Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation

In this work, we explore vicinal dibromide stereoisomers as “probes” of the .... Green Processing and Synthesis 2016, 5 (1) DOI: 10.1515/gps-2015-...
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Environ. Sci. Technol. 2001, 35, 2268-2274

Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zerovalent Metals LISA A. TOTTEN,† URS JANS,‡ AND A. LYNN ROBERTS* Department of Geography and Environmental Engineering, The Johns Hopkins University, 313 Ames Hall, 3400 North Charles Street, Baltimore, Maryland 21218

Whether reductive dehalogenation proceeds via a one- or a two-electron mechanism has been suggested to affect product distributions, hence potentially influencing the success of engineered treatment systems. In this work, we explore vicinal dibromide stereoisomers as “probes” of the concertedness of electron transfer in reduction by aqueous suspensions of iron and zinc metal. Dibromides consisted of 2,3-dibromopentane (diBP) stereoisomers and (()-1,2-dibromo-1,2-diphenylethane. All dibromides reacted with metals to give the same E:Z ratio of olefins observed during dehalogenation by iodide (a two-electron reductant). Reduction by Cr(II) (a one-electron reductant) yielded distinctly different proportions of E and Z olefins. Although this might be construed as evidence that metals function as two-electron reductants, high stereospecificity was also obtained for reduction of diBPs by Fe(II) adsorbed to goethite, a presumed one-electron reductant; this can be explained by two single-electron transfers in rapid succession, facilitated by the locally elevated concentration of reducing equivalents at the oxide-water interface. The results suggest that reduction of alkyl halides by metals is not likely to produce free radicals that persist long enough to undergo radical-radical coupling or hydrogen-atom abstraction from minor dissolved constituents. Apparent free-radical coupling products are more likely to result from (possibly surface-bound) organometallic intermediates.

Introduction Zerovalent metals offer great promise in treating organohalide contaminants through abiotic reductive dehalogenations (1-9). Despite significant progress in recent years, many basic questions about the mechanisms of these reactions remain unanswered. Metal-promoted reductions include hydrogenolysis (replacement of halogen by hydrogen), reductive β-elimination (removal of two vicinal halogens, accompanied by an increase in bond order), reductive R-elimination (e.g., reduction of carbon tetrachloride to dichlorocarbene), and * Corresponding author phone: (410) 516-4387; fax: (410) 5168996; e-mail: [email protected]. † Current affiliation: Department of Environmental Sciences, Rutgers University, 14 College Farm Rd., New Brunswick, NJ 08901. ‡ Current affiliation: Department of Chemistry, City College of CUNY, Convent Avenue at 138th Street, New York, NY 10031. 2268

