA), cetyltrimethylammonium bromide, and sodium tetraphenylborate (TPB) were purchased from Aldrich Chemical Co. (Milwaukee, WI). 2,4-Dinitrotoluene-3sulfonic acid (DNTSA) was obtained from the U.S. Army’s Construction Engineering Research Laboratory (Champaign, IL). Anhydrous magnesium sulfate, boric acid, Borax, sodium phosphate, sodium hydroxide, and sodium phosphate monobasic were obtained from Fisher Scientific; highpurity methylene chloride was obtained from Baxter, Burdick, and Jackson (Muskegon, MI). Sodium borohydride was purchased from Alfa Products (Danvers, MA). Laboratory water was deionized and glass distilled from a solution of KMnO4/KOH before use. In order to stabilize the highly reactive solutions of sodium borohydride, it was necessary to add sodium
0013-936X/96/0930-1192$12.00/0
1996 American Chemical Society
hydroxide to all solutions of sodium borohydride before addition to the solutions of the nitroaromatic compounds. At lower pHs, borohydride decomposes with the loss of hydrogen gas. Since a relatively large amount of NaOH was added, all reaction solutions containing sodium borohydride had a high pH (11.8-13.5). The sodium borohydride solutions were typically prepared by first dissolving 0.1 g of NaOH in 100 mL of water (0.025 M) and then adding 0.25 g of sodium borohydride (0.066 M). Lamps and Irradiation Conditions. All photoreactions were performed using a medium-pressure Pyrex-filtered mercury arc lamp (λ > 290 nm) with power supplied by a 200-W source (Ace Glass, Vineland, NJ). In all experiments, only the nitro compound under test absorbed UV. The lamp was contained in a borosilicate immersion well, which was water cooled. Kinetic studies were carried out using a “merry-go-round” reactor (Ace Glass), a motorized device that allows the illumination of multiple samples at a fixed distance from a central lamp. Aliquots of 5 mL were pipetted into Pyrex disposable screw-cap culture tubes (13 × 100 mm), which were capped, placed into the “merrygo-round” cell holder at a distance of 5 cm from the lamp housing, and illuminated. Prior experiments from our laboratory have indicated that, under these conditions, most photochemical reactions proceed at a rate approximately twice that of reactions performed in full summer sunlight at this latitude (40° N). Aliquots (250 µL) of the reaction mixture were taken at various time intervals and analyzed by the HPLC method described below. The NaBH4-NaOH solution described above was combined with a 2 × 10-4 M solution (in water) of the nitroaromatic compound to be tested on a 1:1 volume ratio to produce the following final concentrations: 1 × 10-4 M nitroaromatic compound, 0.033 M NaBH4, and 0.0125 M NaOH. If an anoxic environment was required, argon gas was slowly bubbled through each 5-mL sample for approximately 5 min before irradiation. All samples were analyzed by HPLC as described later. Solutions of TPB were prepared by dissolving 0.1 g of NaOH in 100 mL of water followed by the addition of 2.26 g of TPB to the solution. 50 mL of this solution was combined with 50 mL of a 2 × 10-4 M solution of a nitroaromatic compound in water to produce the following final concentrations: 1 × 10-4 M nitro compound, 0.0125 M NaOH, and 0.033 M TPB. Argon gas was bubbled through each 5-ml sample for approximately 5 min before irradiation. Larger-volume batch reactions were performed in order to obtain enough material to analyze for reaction products using gas chromatography/mass spectroscopy. In a typical batch reaction, a medium-pressure mercury arc lamp with a surrounding immersion well was immersed in 500 mL of the reaction mixture contained in a 1000-mL Pyrex photochemical reaction vessel (Ace Glass). No attempt was made to correlate the yields of the products observed in the batch process with those from kinetic experiments and, since the initial concentration of the nitro compound (and BH4-) were higher, it is probable that many products could have been formed by favorable, second-order reactions that were not observed in the kinetic experiments. Kinetic and Product Analyses. In preparative reactions, the extraction of hydrophobic products was carried out using a styrene-divinylbenzene Empore solid-phase extraction disk (47 mm; 3M, St. Paul, MN) set in a glass, vacuum-filtering apparatus. Disks were cleaned per the
