Article pubs.acs.org/IC
Heterogeneous Organophosphate Ethanolysis: Degradation of Phosphonothioate Neurotoxin by a Supported Molybdenum Peroxo Polymer Louis Y. Kuo,* Andrew Bennett, and Qianli Miao Department of Chemistry, Lewis & Clark College, Portland, Oregon 97219, United States S Supporting Information *
ABSTRACT: A polystyrene-supported molybdenum peroxo material [Mo−Y(s)] was applied toward the oxidative degradation of the organophosphate neurotoxin O,S-diethylphenyl phosphonothioate (1) through ethanolysis. In addition to the operational advantages of the heterogeneous reactivity, oxidative ethanolysis with a 10-fold excess of hydrogen peroxide yields only P−S bond scission to produce diethylphenyl phosphonate and ethyl sulfate. This is the first report of a molybdenum solid support that promotes the degradation of sulfur-containing organophosphate with the turnover benefits of heterogeneous catalysis. The activation parameters of 1 ethanolysis by Mo−Y(s) (Ea = 57 ± 6 kJ/mol and ΔS⧧ = −124 ± 21 J/mol·K) and by the model compound oxodiperoxo(pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2-carboxylic acid) monohydrate (3; Ea = 55 ± 5 kJ/mol and ΔS⧧ = −154 ± 15 J/mol·K) are almost identical for the oxidation of thioanisole by 3. This suggests that the rate-determining step for 1 ethanolysis is sulfur oxidation to form an intermediate phosphonothioate S-oxide, which subsequently undergoes nucleophilic attack by the ethanol solvent to form diethylphenyl phosphonate and ethyl sulfate. Evidence for the formation of this S-oxide intermediate and the postulated ethanolysis mechanism is provided.
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INTRODUCTION This is the first report of a heterogeneous system for the oxidative degradation of a phosphonothioate neurotoxin. Phosphonothioates of the form RP(O)(SR1)(OR2)1 are acetylcholine esterase inhibitors that have applications as pesticides2 and, most notoriously, as chemical warfare agents.3 As one of the most dangerous toxins, the nerve agent VX [R = CH3, R1 = CH2CH2N(iPr)2, and R2 = Et] has an LD50 = 0.015 mg/kg;4 just 2 drops would kill an average 70 kg adult. Even phosphorothioate pesticides such as parathion5 [ArOP(S)(OEt)2] undergo tautomerization (eq 1)6 to form phosphonothiolate with the same P−S and P−O ligation as VX.7,8
to yield the corresponding S-oxide that subsequently underwent nucleophilic substitution to yield the phosphonate [EtOP(O)(Nu)CH3] and ethylsulfenic acid; the latter quickly oxidized to ethyl sulfonate. In this case, exclusive P−S scission was always seen, which is the desired degradation pathway for phosphonothioates; P−O scission yields another phosphonothioate that sometimes is as toxic as the original compound.14 The S-oxide of phosphonothioate was further investigated by Segall and Casida,15 who found it to be extremely reactive and even implicated it as a potential intermediate in the biological oxidation of phosphonothioates.16 These homogeneous investigations prompted us to look at heterogeneous systems that accomplish the same oxidative degradation of phosphonothioate neurotoxins. To that end, we chose a molybdenum peroxide complex17 immobilized on a polystyrene support via a triethylenetetramine linker. This immobilized molybdenum system is easily made from molybdenum powder/hydrogen peroxide (H2O2) and commercially available CR20 Diaion beads (Mistubushi Chemical Co.); that molybdenum is an abundant metal further adds to the utility of this oxygenation system. The CR20 Diaion beads are polystyrene chelating resins with polyamine functional groups, as drawn in the Table of Contents figure; they have
Several prior reports9,10 highlight the use of the peroxy functionality for oxidative degradation of phosphonothioates. Given the prior literature11,12 on sulfide oxidation by molybdenum peroxo catalysts, the plausible strategy for degrading phosphonothioate neurotoxins entails sulfur oxidation, followed by the decomposition of phosphonothioate S-oxide. Specifically (Scheme 1), Yang and co-workers13 oxidized O,S-diethylmethyl phosphonothioate (30 mM) with excess aqueous m-chloroperoxybenzoic acid (m-CPBA; 240 mM) © 2017 American Chemical Society
Received: June 18, 2017 Published: August 2, 2017 10013
DOI: 10.1021/acs.inorgchem.7b01545 Inorg. Chem. 2017, 56, 10013−10020
Article
Inorganic Chemistry Scheme 1. Oxidative Degradation of Phosphonothioate via S-Oxide
Figure 1. Organophosphate structures including the phosphonothioate chemical warfare agent VX and its analogue 1. The title phosphonothioate, O,S-diethylphenyl phosphonothioate (1), was made according to prior literature preparations21 with reagents that were purchased from TCI (Portland, OR). The synthesis of 13C-enriched phosphonothioate O-ethyl-S-methyl(13CH3)phenyl phosphonothioate (2), an analogue of 1 for 13C NMR studies, was done according to prior literature reports.22 The solution analogues oxodiperoxo(pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2carboxylic acid) monohydrate (3) and oxoperoxo(pyridine-2,6-dicarboxylato)aquomolybdate(VI) (4) were made according to the procedure of Mares and co-workers.23 Characterization was done with 13C NMR and, in the case with the diperoxomolybdate (3), SC-XRD (Supporting Information S1); this compound was previously characterized with single-crystal X-ray crystallography. The supported molybdenum peroxo catalyst, abbreviated as Mo−Y(s), was synthesized according to the procedure of Kurusu and co-workers.17 Molybdenum powder (0.75 g, 7.8 mmol) was gradually added to 10 mL of aqueous 30% H2O2 (327 mmol) over the course of 20 min. The addition process took place over an ice bath to avoid rapid heating or boiling of the solution. The solution initially became opaque with a dark-green hue but transitioned to translucent (bright) yellow over the course of the molybdenum addition, thus the reason for the Mo−Y(s) naming by the original authors. Diaion CR20 polymeric beads (2.15 g) were slowly added to the stirring yellow suspension over the course