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hydrogenation of multiple bonds, all involving a net transfer of two electrons. The principal goal of this research was to explore one important aspect of the mechanism, namely the concertedness of electron transfer during metal-promoted reductive β-elimination. Theoretically, reduction can proceed either via single electron transfer (SET, which may occur via outer-sphere or inner-sphere pathways) or via two-electron pathways, which must necessarily be inner-sphere (10). Whether reactions occur via outer- or inner-sphere mechanisms is of enormous practical significance in designing catalysts or stoichiometric reagents, as inner-sphere reductions occur much more rapidly than outer-sphere reactions (11). The mechanism not only dictates reactivity; it also influences selectivity (i.e., product distributions), potentially affecting the success of treatment approaches. If reduction occurs via SET, the resulting intermediates might persist long enough to undergo characteristic free-radical reactions. Some researchers (12, 13) have accordingly invoked coupling of free radicals to account for products encountered in organohalide reactions with zerovalent metals. A better understanding of the mechanism through which zerovalent metals reduce organohalides would explain why 1,1,1-trichloroethane reacts with Fe(0) to generate significant C4 hydrocarbons, but no such coupling products are observed in reaction with Zn(0) (14). If free radicals were sufficiently persistent, they might encounter H-atom donors, forming hydrogenolysis products; Balko and Tratnyek (15) have, for example, suggested that abstraction of H• from dissolved organic matter by •CCl3 could shift products toward the relatively undesirable species CHCl3 during CCl4 reduction by Fe(0). Other researchers have invoked one- versus twoelectron mechanisms to account for selectivity in reductive dehalogenation. Criddle and McCarty (16) hypothesized parallel one- and two-electron reactions during electrolytic reduction of CCl4 to CHCl3 (via •CCl3) and formate (via dichlorocarbene), respectively, while Buschmann et al. (17) ascribed the increased yield of haloforms obtained in reduction of tetrahalomethanes by cysteine to a two-electron pathway rather than an SET pathway. To date, techniques designed to confirm the existence of free radicals (and hence SET pathways) have not been successfully applied to organohalide reactions with zerovalent metals in aqueous solution. The phrase “two-electron reduction” is somewhat misleading because electrons only can be transferred one at a time (18). Even the classic SN2 reaction, which is not normally perceived as involving ET, can be envisioned as taking place via a process in which bond formation and “single electron shift” (18, 19) or (equivalently) “inner sphere single electron transfer” (20) are concerted events. Nonetheless, there is a clear mechanistic distinction between SET processes resulting in homolytic bond cleavage, and “two-electron” or “polar” processes (such as SN2 reactions) that do not produce freeradical intermediates. To provide indirect evidence concerning the mechanism through which zerovalent metals reduce alkyl halides, we have examined the reduction of a series of stereochemical probe compounds (Figure 1). These include (2R, 3S)-2,3dibromopentane (diBP) and its stereoisomeric enantiomer, (2S, 3R)-2,3-dibromopentane (referred to as erythro-diBP), as well as (2R, 3R)-2,3-dibromopentane and its stereoisomeric enantiomer, (2S, 3S)-2,3-dibromopentane (threo-diBP). The other probe employed was (1R, 2R)-1,2-dibromo-1,2-diphenylethane [and its enantiomeric isomer, abbreviated as (()-SBr2]. 10.1021/es0010195 CCC: $20.00

 2001 American Chemical Society Published on Web 05/04/2001

FIGURE 1. Idealized reaction schemes for the model compounds used in this study: (a) single electron reduction to free radical intermediate, and (b) concerted two-electron reduction by electron donor “D:” to olefin plus (potentially unstable) (DBr)+ complex. For diBPs, R ) methyl, R′ ) ethyl (or vice versa). For (()-SBr2, R ) R′ ) phenyl. If these vicinal dihalides undergo reduction via SET, the first one-electron reduction would yield a radical (Figure 1a) that is partially stabilized by delocalization of the unpaired electron over the remaining bromine. This “bridged” bromine radical could experience rotation around the central C-C bond before being further reduced, resulting in a mixture of E and Z olefins. The E/Z ratio of the products is a function of the rate at which the second electron is transferred relative to the rate of C-C bond rotation, and also of the relative stability of the E-like and Z-like radical conformers. If C-C bond rotation is fast relative to the second electron transfer, the E olefin may be favored slightly over the Z isomer (if the R groups are small and steric interactions are minor), or the E olefin may be formed exclusively (as in the case of bulky R groups). The same E:Z ratio should, however, be attained, regardless of whether the threo or the erythro isomer (or the meso or the (()- pair) is reduced. In contrast to the SET mechanism, if vicinal dihalides were to undergo reduction via a “two-electron” pathway (Figure 1b), the reaction typically would be highly stereospecific, demonstrating a strong preference for anti elimination (21): threo-diBP reacts to (Z)-2-pentene, and erythro-diBP to form (E)-2-pentene. (Numerous exceptions to anti stereospecificity have nonetheless been observed for (()-SBr2 (21), as discussed below.) This “two-electron” reaction may be envisioned as involving transfer of halonium (X+) to the electron donor D. Reductive elimination of vicinal dihalides by I- has commonly been attributed to just such a “twoelectron” mechanism (22-24), leading to description of Ias a “nucleophilic reductant”. Low-valent transition metals, including Fe(I), Fe(“0”), and Co(I) porphyrins, also reduce vicinal dibromides via an inner-sphere two-electron halonium transfer mechanism (11). Several low-valent transition metal complexes have been described as “nucleophiles” in their reactions with alkyl halides (25, 26). Even zerovalent metals have been referred to in the older literature as