manufacturer’s directions. Half of the reaction mixture (250 mL) was extracted immediately after the conclusion of the reaction. Products were eluted with three washings of MeCl2 and collected in a 50-mL test tube (total volume, 20 mL). The disk was also subjected to three washings with MeOH (20 mL) to remove more polar products; these washings were collected in a separate test tube. A brown precipitate accumulated on the surface of the disk and, therefore, the disk was also rinsed with ethyl acetate to solubilize it. The other 250 mL of the reaction mixture was also extracted; however, the pH was adjusted to approximately 7 to collect reaction products that may have been ionized at high pH. The same procedure for extraction was performed. After extraction, aliquots of the MeCl2 and the MeOH fractions were put in a separate test tube and blown to dryness under a stream of argon at 40 °C. Samples were redissolved in a MeCl2 or EtAc (when residue was not soluble in MeCl2) solution containing 10-4 M naphthalene as an external standard. A 2-µL sample was injected onto the GC (FID) for quantification and the GC/MS for identification of products. Both GC instruments used the same type of column, a DB-1 capillary column, 30 m × 0.32 mm i.d. (J&W Scientific, San Jose, CA). Tentative determination of product identities was done by GC/MS using a 5890 HewlettPackard gas chromatograph (Avondale, PA) coupled with a Finnigan MAT ion trap detector (Sunnyvale, CA). Confirmation was achieved, where possible, by matching the spectra to those of reference compounds from literature sources and/or by comparing spectra and retention times to those of authentic standard compounds. All UV-visible spectra were obtained using a Hitachi Model U-2000 double beam spectrophotometer (Hitachi Instruments Inc.; Tokyo, Japan). Samples were pipetted into a 1-cm quartz cuvette, and a wavelength scan was performed with a scan speed of 200 nm/min. HPLC analysis of uncharged nitro compounds was performed using HPLC with a mobile phase of 80:20 MeCN/ H2O (v/v) for DNT (RT ) 4.2 min) and DNB (RT ) 3.5 min). The detector wavelength was set on 254 nm. Concentrations in reaction mixtures were determined using a standard curve. Disappearance rate constants and half-lives were calculated from first-order plots of the loss of the starting materials. HPLC analysis of nitrosulfonic acids was performed by an ion-pairing method (11). In brief, a 200-µL sample was injected and monitored at 254 or 263 nm, which are the absorption maxima of DNBSA and DNTSA, respectively. The mobile phase comprised by volume: 55% acetonitrile, 3% methanol, and 42% of a 0.3% by weight solution of cetyltrimethylammonium bromide in glass distilled/deionized water. Standard curves were produced using DNBSA and DNTSA to determine detection limits and retention times of some typical nitroaromatic sulfonic acids. Some HPLC data were also collected using a diode array detector (Groton PF1 Diode Array Detector System, Groton Technology, Inc., Waltham, MA).
Results and Discussion General. Our experiments with borohydride were performed with several representative nitroaromatic compounds: 1,3-dinitrobenzene (DNB, 1), 2,4-dinitrotoluene (DNT, 2), 2,4-dinitrobenzenesulfonic acid (DNBSA, 3), and 2,4-dinitrotoluene-3-sulfonic acid (DNTSA, 4). The structures for these compounds and their reaction products are given in Figure 1. DNBSA was chosen as a model compound
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FIGURE 1. Structures of compounds in the text.