of 20 min. The suspension was stirred slowly (to avoid damage of the beads) overnight. After ∼18 h of light stirring, the bright-yellow beads were filtered on a fritted funnel and washed with deionized water (100 mL) followed by diethyl ether. Elem anal. Calcd for Mo−Y(s): C, 56.66; H, 5.80; N, 3.67; Mo, 11.68. Elem anal. Calcd for CR20 (no molybdenum added): C, 72.70; H, 8.95; N, 8.35. Kinetic Runs and Oxygen Species Analyses. The title reaction was carried out in 1 dram vials with 100 mg of the Mo−Y(s) support (0.12 mmol of molybdenum). The ethanol (EtOH) solvent (900 μL) and deuterated methanol (MeOH; 50 μL for a lock source) were added to Mo−Y(s), followed by the addition of 15 μL of aqueous 30% H2O2 (0.15 mmol), which turned the solution above the beads slightly yellow. This suspension was allowed to sit for ∼30 min until the mild bubbling ceased before the addition of 1. Kinetics commenced with the addition of 1 (3 μL, 0.014 mmol). At specific time intervals, the solution above the Mo−Y(s) beads/support was pipetted from the vial and into a 5 mm NMR tube without collection of any of the 0.5 mm beads. Following a 31P or 13C NMR spectrum, this solution was pipetted back to the 1 dram reaction vial containing the Mo−Y(s) beads/support. This process was repeated at various time points for kinetic runs. The singlet oxygen sensing experiment followed the same procedure as that of Hwang and co-workers,24 which began with a stock solution of fluorescent SOSG made up of 100 mg of SOSG in
been used to selectively chelate heavy-metal ions. This molybdenum peroxide support successfully epoxidized alkenes with the addition of tert-butyl peroxide and demonstrated recyclability in the epoxidation chemistry, which makes this system catalytic. An exhaustive literature search on supported molybdenum peroxide polymers18,19 did not find their use in the oxidative degradation of phosphonothioate neurotoxins. As such, this is the first report of a polymer-supported molybdenum material that effectively carries out ethanolysis of a phosphonothioate neurotoxin. The title compound of this report is O,S-diethylphenyl phosphonothioate (1),20 which was used to discern the chemistry of phosphonothioates. In addition, 1 serves as an analogue for the chemical warfare agent VX (Figure 1).3 This report describes the polymer-supported molybdenum [abbreviated as Mo−Y(s)] material, followed by its use for heterogeneous 1 ethanolysis; the original reason for calling this support Mo−Y is spelled out in the Materials and Methods section. We conclude with several thermodynamic and NMR studies on a discrete solution model that seeks to elucidate a possible mechanism for this successful neurotoxin oxidation.
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MATERIALS AND METHODS
Equipment and Materials. All 31P, 1H, and 13C NMR spectra were taken on a Bruker Avance-300 spectrometer at 121.495, 300.130, and 75.468 MHz, respectively. IR spectra of the molybdenum compounds were obtained with a Nicolet 380 FT-IR spectrometer as a KBr pellet. Reactions for Arrhenius plots were run either in a Fischer Scientific Isotemp Refrigerated Circulator (model 9000) or on an IKA RCT hot plate/stirrer. All UV−vis spectra were obtained with an Agilent 8453 UV−vis spectrometer, and fluorescence was done on a PerkinElmer LS 55 fluorimeter. Single-crystal X-ray diffraction (SC-XRD) characterization was carried out on a Bruker Smart X2S diffractometer. Molybdenum powder was from Alfa Aesar (99.999%), and aqueous H2O2 (27−30%) was from Sigma-Aldrich. The Diaion CR20 polystyrene beads were a gift from Mitsubishi Chemical Co., and the Singlet Oxygen Sensor Green (SOSG) detection kit was from Molecular Probe (Eugene, OR). Elemental analyses were conducted by ALS Environmental (Tuscon, AZ), and energy-dispersive X-ray (EDX) spectroscopy was carried out at the Center for Electron Microscopy and Nanofabrication facility at Portland State University. Gas chromatography/mass spectrometry (GC/MS) analysis was performed at the Pankow Group Mass Spectrometry Facility at Portland State University. 10014
DOI: 10.1021/acs.inorgchem.7b01545 Inorg. Chem. 2017, 56, 10013−10020
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Inorganic Chemistry 330 μL of MeOH for a final concentration of 5.0 mM. From this stock solution, 0.1 or 1 mM SOSG was added to the aforementioned Mo−Y(s)/MeOH/H2O2 suspension (no 1). The liquid above the Mo−Y beads was transferred to the fluorimeter, and the 525 nm emission was recorded with both 382 and 505 nm excitation.
authentic addition (Supporting Information S5). Throughout this study, these were the main two signals, and at 30 °C, the reaction was complete in ∼3 h with 30 μL of 30% H2O2(aq), as shown in Figure 2. That only the phosphonate is produced
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RESULTS AND DISCUSSION Immobilized Molybdenum Support. The immobilized molybdenum polymer, Mo−Y(s),17 was characterized by elemental analysis and IR spectroscopy. The oxidation of Mo(s) with H2O2 initially generated a canary-yellow solution referred to as Mo−Y(l). This yellow solution was postulated by Kurusu and co-workers to be a polyoxomolybdate consisting of a chain of Mo−O−Mo (μ-oxo) linkages that carried out homogeneous alkene oxidation chemistry with H2O2 addition.25 This polyoxomolybdate [Mo−Y(l)] was immobilized on the CR20 Diaion polyamine support (0.5 mm beads).17 Our analysis results were consistent with the previous report wherein the molybdenum elemental analyses gave 11.68% vis-à-vis the 10%17 reported previously. In addition, the IR spectrum of crushed Mo−Y(s) beads revealed a prominent MoO 943 cm−1 stretch with weaker peroxo stretches at 850 and 580 cm−1. In addition, there is a broad OH stretch at 3450 cm−1, but this prominent stretch was already present in the CR20 Diaion polymer prior to Mo(s)/H2O2(aq) addition (Supporting Information S2). There is a strong 906 cm−1 stretch in Mo−Y(s), and a prior report on a dried solution of Mo−Y(l) attributed this stretch to