“nucleophiles” (27, 28), though the appropriateness of viewing a metal surface in this manner may be questioned. Vicinal dibromides have been extensively used in the past to investigate a variety of reduction reactions; Baciocchi (21) provides a review. For example, Schubert et al. (27) invoked differences in E:Z ratios as evidence that sodium metal in NH3 reduced vicinal dibromoalkanes via a free-radical mechanism, whereas magnesium (in tetrahydrofuran) and zinc metal (in water) were hypothesized to react via a “twoelectron” mechanism. More recently, stereochemical evidence for reactions of vicinal dihalides in homogeneous solution has led to suggestions that polysulfide ions (29), fluorenide ions (30), and diorganotellurides (31) can serve as “two-electron” reductants. Studies have shown that reductive β-elimination mechanisms are a function not only of the reductant, but also of the structure of the dihalide probe, the solvent, and the temperature. Thus, although some reactions of zerovalent metals with vicinal dihalide stereoisomers have previously been investigated (27, 28, 32, 33), experiments typically were conducted in organic solvents or were carried out at high temperatures, and the results may be of limited applicability to environmental transformations. The principal goals of this research were (1) to investigate reactions of vicinal dibromide stereoisomers with a variety of zerovalent metals under environmentally relevant conditions (i.e., at room temperature, in water); and (2) to assess the overall feasibility of vicinal dihalides as “probes” of environmental reduction mechanisms. Reactions with model reductants were therefore also investigated, including a oneelectron reductant [Cr(II) (aq)] (32, 34) and the nucleophilic reductant I-. Some researchers have suggested that surfaceadsorbed Fe(II) species may represent important reductants of organohalides in Fe(0) systems (4, 35, 36) as well as in natural environments (37-39). Reductions promoted by Fe(II) adsorbed to an Fe(III) oxide, goethite, were thus also studied. Hydrolysis of the probe compounds, which competes with reduction, was also examined.