for sulfonic acids from redwater that were not commercially available. DNB and DNT are also reported to be major components of redwater, ranging in concentrations from 0.20 to 8.5 and from 0.04 to 48.6 mg/L, respectively, and have a 100% occurrence rate in measured TNT wastewater effluents (12). DNTSA has been reported to be a constituent of redwater (1), but was not present in a redwater sample that we analyzed (11). Only kinetic studies were performed with DNTSA due to the limited quantity available. Ultraviolet spectra of the nitro compounds showed the characteristic maximum at 240-250 nm ( = 15 000 L mol-1 cm-1). All the compounds also absorbed weakly in the solar UV-B (near 290 nm) region ( for DNT = 2900 and for DNB ≈ 300) due to the extended tailing of this absorption and to weak, longer wavelength-induced transitions. In the solar UV region, it is likely that both π f π* and n f π* transitions are occurring. The type of transition will determine the photochemistry in which the molecule will participate. An n f π* transition may result in the reduction of the nitro group, whereas a π f π* transition may enhance nucleophilic substitution and/or reduction of the aromatic ring (3, 9). Reaction Kinetics. Kinetic data for the photodegradation of the test compounds (analyzed using first-order assumptions) are summarized in Table 1. The reaction is thought to be bimolecular; however, pseudo-first-order kinetics were assumed to apply because the concentration of BH4- was in great excess (>100X) of the concentration of the nitro compounds and, more importantly, of the excited states of these compounds. Correlations were fairly good (r2 ranged from 0.89 to 0.99) and supported the pseudo-first-order assumption.
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TABLE 1
Half-Lives (in Minutes) for Disappearance of Nitro Compounds under Various Conditionsa direct compd
air
DNB DNT DNBSA DNTSA
>1000 83 720 320
NaBH4
NaB(Ph)4
argon
dark
air
argon
argon
55 46 55 32
22 34 38
26 18
350
>1000 >1000 280
a Pseudo-first-order kinetics are assumed to apply. See text for details.
When borohydride was absent but UV light was present, the nitro compounds underwent slow photodecomposition. DNT is already known to react readily in the presence of shorter wavelength UV; the excited-state nitro group abstracts a hydrogen intramolecularly from the methyl group to form an aci-quinoid structure capable of further transformations (4). Under our conditions, its half-life was about 80 min. The first-order rates of direct photolysis for the other compounds were insignificant when compared to those when borohydride was present. Light was also shown to be required for efficient loss of the nitro compounds. Although DNBSA underwent a slow reduction with borohydride in the absence of light (t1/2 280 min), the rate of loss of the other two compounds tested was almost immeasurably slow. The kinetic data indicate that for all cases the light reaction proceeded much more rapidly than the corresponding dark reaction. Table 1 also indicates that the effect of oxygen on the photoreaction was to reduce the rate of loss of all the
FIGURE 2. Generalized mechanism for reduction of aromatic nitro compounds.
compounds. Oxygen is a known quencher of certain free radicals and excited states; it is also a good acceptor of electrons. Therefore, the optimum conditions for the reductive loss of the starting nitro compounds were for borohydride and light to be present and for oxygen to be absent. Products. (1) From DNB. Dark Reactions. Although they proceeded slowly, products could be readily isolated from nitroaromatic borohydride reactions conducted in the absence of light, and product mixtures tended to be simpler. Accordingly, considerable efforts were made to identify the dark reaction products and to compare them to those formed in the presence of light, which accelerated the overall reactions markedly. For the dark reactions, nitroaromatic reduction appears to occur almost exclusively at the nitro group. The formation of reduced species from nitro compounds usually occurs in the presence of reducing agents through sequential reductions to produce nitrosobenzene, the nitrosobenzene radical anion, phenylnitroxide radical, and phenylhydroxyamine (13) (Figure 2). For DNB, extraction of dark reaction mixtures and subsequent analysis by GC/MS showed the expected reduction productssthe partially reduced 3-nitroaniline (5, MW ) 138) and azo (6, MW ) 272) and azoxy (7, MW ) 288) dimers as well as a new product with a strong molecular ion at m/z 259. Mass spectra for the apparent azo and azoxy dimers were very similar to those reported for the p-nitro isomers (14, 15). The formation of these dimers may result from any of a variety of possible mechanisms for the coupling of nitroaromatic reduction intermediates. The unexpected compound with MW 259 was