Hydrodefluorination and Hydrogenation of Fluorobenzene under Mild Aqueous Conditions

Fluorinated organic compounds are increasingly used in many applications, and their release to the environment is expected. It is therefore important to find suitable methods for degradation of fluorinated compounds under environmentally relevant conditions. In this study, a simple heterogeneous rhodium-based catalytic system (Rh/Al₂O₃ and H₂) for hydrodefluorination and hydrogenation of fluorobenzene under mild aqueous conditions (1 atm of H₂, ambient temperature) was developed and the underlying reaction mechanism was investigated. Fluorobenzene degraded rapidly (<i>t</i>_1/2 ≈ 0.2 h) to form cyclohexane and fluoride (F^–) as the stable end products, with benzene and cyclohexene observed as intermediates. Cyclohexadiene intermediates were not observed but were expected to form during the hydrogenation of benzene. Three postulated but unobserved fluorinated intermediates were subjected to the catalytic reaction conditions, and it was concluded that they most likely do not form during the fluorobenzene degradation reaction. Isotope labeling experiments showed that the unsaturated intermediates undergo rapid and reversible hydrogenation/dehydrogenation under the reaction conditions and also that fully saturated compounds are unreactive in the catalytic system. Both molecular hydrogen and water were sources of hydrogen in the final cyclohexane product. Kinetic fitting indicated that sorption/desorption of fluorobenzene onto the catalyst surface plays an important role in the mechanism

Reactivity Differences of Combined and Free Amino Acids: Quantifying the Relationship between Three-Dimensional Protein Structure and Singlet Oxygen Reaction Rates

It has long been appreciated that the photooxidation kinetics of amino acid (AA) residues in an intact protein differ from those of free AAs due to differences in the local steric microenvironment, such as its location in the three-dimensional structure. Yet there are only a few studies that have quantified the effect of protein structure on the photochemical reactivity of its residues. This is important for predicting phototransformation rates of AAs in aquatic environments where AAs in combined forms (e.g., oligopeptides and proteins) are more abundant than free AAs. In this work, the photochemical reactivity differences between free and combined AAs were assessed. Singlet oxygen (¹O₂) reaction kinetics of individual photooxidizable residues in the protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were examined. The results suggest that the ¹O₂ accessibility of residues in intact GAPDH has a profound effect on their photodegradation kinetics and for histidine residues can explain most of the variation in ¹O₂ reactivity. Additionally, ¹O₂-accessibile surface area values of residues calculated from protein crystal structure data are useful in predicting their reaction rates in GAPDH. This work illustrates a new approach to assess the differential photochemical reactivity of AA-based biomolecules in natural environments or engineered applications

Environmental Photoinactivation of Extracellular Phosphatases and the Effects of Dissolved Organic Matter

Alkaline phosphatases are ubiquitous extracellular enzymes in aquatic systems and play a central role in the biogeochemical cycling of phosphorus. Yet, the photochemical stability of phosphatase and effects of natural organic matter (DOM) are not completely understood. We demonstrate that phosphatase activity in natural biofilm samples decreased during sunlight exposure similar to well-defined bacterial phosphatase solutions. Direct photoinactivation was slowed by more than 50% in the presence of redox-active dissolved organic matter (DOM, 10 mg_C L^–1) or a model antioxidant (esculetin, 50 μM), even after light screening effects had been accounted for. Thus, DOM can not only inhibit enzymes (in the dark) or sensitize photodegradation by producing photochemically produced reactive intermediates but can also significantly quench direct photoinactivation of phosphatase. Our data further suggest that direct photooxidation of tryptophan residues within the protein structure are significantly involved in the photoinactivation of phosphatase because a loss of tryptophan-like fluorescence paralleled photoinactivation kinetics and because DOM acted as an antioxidant toward photoinactivation, a phenomenon recently established for the photooxidation of freely dissolved tryptophan. Thus, photoinactivation of phosphatase can be significantly slowed in the presence of naturally occurring antioxidants like DOM. The mechanistic link between tryptophan photooxidation and inactivation of phosphatase may have applicability to other extracellular enzymes but remains to be established

Chlorinated Ethene Reactivity with Vitamin B₁₂ Is Governed by Cobalamin Chloroethylcarbanions as Crossroads of Competing Pathways