a μ-oxo functional group.26 Furthermore, EDX spectroscopy on Mo−Y(s) showed that modification of the CR20 Diaion beads resulted in the surface attachment of molybdenum that was previously absent (Supporting Information S3A). To this end, we were interested in calculating the ratio of oxygen to molybdenum through both elemental and EDX analyses. The actual calculations are detailed in the Supporting Information S4; from the elemental analysis results, the O/Mo ratio comes out as 5.3:1. This is close to the EDX data, which give an O/Mo ratio of ∼3:1. It should be noted that the molybdenum content in the Mo−Y(s) beads does vary from batch to batch, as determined from EDX, and Supporting Information S3B gives a representative variation in the weight percent of molybdenum from a different batch preparation. The oxygen atoms on Mo−Y(s) are a combination of MoO, Mo(OO), and Mo−O−Mo functionalities that originate from the added H2O2(aq). Heterogeneous Ethanolysis Reaction. The title reaction was 1 ethanolysis (0.014 mmol) by excess Mo−Y(s) (0.12 mmol of molybdenum, 100 mg of Mo−Y beads) in EtOH (900 μL of EtOH and 50 μL of MeOH-d4) in excess H2O2 (0.15 mmol, 15 μL of a 30% aqueous solution). Ethanolysis was chosen based on prior reports27 for the heterogeneous sulfur oxidation by a silica-immobilized oxovanadium(IV) salicylidene complex; EtOH was one of the optimal solvents found in the oxidation of sulfides. In addition, the product of ethanolysis was a neutrally charged phosphonate (vide infra); hydrolysis under alkaline conditions yielded an anionic phosphonate that was resistant to further nucleophilic attack. In that connection, an alcohol media allowed further degradation of the product(s) by hydrolysis or incineration. Finally, ethanolysis was chosen because the phosphonothioate 1 was very soluble in EtOH. Ethanolysis was tracked with 31P NMR in solution by monitoring the disappearance of the starting material 1 (47 ppm) with concomitant growth of the product signal (19.5 ppm).28 This product was identified as diethylphenyl phosphonate through
Figure 2. Heterogeneous 1 ethanolysis (0.014 mmol) by Mo−Y(s) support (100 mg, 0.12 mmol) in 900 μL of EtOH (+50 μL MeOH-d4 for the deuterium lock) with the addition of 30 μL of 30% H2O2 (0.30 mmol) at 30 °C. This demonstrates only the production of diethylphenyl phosphonothioate at 19.5 ppm; only P−S scission. Actual kinetic runs were done at room temperature and with 15 μL of H2O2 for a reasonable sampling over the course of a 18 h reaction time.
means that ethanolysis proceeds via selective P−S scission, which is the desired pathway for phosphonothioate degradation.3 Therefore, the overall fate of the phosphorus for phosphonothioate 1 is shown in eq 2. Mo−Y(s) failed to promote the degradation of 1 in acetonitrile or acetone (with 30 μL of H2O2), which underscores the value of ethanolysis29 by Mo−Y(s) in a nucleophilic alcohol solvent; the same reaction
occurs in MeOH to yield ethylmethylphenyl phosphonate. In addition, di-tert-butyl peroxide did not promote any ethanolysis under the same conditions. In this connection, we do see an intermediate 26.7 ppm 31P (broad) singlet when the reaction medium is switched to a less nucleophilic 7:3 EtOH/acetone medium (Supporting Information S6). Upon completion of this slower ethanolysis, this singlet goes away. Prior reports13 have attributed this singlet as the S → O oxide upon oxidation of O,S-diethylmethyl phosphonothioate by m-CPBA (Scheme 1) in a nonnucleophilic solvent (tert-butyl alcohol). Given the oxidative conditions of this work, this phosphonothioate S-oxide of 1 is a plausible intermediate in ethanolysis, which will be addressed at the end of this discussion. Actual kinetic (quantitative) runs in this heterogeneous investigation employed conditions with slower ethanolysis (see the Materials and Methods section) to accurately track the reaction over a longer (∼18 h) time period. An initial lag/delay was observed in the first hour, which corresponded to the time for mild bubbling from the Mo−Y(s) beads (in EtOH and H2O2) to cease. As such, all ethanolysis allowed a half-hour “wait time” for this bubbling to diminish, wherein the reaction medium consisted of only the Mo−Y(s) beads in EtOH/ MeOH-d4 and 15 μL of 30% H2O2. Headspace GC of the gas 10015
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Inorganic Chemistry Table 1. Control Reactions for 1 Ethanolysis (900 μL) by Mo−Y(s) with 50 μL of MeOH-d4 for the NMR Lock control no.
conditions
result
1 2 3 4 5 6
EtOH + H2O2(aq) + Mo−Y(s); for conditions for kinetic runs, see the Materials and Methods section EtOH + H2O2(aq) EtOH + Mo−Y(s); no H2O2 solution/filtrate of the EtOH + H2O2(aq) + Mo−Y(s) reaction EtOH + H2O2(aq) + CR20 Diaion (no Mo/H2O2) beads filtration of control no. 1 to remove EtOH/H2O2(aq), followed by replacement with just EtOH
completion of 1 ethanolysis in 18 h no ethanolysis seen for up to 3 days no ethanolysis seen for up to 3 days no ethanolysis seen for up to 3 days no ethanolysis seen for up to 3 days no ethanolysis seen for up to 3 days
“depleted” Mo−Y(s) was tested for activity on 1, and no ethanolysis was observed. This indicates the observed activity is indeed a function of the supported molybdenum species; whatever material that leaches from Mo−Y(s) in these turnover attempts is decomposed molybdenum that is incapable of 1 ethanolysis. After the second equiv of 1 was added to Mo−Y(s), we employed the protocol set by Kurusu in regenerating the support by washing away the reaction products, followed by incubation of the 100 mg of Mo−Y(s) beads in 10 mL of 30% H2O2(aq) at 40 °C for 3 h. Mo−Y(s) was filtered and then dried in vacuo. These regenerated Mo−Y(s) beads were used for another three complete cycles of 1 ethanolysis. Altogether, regeneration of the beads with this protocol achieves five complete cycles of 1 ethanolysis. This result demonstrates a proof-of-concept feature for the regenerative utility of the Mo− Y(s) beads. Product and Singlet Oxygen Species Analysis. Given