tentatively identified as 3,3′-dinitrodiphenylamine (8). Its formation requires the loss of a nitrogen functional group. Dale and Vikersveen (16) reported the formation of 3,3′-diphenylamine from a reaction of DNB with NaBH4 in a strongly basic medium (refluxing MeOH with NaOH). Products similar to those already described were observed for a dark reaction carried out in an argon-purged solution, except for the detection of an additional compound having the following fragmentation pattern: m/z ) 76, 152, 50, 180, 312, 63, 90, and 122, given in decreasing abundance. The structure of this compound is as yet undetermined. Light Reactions. Products from the dark- and lightinduced reactions were generally similar. The major products isolated from the dichloromethane extracts of the photoreactions were 6 and 7. The 3,3′-diphenylamine product (8) was not observed in the photoreactions. A new product was detected from the “polar” extracts of a batch reaction with argon purging. The product had a retention
time of 55 min with the following m/z ratios (in decreasing abundance): 121, 91, 159, 173, 105, 69, 57, 145, and 135. In addition, in batch reactions small amounts of ring denitration products were observedsnitrobenzene (9, MW 123) and a reduced nitrobenzene derivative, probably 10 or another nitrocyclohexadiene isomer (MW 125). These compounds were not formed in dark reactions. Overall, the data for DNB show that reduction occurs on the nitro group to form intermediates that principally couple to form dimers. Fully reduced diaminobenzene was not observed, suggesting that coupling between partially reduced monomers was favored under our conditions over complete reduction. Consistent with this hypothesis, it has been reported that, in a strongly basic solution containing phenylhydroxyamine and nitrosobenzene, azoxybenzene was rapidly produced (17). (2) From DNT. The reactions performed with DNT would be expected to give products analogous to those formed in reactions with DNB as well as products formed from reactions characteristic of ortho-alkyl-substituted nitroaromatics (6). These expectations were borne out. Dark Reactions. Dark reactions were performed in aerated and argon- purged solutions. Argon-purged reaction mixtures contained the greater number of identifiable products. The major products formed were tentatively identified as having molecular weights of 152, 461, and 287 and occurred in both argon purged and air-containing solutions. For MW ) 152 and 287, more than one peak in the mixture was found to have fragments at this mass, indicating that multiple isomers were formed. This occurrence was also observed in the batch reactions (photolytic). The compounds having a molecular weight of 152 were identified as the mononitro reduction products, 2-methyl5-nitroaniline and 4-methyl-3-nitroaniline (11 and 12). Injections of authentic standards confirmed these structures. The formation of these two products was only observed in reactions that were argon purged. (Oxygen would probably oxidize the radical anion formed from successive reduction of the nitro group and inhibit the production of the aniline. This effect may occur in DNT and not in DNB because of the presence of the methyl group, whose electron-donating character may increase the electron density so that oxidation can occur more readily.) No evidence was found for the formation of either an azoxy or azo dimer in either of the dark reactions. As in the dark reactions for DNB, however, presumed diphenylamine derivatives (13) were present (MW ) 287). Standards were unavailable for confirmation, although fragmentation patterns generally agreed with what would be expected for such products. Another product, thought to have a molecular weight of 461, was not identified; its fragmentation pattern given in decreasing abundance is m/z ) 207, 461, 460, 75, 193, 133, and 127. Light Reactions. As in the dark reactions, the argonpurged solution exposed to light and containing BH4- and DNT contained the greatest number of detectable products. Both reaction mixtures (with and without argon purging) contained 11 and 12. Mixtures from argon purging contained dinitrodimethylazo (14; MW ) 300) and dinitrodimethylazoxy products (15; MW ) 316), but solutions containing oxygen contained only traces of MW ) 300. However, oxygenated solutions contained two other apparent azo compounds (MW ) 298) with the following mass fragments: m/z )