Experimental Methods Reagents. Threo- and erythro-diBPs and (()-SBr2 were synthesized from (Z)- and (E)-2-pentene and (Z)-stilbene by stereospecifically brominating the olefins with pyridinium tribromide (40). The (()-SBr2 product was purified by recrystallization in methanol, and was found by gas chromatographic/mass spectrometric (GC/MS) analysis to be free of significant impurities. The threo- and erythro-2,3-diBPs each contained less than 0.5% impurity of the undesired isomer. The identities of the synthesis products were verified by MS and proton NMR analysis (41). Preparation of Metals. Each metal was acid-washed before use (41). Fe(0) (Fisher, 100 mesh, 95.3%) was cleaned with 1 N HCl. Zn(0) (Baker, 30 mesh, 100.5%), and Cu(0) (Aldrich, 40 mesh, 99.5%) were cleaned with 0.4% sulfuric acid; and Al(0) (Baker, 40 mesh, >99%) was cleaned with a solution of chromic and sulfuric acids. All cleaning procedures were conducted in an anaerobic glovebox (95% N2/5% H2). The surface areas of the Cu(0) and Fe(0) were measured by nitrogen BET and found to be 0.13 and 0.76 m2/g, respectively. The lower surface area of Zn(0) (0.035 m2/g) required measurement by krypton BET. Goethite. Goethite slurry (44.2 g goethite/L) was synthesized by Barbara Coughlin (42) and was provided by Alan T. Stone. The goethite had a specific surface area of 47.5 m2/g (42). For each experiment, 9.13 mM FeCl2‚4H2O was equilibrated overnight with 2.95 g/L of goethite (to allow sufficient time for the Fe(II) to adsorb and to develop significant reactivity; ref 37) before spiking with a vicinal dibromide. The Fe(II) was present at 14 times molar excess of the concentration of available sites (42). Experiments at pH 7.5 employed 5 mM tris(hydroxymethyl)aminomethane (Tris) VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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buffer plus 0.1 M NaCl. Experiments at pH 10.5 used 5 mM 2-amino-2-methyl-1-propanol buffer plus 0.1 M NaCl. Above pH 6, all surface sites on the goethite should be occupied by Fe(II) (42). Cr(II) Solutions. Cr(II) solutions were prepared in the glovebox by the method of Castro (43). Cr(II) reactions were carried out in 5 mM H2SO4 (pH 2.0-2.1) in a water bath at 25.0 (( 0.1) °C. Initial Cr(II) concentrations were determined by oxidation with Fe(III), with quantitation of the resulting Fe(II) by a bathophenanthroline disulfonic acid colorimetric assay (44). Iodide. Reactions with iodide were conducted in 1 M KI, 0.1 M NaCl, and 50 mM sodium tetraborate buffer (pH 9.3) in a water bath at 25.0 (( 0.1) °C. This pH value was employed to provide consistency with other experiments involving strong nucleophiles (45). Reactions of diBP Isomers. Reactors were prepared in a glovebox to minimize contamination with O2. Reactions with metals were carried out in 1-L flasks. Reactions with goethite, Cr(II), iodide, and hydrolysis experiments were carried out in 125-mL flasks. Each flask was fitted with a ground-glass stopcock adapter (Ace Glass) plugged with an NMR-type septum (Aldrich catalog #Z10,070-6). The reaction solution (typically unbuffered 0.1 M NaCl for reactions with metals; 50 mM Tris buffer/0.1 M NaCl for hydrolysis experiments at pH 7.5; or 50 mM sodium tetraborate/0.1 M NaCl for hydrolysis experiments at pH 9.3) was deoxygenated via sparging with purified argon. The reductant was weighed or pipetted into each flask, which was then filled completely with aqueous solution. For reactions with zerovalent metals, an aliquot was removed for pH measurement after equilibrating with the metal overnight. The flask was then filled completely with aqueous solution, leaving no headspace. Flasks were spiked with either threo- or erythro-diBP in methanol to achieve an initial dibromide concentration of about 0.2 mM and a methanol concentration of 0.025 M. Flasks were then removed from the glovebox and were placed on a roller table at 40 rpm for mixing at room temperature. Samples (1 mL) were taken without introducing headspace by injecting an aliquot of deoxygenated solution through the septum, forcing the liquid samples into a second syringe. Aliquots (1 mL) were extracted into 1 mL hexane. Extracts were analyzed on a Carlo-Erba Mega 2 GC equipped with a flame-ionization detector (FID) and a 30 m × 0.32 mm × 5 µm Rtx-1 capillary column (Restek). The same analytical column was used to confirm the identities of the reaction products via GC/MS. Reactions of (()-SBr2. Experimental procedures were as described for the diBPs except that all reactions were conducted in 1-L flasks. Hydrolysis experiments were conducted in 50 mM borate/0.1 M NaCl buffer at pH values between 8.2 and 9.7. Each flask was spiked (via a methanol carrier) to give an initial (()-SBr2 concentration of 4 µM (methanol concentration 0.025 M). No attempt was made to keep these systems headspace free, as the parent compound and olefin products have relatively low Henry’s law constants. Samples (10 mL) were taken from each flask, with the volume being displaced by purified N2 or an N2/H2 mixture. Samples were extracted with 1 mL of hexane, and 1-µL hexane aliquots were injected onto a 15 m × 0.25 mm × 0.25 µm EC-5 capillary column (Alltech) for GC/FID analysis. Selected samples were also analyzed using a Hewlett-Packard (HP) 5890 GC equipped with an HP 5970 mass selective detector.

Results and Discussion Hydrolysis of the Probe Compounds. All of the dibromide probe compounds hydrolyzed at a rate independent of pH within the pH ranges tested. Hydrolysis rate constants for the diBPs were 0.02 hr-1 (erythro) and 0.006 hr-1 (threo) at pH 7.5. Products were tentatively identified by GC/MS (after 2270