the oxidative conditions of the ethanolysis, we examined the fate of the alkylthiolate leaving group with 13C NMR for methanolysis of O-ethyl-S-methyl(13 CH 3 )phenylphosphonothioate (2)22 in a MeOH-d4 solvent. Compound 2 differs from 1 by just one carbon on the alkylthiolate leaving group, and prior synthetic protocols22 placed a 13C-labeled methylthiolate leaving group for 13C NMR tracking. Consistent with prior reports9,30 on the oxidative degradation of phosphonothioates, 13C NMR reveals the alkylthioate leaving group becomes alkylsulfate; in the case for methanolysis of 2, methanesulfonic acid is formed (i.e., 13C NMR singlet at 38 ppm), as shown in Figure 3. The additional minor peak at 35.5 ppm is from esterification of the methanesulfonic acid with CD3OD (solvent) to form methyl-d3-methanesulfonate, which displaces the methyl signal upfield by ∼3 ppm.31 The CD3 carbon signal (not 13C-enriched) of methyl-d3methanesulfonate is not visible because of the absence of the 1 H−13C NOE. These 13C chemical shifts were also found for
generated in the bubbling revealed it to be only air; no enrichment of oxygen was detected. The allowance of this wait time resulted in first-order kinetics for the fraction of the 1 signal versus time. Ethanolysis has a first-order dependence in the H2O2 concentration [15−30 μL of H2O2(aq)] and in the Mo−Y(s) polymer (25−100 mg). The upper limit of 100 mg (0.12 mmol of molybdenum) of Mo−Y(s) was used because this was the saturation point; any further increase in the amount of Mo−Y(s) did not yield increased 1 ethanolysis rates by the solid support. A few critical controls were run to confirm that ethanolysis was indeed carried out by the supported molybdenum polymer. The controls sought to address possible leaching of the molybdenum active species from the support and that 1 ethanolysis was done by H2O2 + EtOH solvent alone. The conditions of ethanolysis by Mo−Y(s) in EtOH/H2O2 resulted in the complete degradation of 1 overnight (∼18 h) at room temperature. The negative controls set out in Table 1 are the result of at least 3 days of monitoring (in 50 μL of MeOH-d4 for the NMR lock) that revealed no ethanolysis (i.e., no product peak at 19.5 ppm). These controls clearly show that (1) H2O2 is necessary for any ethanolysis to take place, (2) the unmodified CR20 Diaion beads are incapable of ethanolysis without the bound molybdenum, and (3) whatever material that leaches from the Mo−Y(s) beads is incapable of ethanolysis. Taken together, they reveal that the molybdenum complex on the CR20 Diaion support is responsible for ethanolysis. The H2O2 requirement is further underscored by the last control (no. 6) in Table 1. This control is identical with the general format of the reaction where H2O2 (15 μL) is incubated with Mo−Y(s) in EtOH for half an hour prior to the addition of 1. However, when the ethanolic solvent (including 15 μL of H2O2) is removed and replaced with only EtOH, no reaction takes place, which shows the necessity of having H2O2 around at all times during ethanolysis. The question of regenerating the support lies at the heart of heterogeneous reactions, and this was addressed by Kurusu and co-workers17 for the epoxidation of alkenes by the Mo−Y(s) polymer. As such, we found that 100 mg of Mo−Y(s) (0.12 mmol of bulk molybdenum) degrades almost an equimolar amount of phosphonothioate (30 μL of 1, 0.14 mmol) through ethanolysis (15 μL of 30% H2O2) at 40 °C (∼24 h) twice. Upon the third addition of 30 μL of 1 to Mo−Y(s) (0.28 mmol of 1 was already degraded), ethanolysis went to 50% completion when H2O2 was present. An EDX analysis of the Mo−Y(s) polymer after the second round of 1 ethanolysis revealed no significant drop in the molybdenum content with the depleted Mo−Y(s). The Mo−Y(s) sample started with 36.1 ± 10.5 wt % molybdenum, and after the second run, it was measured to be 32.3 ± 3.1 wt % (Supporting Information 3C), a small but statistically insignificant drop in the molybdenum content after the second round of 1 ethanolysis. Nevertheless, the solution from this
phosphonothioate oxidation by m-CPBA.13 Possible mechanistic pathways for oxidative ethanolysis will be discussed later. Prior results32 show that MoO42− catalyzes the disproportionation of H2O2 to water and singlet oxygen in micelles.33 This prompted an investigation of singlet oxygen formation by Mo−Y(s) that could oxidize the sulfur34 of 1. As such, we used the commercially available SOSG35 reagent to detect the presence of 1O2 when Mo−Y(s) is treated with H2O2 in EtOH. SOSG is specific for 1O2, does not fluoresce with radicals or hydroxyl radicals, and has a picomole detection level. The results clearly show no fluorescence (525 nm emission) in 10016
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Inorganic Chemistry
and H2O2 contents) be comparable between the homogeneous Mo−Y(l) and the heterogeneous Mo−Y(s) ethanolysis of 1. The additional water actually slows the Mo−Y(s)-mediated reactions relative to the NMR in Figure 2. To a first approximation, 1 ethanolysis in the homogeneous liquid phase [Mo−Y(l)] is slower by 2-fold compared to the heterogeneous reaction with the Mo−Y(s) beads under the same molar amount of H2O2 and molybdenum (Figure 4). While this is not a dramatic difference, the operational and regenerative advantages with heterogeneous Mo−Y(s) for 1 ethanolysis make these beads superior to the solution polymeric Mo−Y(l) homogeneous system. We were interested in a discrete EtOH-soluble molybdenum oxo complex that could test the role of key functional groups in sulfur oxidation chemistry.37 Specifically, this solution model seeks to examine the role of a molybdenum peroxo functionality with known sulfide oxidization chemistry.11,12 To that end, Di Furia and co-workers used oxodiperoxo(pyridine-2-carboxylato)molydate(VI) (3) to oxidize thioanisole to the corresponding sulfide (eq 5).38 The hypothesis seeks to test whether this sulfur oxidation could be employed in the oxidative degradation of phosphonothioate 1.