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298, 89, 251, 205, 77, 63, 151, 102, 119, 133, and 165 and (4 min later) m/z ) ,298, 63, 253, 205, 281, 89, 73 (75), 164, 235, and 147. These compounds have been tentatively identified as isoxazole derivatives (anthranils) resulting from the intramolecular photocyclization of the nitro group and the ortho-methyl group on one of the aromatic rings of the dimer. Related compounds are known to form in thermal redox reactions of o-nitroalkylbenzenes (18, 19). The mass spectrum of the earlier eluting peak showed a loss of 47 from 298 to give the m/z 251 ion, indicating that an intramolecular cyclization had probably occurred with the ortho-nitro-methyl group (16). The later peak had a typical fragmentation pattern [loss of 17 (m/z ) 281); loss of 28 (m/z ) 253)] for a nitroaromatic compound having an orthomethyl group such as 17 (20). The identification of 4-nitroso-2-nitrotoluene or 2-nitroso-4-nitrotoluene (18; MW ) 166) in both argon purged and oxygen-containing solutions and of nitrosomethylanilines (19 and 20; MW ) 136) in the argon-purged solutions supports the stepwise reduction process occurring on the nitro group to form reactive, radical intermediates. The absence of nitrosoanilines or other similarly reduced species in the oxygen-containing solutions suggests either that their formation is inhibited by oxygen scavenging of reactive, radical intermediates or that they are oxidized by oxygen. (3) From DNBSA. The most noticeable product from the illumination of DNBSA in the presence of borohydride was a violet material that appeared shortly after illumination began. It was revealed by its characteristic long-wavelength maximum at about 540 nm in reaction mixtures. The product was also formed in the dark, suggesting that it could have been either a Meisenheimer complex (21) (21) or an azo dye, products of ring and nitro group attack, respectively. However, attempts to purify or to analyze the compound in order to determine its structure were unsuccessful; it appeared to be highly polar and to resist derivatization by common alkylating agents such as methyl iodide, dimethyl sulfate, and diazomethane. Photodiode array detector analysis of illuminated 5 × 10-4 M DNBSA and 0.033 M sodium borohydride solutions also revealed (using standard compounds) the production of denitration and desulfonation products: a mononitrobenzene sulfonic acid (22), DNB, and m-nitroaniline (5). The sulfonic acid was also observed by GC/MS of its derivative with diethylamine (11). Liquid/liquid extractions of the reaction mixture with dichloromethane, however, failed to confirm the latter two products by GC/MS. Mechanistic Studies. The literature of borohydride reduction (13) contains examples of one- and two-electron mechanisms that invoke electrons, hydride ions, and hydrogen atoms as reducing agents. The electron transfer mechanism was tested with our compounds using sodium tetraphenylborate (TPB), a so-called “pure electron transfer agent” (22). The presence of four phenyl groups, as opposed to the four hydrogens, prevents the transfer of hydrogen as observed with BH4- as well as the hydrogen abstraction reaction. TPB has minimal absorption above λ ) 290 nm; therefore, the use of TPB can help determine the rate of reactions involving pure electron transfer. The measured rate constant for reaction of DNB with TPB in an argon-0purged solution (0.026/min) was very similar to that obtained for reaction with BH4- (0.032/min; Figure 3). DNT also reacted with TPB in an argon-purged solution with a rate constant of 0.037/min, almost twice as
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FIGURE 3. First-order plot of loss of DNB in the presence of borohydride (open squares) amd TPB (dark squares).
large as that for reaction of DNT with BH4- (0.020/min) under the same conditions. It is not clear what may be causing the difference in rates for the DNT reactions. Hutchins et al. (23) reported that electron-donating substituents retarded reduction reactions on nitroaromatic compounds; the slowing of the reaction rates for DNT with BH4- may reflect this. Therefore, the kinetic results with TPB, a reducing agent without hydrogen atoms, indicate that the most likely possibility for the initial photoreduction step is a photoinduced electron transfer. Obviously the precise rate of electron transfer from a borate anion will be a function of the steric and electronic properties of the groups around the central boron atom; however, it is worth noting that a phenyl group often has virtually the same effect as a hydrogen atom in many aromatic substitution reactions (16). Therefore, the result that the reaction rate with TPB was equal to or greater than reaction rates with BH4- for either DNB or DNT supports the assumption that reactions in these systems are occurring via electron transfer to the n-π* excited states, since the bulky TPB molecule is unlikely to attack the ring nor is there hydride present. Reaction with BH4- in the presence of light would be expected to result in the reduction of the nitro group to form reduced nitro products such as nitroso compounds, nitroxides, hydroxylamines, and fully reduced amines as well as products resulting from the reactions (photochemical or otherwise) between these intermediates.