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FIGURE 2. E and Z olefins (as a percent of total olefin) formed from the reaction of various reductants (top) with (()-SBr2 and (bottom) with threo-diBP. Fe (neutral) refers to the reduction carried out in 0.1 M NaCl at pH 6.5-8.5. Fe (acidic) refers to the reduction carried out in 0.1 M NaCl solution initially adjusted to pH 4 with HCl. Fe (basic) refers to the reduction carried out in 0.1 M NaCl solution initially adjusted to pH 10 with NaOH. Fe(II)/goethite experiments were conducted at pH 10.5. See text for other reaction conditions. Error bars represent 95% confidence limits. extraction into hexane) as 2-bromo-3-pentanol and 3-bromo2-pentanol. The rate constant for (()-SBr2 hydrolysis was 0.17 hr-1 at pH 8.2-9.7. The major product was the epoxide, (E)-stilbene oxide (95%). Traces of (Z)-stilbene oxide (less than 10% of the total epoxide yield; confirmed by GC/MS) were also formed, along with minor amounts of (E)-stilbene. (Z)Stilbene was not detected as a hydrolysis product. Both (E)and (Z)-stilbene oxide were found to be stable in buffered aqueous solutions at pH 9.2, and no isomerization of (Z)stilbene oxide to the E isomer was observed. The distribution of epoxides obtained from (()-SBr2 and the rapid nature of the hydrolysis together point to an SN1 mechanism to form a relatively stable benzyl carbocation that may be in equilibrium with a bridged bromonium ion. Reaction of the solvent or Br- with such a cationic intermediate has been invoked to account for (E)-stilbene obtained in the solvolysis of (()-SBr2 (46). Benzyl carbocations are thought to be capable of extensive C-C bond rotation (21), and the high E:Z ratio of the epoxides likely signifies that C-C bond rotation approaches an equilibrium in which the phenyl groups are largely trans to one another. This is an important observation because nucleophilic reduction of (()SBr2 may also involve intermediate benzyl carbocations. Reductive Dehalogenation of the Probe Compounds. The stereospecificity of the reductive elimination reactions is expressed as the percent E olefin present in the total olefin yield. Results for (()-SBr2 and threo-diBP are summarized in Figure 2. Note that the E and Z olefin products were stable

under the reaction conditions, neither undergoing reduction nor isomerization. Reduction by Cr(II). For (()-SBr2 and for both the diBPs, the rate of reduction was fast relative to that of hydrolysis, and disappearance exhibited pseudo first-order kinetics. (()SBr2 reacted with 0.06 mM Cr(II) to give (E)-stilbene as the sole observed product (Figure 2), consistent with the results of Mathai et al. (32). The second-order rate constant for this reaction was 3.7 M-1s-1. The E:Z ratio results from the eclipsing interactions in the intermediate radical, which drive bond rotation (see Figure 1). These radicals apparently persist long enough to attain a distribution in which essentially 100% are in the E-like configuration. High E:Z ratios (up to 17.9:1) have also been observed for reduction of (()-SBr2 by aromatic radical anions (30), which are believed to serve as outersphere SET reductants (11). Second-order rate constants for reaction of 10 mM Cr(II) with threo- and erythro-diBP were ∼0.012 M-1s-1 and 0.018 M-1s-1, respectively. Reduction of 2,3-diBPs by Cr(II) resulted in 68% (E)-2-pentene from the threo isomer (Figure 2) and 73% from the erythro isomer. These results parallel those of Kochi and Singleton (34) who observed a similar E:Z ratio in 2-butenes from the reduction of (()-2,3-dibromobutane by 68 mM Cr(II) in acetonitrile/water (30:10). The formation of both (Z)- and (E)-2-pentenes indicates that the free-radical intermediate persists long enough to undergo C-C bond rotation. Alkyl-substituted vicinal dibromides, unlike (()SBr2, are reduced to a mixture of E and Z olefins because the radical rotamers are closer in energy and therefore significant amounts of each exist at equilibrium. Reduction by Iodide. (()-SBr2 was reduced by iodide to form both (Z)-and (E)-stilbene (Figure 2; k ∼5.9 × 10-5 M-1s-1). Hydrolysis competed with reductive elimination. Stilbenes comprised 55% of the total products, of which 70% was (E)-stilbene. Mass balances were good (100-110%). The observation of some (E)-stilbene from the reduction of (()-SBr2 by I- might initially appear at odds with a nucleophilic mechanism (Figure 1b), which should yield only (Z)-stilbene. In contrast, a one-electron reductant such as Cr(II) yields only the E isomer. Lee (24) notes, however, that nucleophilic dehalogenation of (()-SBr2 in methanol most likely occurs via a mechanism in which a bridged halonium ion is in equilibrium with a benzyl carbocation. Because rotation about the C-C bond of such an open carbocation is hypothesized to occur freely, a halonium ion mechanism would also account for the formation of (E)-stilbene during the reduction of (()-SBr2 by I- in water. Mathai et al. (32) have also observed (E)-stilbene in the reduction of (()-SBr2 by iodide in organic solvents, ranging from 4% to 35% of the total stilbene. Close scrutiny of the data reveals that the % (E)-stilbene increases as the solvent becomes more polar. This may reflect increasing stabilization of charged intermediates in more polar solvents, favoring a mechanism involving a cationic intermediate. In contrast to the results obtained with (()-SBr2, threodiBP was reduced by iodide to form exclusively (Z)-2-pentene (with a second-order rate constant kI- of approximately 6 × 10-6 M-1s-1), and erythro-diBP was reduced by iodide to form exclusively (E)-2-pentene (with kI- ∼ 1 × 10-5 M-1s-1). Mass balances were essentially 100%. These results agree with previous studies, which demonstrated that simple aliphatic dibromides of the type RCHBrCHBrR′ (R, R′ ) alkyl) react with iodide primarily via concerted nucleophilic reductive elimination (Figure 1b) rather than via a halonium ion mechanism (21). For example, reduction of (()-2,3-dibromobutane by iodide yields 95-100% (Z)-2-butene in methanol/water (90:10, 59 °C) (47). The greater tendency of the diBPs to react via a concerted mechanism probably reflects lesser stabilization of a charged intermediate by alkyl versus phenyl substituents.