Figure 3. 13C NMR spectrum of 2 methanolysis in CD3OD by Mo−Y(s) that has the 13C-enriched methylthiolate leaving group. Starting 2 has a doublet (2JCP = 6.7 Hz) at 11 ppm that completely disappears after 18 h (30 °C in CD3OD) to yield only two prominent product singlets at 38 and 35.5 ppm described in eq 3.
the presence of SOSG that is above the background when excited at both 382 and 505 nm. The possibility of a radical reaction mechanism was also addressed, and the rate of 1 ethanolysis by Mo−Y(s) was examined with an equimolar amount of 2,2,6,6tetramethylpiperdine-1-oxyl,36 a known radical scavenger. Under the identical conditions set in Figure 3, the radical scavenger had no effect on 1 ethanolysis into diethylphenyl phosphonate. A similar observation was made by Ren and co-workers12 in their thioanisole oxidation by a polyoxomolybdate, and it suggests that while radical intermediates may be present, they do not affect the oxidative degradation reaction (eq 2). Solution Model with a Discrete Coordination Complex. Early work by Kurusu examined homogeneous alkene oxidation by a homogeneous solution of molybdenum powder in H2O2(aq),25,26 which was called Mo−Y(l). It was postulated that this Mo−Y(l) liquid has the Mo−O−Mo (μ-oxo) array that was disrupted by H2O2 to form a molybdenum perhydroxyl group (eq 4); the molybdenum-bound hydroperoxide was the active species in the homogeneous oxidation of alkenes.25
Compound 3, a diperoxo anion, was soluble in EtOH, and its chemistry was compared to Mo−Y(s). There were minor differences in the IR spectrum (Supporting Information S7) of 3 and Mo−Y(s) with regard to the intensity of the peroxo stretches. Nevertheless, 0.10 mmol of 3 successfully degraded 1 (0.014 mmol) in a homogeneous solution. Specifically, hydrolysis of 1 in water was rapid (∼3 h) with excess (7 fold) 3 (no H2O2). Ethanolysis of 1 by 3 was 3 times as slow as the heterogeneous [Mo−Y(s)] system under the same conditions. The relatively slower rate of 1 ethanolysis by this solution model (3) underscores that the active species in the heterogeneous reaction [by Mo−Y(s)] is the supported molybdenum oxoperoxo complex. Had the active species in the heterogeneous ethanolysis been a leached complex like the model solution complex, then 3 would have exhibited a much faster reaction than Mo−Y(s). Ethanolysis of 1 by 3 in EtOH yielded only the 19.5 ppm product 31P NMR signal (Figure 2), which demonstrated specific P−S scission. In addition, the 13C NMR spectrum (Supporting Information S8) for methanolysis (CD3OD) of 13C-enriched 2 by 3 gave the same spectral features as Figure 4 for the Mo−Y(s) heterogeneous system. Ethanolysis of 1 was first-order in 3 and H2O2 within the concentrations used in the study; similar to the case with Mo−Y(s). It should be noted that the monoperoxo complex 4 was totally inactive in degrading 1 through either hydrolysis or ethanolysis. Complex 3 is a convenient discrete solution model for the chemistry Mo−Y(s) in terms of the thermodynamics of ethanolysis
Therefore, we were interested in seeing how the solution form of the Mo−Y(s) beads functions in carrying out 1 ethanolysis under identical conditions. Thus, prior to adding the Diaion support to the canary-yellow molybdenum powder/H2O2(aq) solution, a fraction of Mo−Y(l) was used to carry out 1 ethanolysis. The amount of Mo−Y(l) used corresponded to the molar amount of molybdenum in Mo−Y(s) employed in a heterogeneous 1 ethanolysis reaction. Specifically, 167 μL of a 0.79 M molybdenum solution in 30% H2O2(aq) [0.375 g of Mo(s) in 5 mL of 30% H2O2(aq)] gives 0.13 mmol of molybdenum and 1.5 mmol of H2O2. This is similar in the molybdenum content (Figure 2) for the Mo−Y(s) heterogeneous reactions, but there is 5 times more H2O2. As such, the heterogeneous Mo−Y(s) reactions employed 150 μL of 30% H2O2(aq) in order to have all conditions (i.e., the molybdenum 10017
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Figure 4. Comparison of 1 ethanolysis (30 °C) by Mo−Y(l) in the homogeneous liquid phase versus the Mo−Y(s) heterogeneous reaction under comparable conditions where the molar amounts of molybdenum (0.13 mmol) and H2O2 (15 mmol) are equal. For the homogeneous Mo−Y(l) reactions, 167 μL of the molybdenum/H2O2(aq) solution (0.375 g of molybdenum powder in 5 mL of 30% H2O2) was used to degrade 3 μL of 1 (0.014 mmol) in 1 mL of EtOH. In order to keep the same H2O2, it entailed using more (150 μL) H2O2(aq) than the usual kinetic runs (15 μL) for the heterogeneous Mo−Y(s) reactions. The additional signal at 26 ppm for both cases is the intermediate S → O oxide (Supporting Information S6). The small upfield 17 ppm signal is possibly the result of hydrolysis of 1.
against phosphonothioate 1. Arrhenius plots (30−70 °C) for 1 ethanolysis by 3 and by Mo−Y(s) (Supporting Information S9) gave activation parameters set in Table 2.
activation parameters in Table 2. These thermodynamic results suggest 3 serves as a solution model for ethanolysis of phosphonothioate 1 by the Mo−Y(s) polymer through the assumption that sulfur oxidation is the rate-determining step in the ethanolysis reaction. In terms of presenting a hypothesis for the ethanolysis mechanism, a critical clue is the fact that H2O2 needs to be present in 1 ethanolysis by Mo−Y(s) at all times (Table 1,
Table 2. Activation Parameters of 1 Ethanolysis by Mo−Y(s) and Solution Model 3 Ea(EtOH) (kJ/mol) ΔS⧧(EtOH) (J/mol·K)
3 solution model
Mo−Y(s)
55 ± 5 −154 ± 15
57 ± 6 −124 ± 21
Interestingly, the two systems have almost identical activation energies and the large negative entropies of activation for both Mo−Y(s) and 3 are reasonably close to one another. A similar thermodynamic observation was noted in hydrolysis of the pesticide disulfoton in homogeneous and heterogeneous (iron oxide) media;39 the respective ΔS⧧ were −42 and −43 J/mol·K. The large negative ΔS⧧ in Table 2 indicates a bimolecular process or one that goes through an ordered transition state. Prior 17O NMR studies40 showed the peroxo oxygen atoms in 3 to be electrophilic, and this has a useful purpose in sulfur oxidation.40 Therefore, molybdenum peroxide 3 oxidizes sulfides, phosphines, and alkenes, wherein the electrophilc peroxo group undergoes nucleophilic attack.40 Indeed, our activation parameters in Table 2 for 1 ethanolysis by both Mo−Y(s) and 3 are similar to those of the thioanisole oxidation (eq 5) reported by Di Furia and co-workers.41 In their study, they used 3 and obtained ΔH⧧ = 53 kJ/mol and ΔS⧧ = −125 J/mol·K, which are close to our ethanolysis
control no. 6). This implies that the first step is formation of the active molybdenum peroxo functionality with the addition of H2O2. This could either occur from conversion of the MoO group (prominent IR stretch at 943 cm−1) to a Mo(OO) peroxo42 (eq 6) or scission of a Mo−O−Mo group (prominent IR stretch at 910 cm−1) to a molybdenum-bound perhydroxy group, as described earlier (eq 4). Both proposed Mo−Y(s) activations have prior literature precedence. In the case of eq 5, Kurusu correlated the active oxygen content with the IR signature of this key functional group for alkene oxidation by Mo−Y(l) in a homogeneous system.25,26 Another precedence for this work lies with VX oxidation by MoO42− in H2O2 in microemulsions.43 It was discovered that, above 0.10 M H2O2, MoO4 2− exists as an equilibrium mixture of the molybdate triperoxo and molybdate tetraperoxo species.