Acknowledgments We thank Michael Cook, Don Cropek, Gary Epling, Steve Maloney, Karen Marley, and Gary Peyton for helpful discussions and assistance and the U.S. Army Construction Engineering Research Laboratory for financial support (Contracts 88-90M-0041, 88-90D-0026, and 88-93D-0018).
Literature Cited (1) Spanggord, R. J.; Suta, B. E. Environ. Sci. Technol. 1982, 16, 233236. (2) Spanggord, R. J.; Spain, J. C.; Nishino, S. F.; Mortelmans, K. E. Appl. Environ. Microbiol. 1991, 57, 3200-3205. (3) Do¨pp, D. Topics Curr. Chem. 1975, 55, 49-85. (4) Chow, Y. L. In The Chemistry of Functional Groups Supplement F: The Chemistry of Amino, Nitroso and Nitro Compounds and their Derivatives Part 1; Patai, S., Ed.; John Wiley & Sons: Chichester, 1982; pp 181-290. (5) Pietra, F.; Vitali, D. J. Chem. Soc. Perkin Trans. 2 1972, 385-389. (6) Gold, V. C.; Rochester, C. H. J. Chem. Soc. 1964, 1704-1717. (7) Letsinger, R. L.; McCain, J. H. J. Am. Chem. Soc. 1966, 88, 28842885. (8) Letsinger, R. L.; Hautala, R. H. Tetrahedron Lett. 1969, 48, 42054208. (9) Petersen, W. C.; Letsinger, R. L. Tetrahedron Lett. 1971, 24, 21972200. (10) Epling, G. E.; Florio, E. M.; Bourque, A. J.; Qian, X.; Stuart, J. D. Environ. Sci. Technol. 1988, 22, 952-956. (11) Crowley, T. O.; Larson, R. A. J. Chromatogr. Sci. 1994, 32, 57-60. (12) Spanggord, R. J.; Gibson, B. W.; Keck, R. J.; Thomas, D. W.; Barkley, J. J., Jr. Environ. Sci. Technol. 1982, 16, 229-232.
(13) March, J. Advanced organic chemistrysreactions, mechanisms, and structure; Wiley-Interscience: New York, 1992. (14) Mass Spectrometry Data Centre. Eight Peak Index of Mass Spectra; MSDC: Nottingham, U.K., 1988 (15) McLafferty, F. W. The Wiley/NBS Registry of Mass Spectral Data Wiley: New York, 1988. (16) Dale, J.; Vikersveen, L. Acta Chem. Scand. 1988, B42, 354-361. (17) Russell, G. A.; Geels, E. J.; Smentowski, F. J.; Chang, K.-Y.; Reynolds, J.; Kaupp, G. J. Am. Chem. Soc. 1967, 89, 3821-3827. (18) He, Y. Z.; Cui, J. P.; Mallard, W. G. J. Am. Chem. Soc. 1988, 110, 3754-3759. (19) Austin, R. P.; Ridd, J. H. J. Chem. Soc. Perkin 2 1994, 1411-1414. (20) Benoit, F.; Holmes, J. L. Org. Mass Spectrosc. 1970, 3, 993-1007. (21) Pietra, F. Q. Rev. Chem. Soc. 1969, 4, 504-521. (22) Kropp, M.; Schuster, G. B. Tetrahedron Lett. 1987, 2, 5295-5298. (23) Hutchins, R. O.; Lawson, D. W.; Rua, L.; Milewski, C.; Maryanoff, B. J. Org. Chem. 1971, 36, 803-806.
Received for review June 15, 1995. Revised manuscript received November 28, 1995. Accepted December 15, 1995.X ES950415Y X
Abstract published in Advance ACS Abstracts, March 1, 1996.
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