FIGURE 3. Reaction of threo-diBP (b) (a) with 4.2 g/L iron (3.2 m2/L) at pH 6.5-8.5; (b) with 0.83 g/L zinc (0.029 m2/L) at pH 6-7.5. All experiments were conducted in 0.1 M NaCl. Products are (Z)-2pentene (9) and (E)-2-pentene (0). Solid lines represent fits to data assuming pseudo first-order decay to observed products. Dashed lines represent mass balances measured at each timepoint. Reduction by Zerovalent Metals. Reduction of the diBPs followed pseudo first-order kinetics with Fe, Zn, Cu, and Al. Results for Fe and Zn are shown in Figure 3. Reaction rate constants normalized to metal surface area (kSA) for threodiBP varied somewhat with metal loading and with mixing speed, and ranged from 3 × 10-6 L‚s-1‚m-2 for Cu(0), to 0.7-1 × 10-4 L‚s-1‚m-2 for Fe(0), and 3-6 × 10-3 L‚s-1‚m-2 for Zn(0). The surface area of the Al(0) could not be measured, but the pseudo first-order rate constant for reaction of threodiBP with 4.2 g/L of Al(0) was 0.05 hr-1 (after accounting for hydrolysis). For Al(0) and Cu(0), throughout the reaction, the pH remained relatively stable at 8.5 and 6.5, respectively. For Fe(0), the pH at the beginning of the experiment was about 6.5 and drifted to 8.5 by the end of the timecourse. For Zn(0), the pH ranged from about 6 at the beginning of the experiment to 7.5 by the end. Each metal reduced both threo- and erythro-2,3-diBP in a highly stereospecific manner, resulting in >95% of the anti elimination product (Figure 2), similar to results obtained for reaction with iodide and consistent with earlier studies (27, 28, 32, 33). Mass balances were close to 100% for all metals except Al(0). The low mass balance (about 65%) observed for Al(0) was probably due to partitioning of the volatile pentene products into the large amount of H2 headspace that formed. No hydrogenolysis products (2- or 3-bromopentanes) were detected. The absence of products resulting from transfer of a proton from the solvent argues against a mechanism involving a free carbanion intermediate. VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Reaction of (()-SBr2 (O) (a) with 0.42 g/L iron (0.32 m2/L) at pH 7.8-8.6; (b) with 0.83 g/L zinc (0.029 m2/L) at pH 7.8. All experiments were conducted in 0.1 M NaCl. Products are (E)-stilbene (1) and (Z)-stilbene ([). Solid lines represent fits to data assuming pseudo first-order decay to observed products. Dashed lines represent mass balances measured at each timepoint. Because the adsorption and reactivity of surface-associated Fe(II) is highly pH-dependent (37, 42), as are the structure and thickness of the passivated layer (48), differences in reactivity and stereospecificity might be anticipated if adsorbed Fe(II) represented the principal reactive species in systems containing Fe(0). Hence, limited experiments were conducted with diBPs in which initial solution pH was varied by adding NaOH or HCl. Although the initial pH was not maintained, the rate of reaction was affected to a modest extent; the reaction was roughly 2 times faster at either high or low pH than at neutral pH. The stereospecificity changed at best only slightly with pH (Figure 2). The disappearance of (()-SBr2 in the presence of iron and zinc exhibited pseudo first-order behavior (Figure 4). Rate constants kSA were 1 × 10-3 and 7 × 10-3 L‚s-1‚m-2 for iron and zinc, respectively. The somewhat higher reactivity of (()-SBr2 relative to the less sterically hindered diBPs is counter to the trend that might be anticipated if reaction were initiated by an SN2 attack on carbon by the metal to form an organometallic intermediate, followed by a concerted syn-elimination of a metal bromide. Hydrolysis did not contribute significantly to the loss of (()-SBr2. Mass balances were essentially 100%, and hydrogenolysis products that might be indicative of a free carbanion intermediate were not detected. (()-SBr2 underwent reduction to a mixture of (Z)- and (E)-stilbenes. The percentage of (E)-stilbene in the olefin 2272