Scheme 2
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Inorganic Chemistry The latter complex, Mo(OO)42−, was mainly responsible for the oxidative degradation of VX through direct oxygen transfer to the sulfur; the triperoxo MoO(OO)32− reacts much slower.43 At the bare minimum, a diperoxo functionality is necessary, which is consistent with solution work by Woo and co-workers, who used oxazine complexes of molybdenum diperoxo functionalities for sulfide oxidation.44 These two observations suggest that the first step is the conversion of MoO and/or Mo−O−Mo functionalities of Mo−Y(s) to oxygenated molybdenum complexes with H2O2. Following the formation of “activated” species, the nucleophilic sulfur of 1 attacks the electrophilic oxygen atoms of the molybdenum peroxo functional group to yield the phosphonothioate S-oxide (Scheme 2). It is proposed that this sulfur oxidation is the rate-determining step based on the measured activation parameters (Table 2) for 1 ethanolysis by 3. Indeed, a similar phosphonothioate S-oxide has been postulated in the oxidative detoxification of O,S-diethylmethyl phosphonothioate (Scheme 1),13 which we see by 31P NMR spectroscopy in a 7:3 EtOH/acetone mixture. This unstable phosphonothioate S-oxide then undergoes rapid ethanolysis with the solvent to form diethylphenyl phosphonate and ethylsulfenic acid. The sulfenic acids are known to oxidize readily to the sulfate,45 which is evident in our 13C NMR spectrum of the final product when both 1 and 13C-enriched 2 undergo ethanolysis with Mo−Y(s).
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of chiral organophosphorus (OP) compounds for delayed polyneuropathy in a regulatory setting. Food Chem. Toxicol. 2004, 42, 1279− 1285. (b) Larson, R. A.; Weber, E. J. Reaction Mechanisms in Environmental Organic Chemistry; Lewis Press: Birżebbuġa, Malta, 1994; pp 122−125. (c) Gordon, J. J.; Leadbeater, L. The prophylactic use of 1-methyl, 2-hydroxyiminomethyl- pyridiunium methanesulfonate (P2S) in the treatment of organophosphate poisoning. Toxicol. Appl. Pharmacol. 1977, 40, 109−114. (3) (a) Kim, K.; Tsay, O. G.; Atwood, D. A.; Churchill, D. G. Destruction and detection of chemical warfare agents. Chem. Rev. 2011, 111 (9), 5345−5403. (b) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Decontamination of chemical warfare agents. Chem. Rev. 1992, 92 (8), 1729−1743. (4) Munro, N. B. Toxicity of the organophosphate chemical warfare agents GA, GB, and VX: implications for public protection. Environ. Health Perspect. 1994, 102, 18−37. (5) Seger, M. R.; Maciel, G. E. NMR investigation of the behavior of an organothiophosphate pesticide, methyl parathion, sorbed on clays. Environ. Sci. Technol. 2006, 40 (2), 552−558. (6) Bielawski, J.; Casida, J. E. Phosphorylating intermediates in the peracid oxidation of phosphorothionates, phosphorothiolates, and phosphorodithioates. J. Agric. Food Chem. 1988, 36 (3), 610−615. (7) (a) Parathion toxicity comes from the PS → PO oxidation that takes place in insects and converts the applied parathion (LD50 = 13 mg/kg) to the more toxic paraoxon (LD50 = 1.8 mg/kg).. (b) Spiro, T. G.; Stigliani, W. M. Chemistry of the Environment; Prentice Hall: Upper Saddle River, NJ, 1996; pp 281−282. (8) Sperling, F.; Ewenike, H. K.; Farber, T. Changes in LD50 of parathion and heptachlor following turpentine pretreatment. Environ. Res. 1972, 5, 164−171. (9) Yang, Y.-C.; Berg, F.; Szafraniec, L.; Beaudry, W.; Bunton, C.; Kumar, A. Peroxyhydrolysis of nerve agent VX and model compounds and related nucleophilic reactions. J. Chem. Soc., Perkin Trans. 2 1997, 3, 607−614. (10) Ohashi, N.; Tsuchiya, Y.; Sasano, H.; Hamada, A. Ozonation Products of Organophosphorous Pesticides in Water. Jpn. J. Toxicol. Environ. Health 1994, 40 (2), 185−192. (11) (a) Carrasco, C. J.; Montilla, F.; Alvarez, E.; Mealli, C.; Manca, G.; Galindo, A. Experimental and Theoretical Insights into the Oxodiperoxomolybdenum-catalysed Sulphide Oxidation Using Hydrogen Peroxide in Ionic Liquids. Dalton Trans. 2014, 43 (36), 13711− 13730. (b) Ruff, R.; Kucsman, A. Mechanism of the oxidation of sulphides with sodium periodate. J. Chem. Soc., Perkin Trans. 2 1985, No. 5, 683−687. (12) Thompson, D.; Cao, Z.; Judkins, E.; Fanwick, P.; Ren, T. Peroxo-dimolybdate catalyst for the oxygenation of organic sulfides by hydrogen peroxide. Inorg. Chim. Acta 2015, 437, 103−109. (13) Yang, Y.-C.; Szafraniec, L.; Beaudry, W.; Rohrbaugh, D. Oxidative detoxification of phosphonothiolates. J. Am. Chem. Soc. 1990, 112 (18), 6621−6627. (14) Yang, Y.-C. Chemical detoxification of nerve agent VX. Acc. Chem. Res. 1999, 32 (2), 109−115. (15) (a) Segall, Y.; Casida, J. E. Oxidative conversion of phosphorothioates to phosphinyloxysulfonates probably via phosphorothiolate S-oxides. Tetrahedron Lett. 1982, 23 (2), 139−142. (b) Bielawski, J.; Casida, J. E. Phosphorylating intermediates in the peracid oxidation of phosphorothionates, phosphorothiolates, and phosphorodithioates. J. Agric. Food Chem. 1988, 36 (3), 610−615. (16) (a) Bellet, E. M.; Casida, J. E. Products of peracid oxidation of organothiophosphorus compounds. J. Agric. Food Chem. 1974, 22 (2), 207−211. (b) Segall, Y.; Casida, J. E. Reaction of proposed phosphorothiolate s-oxide intermediates with alcohols. Phosphorus Sulfur Relat. Elem. 1983, 18, 209−212. (17) Kurusu, Y.; Masuyama, Y.; Saito, M.; Saito, S. Epoxidation with t-butyl hydroperoxide in the presence of molybdenum peroxide and polymer-immobilized molybdenum peroxide. J. Mol. Catal. 1986, 37 (2), 235−242. (18) Ambroziak, K.; Mbeleck, R.; He, Y.; Saha, B.; Sherrington, D. C. Investigation of Batch Alkene Epoxidations Catalyzed by Polymer-