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mixture was 71% for iron and 70% for zinc. The E:Z ratio resulting from reduction of (()-SBr2 by Fe(0), Zn(0), and iodide is the same (within experimental error; Figure 2). Furthermore, reduction of (()-SBr2 in water by a variety of strong nucleophiles (including HS- and polysulfides) results in a similar percentage of (Z)-stilbene (45). This similarity in product distributions may indicate that reactions with metals and “nucleophilic” reductants all involve a common intermediate. Alternatively, it is possible that the mechanism varies but that exactly the same selectivity nevertheless results. Reduction by Surface-Associated Fe(II). Reduction of diBPs in the presence of goethite plus Fe(II) proved slow, even though the surface area of the goethite suspension (140 m2/L) was 40-200× greater than that of the Fe(0) particles. At pH 7.5, the yield of pentenes was less than 10%, but only the stereospecific isomer [(Z)-2-Pentene from threo-diBP and (E)-2-pentene from erythro-diBP] was detected. Because reduction was sluggish, hydrolysis of the probe compounds consumed most of the spiked mass. To speed up the reaction between the Fe(II)/goethite and the diBPs, some experiments were conducted at pH 10.5. Modest (17%) conversion of threo-diBP to 2-pentenes was achieved in 8 h. (E)-2-Pentene represented 3% of the 2-pentene products. The mass balance was 95% (consisting mostly of unreacted threo-diBP). At this high pH, however, Fe(II) is expected to precipitate as Fe(OH)2 (s). Control experiments in which a solution of Fe(II) at pH 10.5 was spiked with threo-diBP showed no production of pentenes despite the formation of a cloudy gray-green precipitate [presumably Fe(OH)2 (s)]. The rate of (()-SBr2 dehalogenation in goethite/Fe(II) systems could not be determined because of competition from hydrolysis. Neither the diBPs nor (()-SBr2 underwent reduction in goethite suspensions in the absence of Fe(II), or in homogeneous Fe(II) solutions. Mechanistic Interpretation. For both the diBPs and (()SBr2, the products observed from reduction by iron and zinc have essentially the same E:Z ratio as those encountered with the classic nucleophilic reductant, iodide. Although this might indicate that reactions occur in each case via a “twoelectron” transfer of halonium (X+), the pronounced anti stereospecificity could equally well reflect a situation in which transfer of the second electron is fast compared to the rate of C-C bond rotation in an intermediate radical. If we postulate that reactions in the heterogeneous systems proceed via SET, estimates of the energy barriers for C-C bond rotation in the intermediate radicals allow limits to be placed on the pseudo first-order rate constants corresponding to transfer of a second electron to the radical (k2). Lexa et al. (11) used the E:Z ratios of 4-octenes obtained in the reduction of D,L- and (()-4,5-dibromooctanes by radical anions to estimate a rotational barrier of 19-29 kJ/mol for the 4-bromooctyl radical. A similar rotational barrier (18-21 kJ/ mol) can be predicted (41) for the less-hindered 2- and 3-bromopentyl radicals using computational chemical techniques employing density functional theory (DFT), corresponding to a rate constant for C-C bond rotation of ∼109 s-1. To produce 70% (E)-stilbene from reduction of (()-SBr2 via SET, k2 would thus have to be of this magnitude. For the bromostilbene radical, DFT calculations (41) yield an estimated C-C bond rotation rate of 108 s-1. The reduction of the diBPs via stereospecific anti elimination by Fe(II) adsorbed to goethite might initially seem somewhat surprising, because Fe(II) would presumably serve as a one-electron reductant. Indeed, the Fe(II) organometallic species C3H5Fe(CO)2P(C6H5)3 has been shown to reduce both meso- and (()-3,4-dibromohexane to (E)-3-hexene in homogeneous organic solution (49). Amonette et al. (39) have hypothesized that the rate-determining step of the reaction of CCl4 with Fe(II) adsorbed to goethite involves simultaneous

transfer of two electrons within a termolecular complex involving two adsorbed Fe(II) atoms; this suggestion was based on the second-order nature of the reaction in adsorbed Fe(II) concentration. We note that a second-order dependence on the reductant concentration is characteristic of noncomplementary redox reactions (50); an alternative explanation could be simply that the reaction occurs via a rapidly reversible transfer of a halogen atom, followed by slower reaction of the resulting radical with a second adsorbed Fe(II) atom. For the Fe(II)/goethite system, we can evaluate the likelihood that transfer of a second electron could be faster than C-C bond rotation. Given the Fe(II) binding-site density on goethite (2.8 sites/nm2 from ref. 42), a simple calculation shows the sorbed Fe(II) atoms to be only about 0.44 nm apart on average. For a bromopentyl radical, the calculated characteristic diffusion time tD (computed as L2/2D, where D is the aqueous diffusion coefficient estimated according to ref 51) over such a distance L is 10-10 s. This is sufficiently fast relative to C-C bond rotation to account for the high stereospecificity observed in the Fe(II)/goethite system. Transfer of the second electron is even more likely to compete with C-C bond rotation for reduction of (()-SBr2, and also for reductions in the zerovalent metal systems, for which reaction may occur with the naked metal (in corrosion pits). The heterogeneous reduction of the vicinal dibromides studied herein may be comparable to their reduction by glassy carbon electrodes (11). Even though the latter reaction occurs via outer-sphere SET, vicinal dibromides react completely stereospecifically, in marked contrast to their outer-sphere SET reduction by aromatic radical anions in solution. A comparable situation may occur in the photoreductive degradation of CCl4 on TiO2 surfaces (52), for which detailed kinetic studies indicated that CCl4 reduction to dichlorocarbene occurred via two single-electron transfers rather than via a concerted two-electron-transfer step. In all of these cases, reduction occurs at a surface containing an excess of reducing equivalents, and transfer of the second electron is fast relative to radical isomerization or diffusion away from the surface. For these heterogeneous systems, the stereospecificity therefore cannot be used to discriminate between a “twoelectron” versus an SET mechanism, nor can we conclude from our results whether electron transfer is an inner- or an outer-sphere process. Nonetheless, our studies indicate that the lifetime of a putative radical intermediate formed by reactions with zerovalent metals is so short as to preclude characteristic free-radical reactions such as self-coupling and hydrogen-atom abstraction from dissolved organic matter. In order for other bimolecular reactions to compete with transfer of the second electron, they would have to be extremely fast, and one reagent would have to be present at an abundance greater than typically observed in the dilute aqueous solution that even contaminated groundwater comprises. For example, a diffusion-limited reaction, such as self-coupling of free radicals (which occurs with a secondorder rate constant on the order of 1010 M-1s-1) could begin to compete with the second one-electron transfer only if the local concentration of the radical species was greater than ∼10-2 M. Such a high concentration of reactive transients is extremely unlikely in the presence of powerful reductants such as zerovalent metals. Any intermediate radical that may be formed will be short-lived and unlikely to escape from the metal [or (hydr)oxide] surface before being reduced to a relatively stable closed-shell species. This raises the possibility that observed coupling products may result from a process other than the collision of two free radicals, perhaps via an intramolecular mechanism involving rearrangement of a dialkyl organometallic intermediate, potentially one bonded to the metal-water interface rather than free in solution.

This possibility will be explored further in a subsequent publication.

Acknowledgments We are grateful to Dr. Gary Posner and Ben Woodard for their help in obtaining NMR spectra of synthesis products. We are also indebted to Dr. Alan Stone and three anonymous reviewers for their helpful insights and suggestions, which greatly improved this work. This study was partially funded through Contract No. F08637 95 C6037 from the Department of the Air Force, as well as through an NSF Graduate Student Fellowship to L. A. T. and an NSF Young Investigator Award (Grant No. BES-9457260) to A. L. R.

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Received for review February 18, 2000. Revised manuscript received February 22, 2001. Accepted March 5, 2001. ES0010195