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01545. Detailed crystallographic data, IR spectra, EDX, calculations for determining the oxygen-to-molybdenum atomic ratio, authentic addition of ethylmethylphenyl phosphate and diethylphenyl phosphate, 31P and 13C NMR spectra, and Arrhenius plots (PDF) Accession Codes
CCDC 1557943 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the NSF-RUI (Award CHE1413090), and crystallographic data collection was done on an SC-XRD funded from NSF-TUES (Grant DUE-1140940). We acknowledge crystallographic advice from Dr. Edward Valente (University of Portland).
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REFERENCES
(1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed., John Wiley & Sons: New York, 1980; pp 472−482. (2) (a) Battershill, J. M.; Edwards, P. M.; Johnson, M. K. Toxicological assessment of isomeric pesticides: a strategy for testing 10019
DOI: 10.1021/acs.inorgchem.7b01545 Inorg. Chem. 2017, 56, 10013−10020
Article
Inorganic Chemistry Supported Mo(VI) Complexes. Ind. Eng. Chem. Res. 2009, 48 (7), 3293−3302. (19) Maiti, S. K.; Malik, K. M. A.; Gupta, S.; Chakraborty, S.; Ganguli, A. K.; Mukherjee, A. K.; Bhattacharyya, R. Oxo- and oxoperoxo-molybdenum(VI) complexes with Aryl Hydroxamates: Synthesis, structure, and catalytic uses in highly efficient, selective, and ecological benign peroxidic epoxidation of olefins. Inorg. Chem. 2006, 45 (24), 9843−9857. (20) DeBruin, K. E.; Johnson, E. M. Preferential mode for nucleophilic attack by methoxide ion on O,S-dimethyl phenylphosphonothiolate. Contrasting behavior to reactions on analogous phosphonium salts. J. Am. Chem. Soc. 1973, 95 (23), 7921−7922. (21) DeBruin, K. E.; Tang, C.-I. W.; Johnson, D. M.; Wilde, R. L. Kinetic facial selectivity in nucleophilic displacements at tetracoordinate phosphorus: kinetics and stereochemistry in the reaction of sodium ethoxide with O,S-dimethyl phenylphosphonothioate. J. Am. Chem. Soc. 1989, 111 (15), 5871−5879. (22) Kuo, L. Y.; Bentley, A. K.; Shari’ati, Y. A.; Smith, C. P. Phosphonothioate Hydrolysis Turnover by Cp2MoCl2 and Silver Nanoparticles. Organometallics 2012, 31 (15), 5294−5301. (23) Jacobson, S.; Tang, R.; Mares, F. Group 6 Transition Metal Peroxo Complexes Stabilized by Polydentate Pyridinecarboxylate Ligands. Inorg. Chem. 1978, 17 (11), 3055−3063. (24) Vankayala, R.; Sagadevan, A.; Vijayaraghavan, P.; Kuo, C.-L.; Hwang, K. C. Metal Nanoparticles Sensitize the Formation of Singlet Oxygen. Angew. Chem., Int. Ed. 2011, 50 (45), 10640−10644. (25) Kurusu, Y.; Masuyama, Y.; Nishio, T. Organic synthetic reactions with activated molybdenum oxide. Proceedings of 4th International Conference on the Chemical Uses of Molybdenum; 1982; pp 241−245. (26) Kurusu, Y. Thermal behavior of a new type molybdenum oxide obtained by oxidation of molybdenum powder or molybdenum trioxide with hydrogen peroxide. Bull. Chem. Soc. Jpn. 1981, 54 (1), 293−294. (27) Jabbari, A.; Mahdavi, H.; Nikoorazm, M.; GhorbaniChoghamarani, A. Oxovanadium(IV) salicylidene Schiff base complex anchored on mesoporous silica MCM-41 as hybrid materials: a robust catalyst for the oxidation of sulfides. J. Porous Mater. 2015, 22, 1111− 1118. (28) Kuo, L. Y.; Glazier, S. K. Stereochemical Inversion of Phosphonothioate Methanolysis by La(III) and Zn(II): Mechanistic Implications for the Degradation of Organophosphate Neurotoxins. Inorg. Chem. 2012, 51 (1), 328−335. (29) Melnychuk, S.; Neverov, A.; Brown, R. S. Catalytic decomposition of simulants for chemical warfare V agents: highly efficient catalysis of the methanolysis of phosphonothioate esters. Angew. Chem., Int. Ed. 2006, 45 (11), 1767−1770. (30) Leslie, D.; Beaudry, W.; Szafraniec, L.; Rohrbaugh, D. Mechanistic implications of pyrophosphate formation in the oxidation of O,S-dimethyl phosphoramidothioate. J. Org. Chem. 1991, 56 (10), 3459−3462. (31) (a) Pretsch, E.; Buhlmann, P.; Badertscher, M. Structure Determination of Organic Compounds; Springer-Verlag: Berlin, 2009; p 126. (b) www.chem.wisc.edu/areas/reich/nmr/C13-data/cdata.htm. (32) (a) Aubry, J. M.; Cazin, B. Chemical sources of singlet oxygen. 2. Quantitative generation of singlet oxygen from hydrogen peroxide disproportionation catalyzed by molybdate ions. Inorg. Chem. 1988, 27 (12), 2013−2014. (b) Aubry, J.-M.; Bouttemy, S. Preparative oxidation of organic compounds in microemulsions with singlet oxygen generated chemically by the sodium molybdate/hydrogen peroxide system. J. Am. Chem. Soc. 1997, 119 (23), 5286−5294. (33) (a) Chiarini, M.; Bunton, C. Oxidation of thioanisole by peroxomolybdate in alcohol-modified micelles of cetylpyridinium chloride. Langmuir 2002, 18 (23), 8806−8812. (b) Chiarini, M.; Cerichelli, G.; Foroudian, H.; Gillitt, N.; Yunes, S.; Bunton, C. Oxidation of thioanisole by peroxomolybdate in assemblies of cetylpyridinium chloride and methyltri-n-octylammonium chloride. Langmuir 2004, 20 (13), 5201−5208.
(34) Liang, J. J.; Gu, C. L.; Kacher, M. L.; Foote, C. Chemistry of singlet oxygen. 45. Mechanism of the photooxidation of sulfides. J. Am. Chem. Soc. 1983, 105 (14), 4717−4721. (35) Lin, H.; Shen, Y.; Chen, D.; Lin, L.; Wilson, B. C.; Li, B.; Xie, S. Feasibility study on quantitative measurements of singlet oxygen generation using singlet oxygen green sensor. J. Fluoresc. 2013, 23, 41− 47. (36) Nardi, S.; Manet, I.; Monti, S.; Miranda, M. A.; Lhiaubet-Vallet, V. Scope and limitations of the TEMPO/EPR method for singlet oxygen detection: The misleading role of electron transfer. Free Radical Biol. Med. 2014, 77, 64−70. (37) (a) Craig, J. A.; Harlan, E. W.; Snyder, B. S.; Whitener, M. A.; Holm, R. H. Oxomolybdenum(IV, V, VI) complexes: Structure, reactivity, and criteria of detection of binuclear (m-oxo)molybdenum(V) products in oxygen atom transfer systems. Inorg. Chem. 1989, 28 (11), 2082−2091. (b) Arzoumanian, H.; Agrifoglio, G.; Krentzien, H. Reaction of a tetrathiocyanatodioxo-molybdate(VI) anion with bipyridines. Synthesis of a highly efficient oxo-transfer catalyst. New J. Chem. 1995, 20 (6), 699−705. (c) Arzoumanian, H.; Lopez, R.; Agrifoglio, G. Synthesis and X-ray characterization of tetraphenylphosphonium tetrathiocyanatodioxomolybdate(VI); A remarkable oxo transfer agent. Inorg. Chem. 1994, 33 (14), 3177−3179. (d) Kuhn, F. E.; Santos, A. M.; Lopes, A.; Gonçalves, I. S.; Herdtweck, E.; Romao, C. (Dimethyl)dioxomolybdenum(VI) complexes: syntheses and catalytic applications. J. Mol. Catal. A: Chem. 2000, 164, 25−38. (38) (a) Bortolini, O.; Campestrini, S.; Di Furia, F.; Modena, G. Metal catalysis in oxidation by peroxides. Kinetics and mechanism of the molybdenum-catalyzed oxidation of sulfoxides to sulfones with hydrogen peroxide. J. Org. Chem. 1987, 52 (23), 5093−5095. (b) Bortolini, O.; Campestrini, S.; Di Furia, F.; Modena, G.; Valle, G. Metal catalysis in oxidation by peroxides. Anionic molybdenumpicolinate N-oxido-peroxo complex: an effective oxidant of primary and secondary alcohols in nonpolar solvents. J. Org. Chem. 1987, 52 (24), 5467−5469. (39) Dannenberg, A.; Pehkonen, S. O. Investigation of the heterogeneously catalyzed hydrolysis of organophosphorus pesticides. J. Agric. Food Chem. 1998, 46 (1), 325−334. (40) Conte, V.; Di Furia, F.; Modena, G.; Bortolini, O. Metal catalysis in oxidation by peroxides. A 17O NMR spectroscopic investigation of neutral and anionic molybdenum peroxo complexes. J. Org. Chem. 1988, 53 (19), 4581−4582. (41) Campestrini, S.; Conte, V.; Di Furia, F.; Modena, G.; Bortolini, O. Metal catalysis in oxidation by peroxides. 30. Electrophilic oxygen transfer from anionic, coordinatively saturated molybdenum peroxo complexes. J. Org. Chem. 1988, 53 (24), 5721−5724. (42) Mimoun, H. The role of peroxymetallation in selective oxidation processes. J. Mol. Catal. 1980, 7, 1−29. (43) Bunton, C. A.; Gillitt, N. D. Oxidation of thioanisole by peroxomolybdate ions: direct oxygen transfer from tetraperoxomolybdate ion. J. Phys. Org. Chem. 2002, 15 (1), 29−35. (44) (a) Amini, M.; Bagherzadeh, M.; Atabaki, B.; Derakhshandeh, P. G.; Ellern, A.; Woo, L. K. Molybdenum(VI)−oxodiperoxo complex containing an oxazine ligand: synthesis, X-ray studies, and catalytic activity. J. Coord. Chem. 2014, 67 (8), 1429−1436. (b) Amini, M.; Haghdoost, M. M.; Bagherzadeh, M. Oxido-peroxido molybdenum(VI) complexes in catalytic and stoichiometric oxidations. Coord. Chem. Rev. 2013, 257, 1093−1121. (45) (a) Gupta, V.; Carroll, K. S. Sulfenic acid chemistry, detection and cellular lifetime. Biochim. Biophys. Acta, Gen. Subj. 2014, 1840, 847−875. (b) McAnoy, A. M.; Williams, J.; Paine, M. R. L.; Rogers, M. L.; Blanksby, S. J. Ion−Molecule reactions of O,S-dimethyl methylphosphonothioate: Evidence for intramolecular sulfur oxidation during VX perhydrolysis. J. Org. Chem. 2009, 74 (24), 9319−9327.
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