Chlorinated ethenes are toxic groundwater contaminants. Although they can be dechlorinated by microorganisms, reductive dehalogenases, and their corrinoid cofactor, biochemical reaction mechanisms remain unsolved. This study uncovers a mechanistic shift revealed by contrasting compound-specific carbon (ε¹³C) and chlorine (ε³⁷Cl) isotope effects between perchloroethene, PCE (ε³⁷Cl = −4.0‰) and <i>cis</i>-dichloroethene, <i>cis</i>-DCE (ε³⁷Cl = −1.5‰), and a pH-dependent shift for trichloroethene, TCE (from ε³⁷Cl = −5.2‰ at pH 12 to ε³⁷Cl = −1.2‰ at pH 5). Different pathways are supported also by pH-dependent reaction rates, TCE product distribution, and hydrogen isotope effects. Mass balance deficits revealed reversible and irreversible cobalamin-substrate association, whereas high-resolution mass spectrometry narrowed down possible structures to chloroalkyl and chlorovinyl cobalamin complexes. Combined experimental evidence is inconsistent with initial electron transfer or alkyl or vinyl complexes as shared intermediates of both pathways. In contrast, it supports cobalamin chlorocarbanions as key intermediates from which Cl^– elimination produces vinyl complexes (explaining rates and products of TCE at high pH), whereas protonation generates less reactive alkyl complexes (explaining rates and products of TCE at low pH). Multielement isotope effect analysis holds promise to identify these competing mechanisms also in real dehalogenases, microorganisms, and even contaminated aquifers

Dehalogenation of Aromatics by Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution has been implicated as a mechanism for both the biotic and abiotic hydrodehalogenation of aromatics. Two mechanisms for the aqueous dehalogenation of aromatics involving nucleophilic aromatic substitution with hydride as a nucleophile are investigated using a validated density functional and continuum solvation protocol. For chlorinated and brominated aromatics, nucleophilic addition <i>ortho</i> to carbon–halogen bonds via an anionic intermediate is predicted to be the preferred mechanism in the majority of cases, while concerted substitution is predicted to be preferred for most fluorinated aromatics. Nucleophilic aromatic substitution reactions with the hydroxide and hydrosulfide anions as nucleophiles are also investigated and compared

Fluorescent Molecular Probes for Detection of One-Electron Oxidants Photochemically Generated by Dissolved Organic Matter

We report a dual probe system based on 4′-substituted biphenyl-2-carboxylic acids (BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes are converted to the corresponding benzocoumarins (BZCs) at different rates depending on the mechanism of oxidation; thus, two probes used simultaneously can differentiate strong triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (^•OH) present in dissolved organic matter (DOM). Comparison of the ratios of BZC–CH₃ and BZC–CF₃ product formation using model photooxidants such as NaNO₂, a ^•OH precursor, and model triplet sensitizer lumichrome gave a range of 2 to 250. Application of these probes to DOM isolates and whole natural waters afforded intermediate ratios. Although the oxidation potential of BPAs (>ca. 1.80 V SHE) is significantly higher than the estimated average reduction potential of typical ³CDOM* samples, these results have demonstrated the presence of a small pool of oxidants in the selected DOM isolates and whole water samples that is capable of oxidizing aromatic carboxylates. As an analytical tool, this probe pair can be used between pH 4–6 without affecting the product formation ratio and may find applications in various systems involving complex mixtures of photochemically produced oxidants of differing natures

Complete Hydrodehalogenation of Polyfluorinated and Other Polyhalogenated Benzenes under Mild Catalytic Conditions

Polyfluorinated arenes are increasingly used in industry and can be considered emerging contaminants. Environmentally applicable degradation methods leading to full defluorination are not reported in the literature. In this study, it is demonstrated that the heterogeneous catalyst Rh/Al₂O₃ is capable of fully defluorinating and hydrogenating polyfluorinated benzenes in water under mild conditions (1 atm H₂, ambient temperature) with degradation half-lives between 11 and 42 min. Analysis of the degradation rates of the 12 fluorobenzene congeners showed two trends: slower degradation with increasing number of fluorine substituents and increasing degradation rates with increasing number of adjacent fluorine substituents. The observed fluorinated intermediates indicated that adjacent fluorine substituents are preferably removed. Besides defluorination and hydrogenation, the scope of the catalyst includes dehalogenation of polychlorinated benzenes, bromobenzene, iodobenzene, and selected mixed dihalobenzenes. Polychlorobenzene degradation rates, like their fluorinated counterparts, decreased with increasing halogen substitution. In contrast to the polyfluorobenzenes though, removal of chlorine substituents was sterically driven. All monohalobenzenes were degraded at similar rates; however, when two carbon–halogen bonds were in direct intramolecular competition, the weaker bond was broken first. Differences in sorption affinities of the substrates are suggested to play a major role in determining the relative rates of transformation of halobenzenes by Rh/Al₂O₃ and H₂

On the Use of Hydroxyl Radical Kinetics to Assess the Number-Average Molecular Weight of Dissolved Organic Matter

Dissolved organic matter (DOM) is involved in numerous environmental processes, and its molecular size is important in many of these processes, such as DOM bioavailability, DOM sorptive capacity, and the formation of disinfection byproducts during water treatment. The size and size distribution of the molecules composing DOM remains an open question. In this contribution, an indirect method to assess the average size of DOM is described, which is based on the reaction of hydroxyl radical (HO^•) quenching by DOM. HO^• is often assumed to be relatively unselective, reacting with nearly all organic molecules with similar rate constants. Literature values for HO^• reaction with organic molecules were surveyed to assess the unselectivity of DOM and to determine a representative quenching rate constant (<i>k</i>_rep = 5.6 × 10⁹ M^–1 s^–1). This value was used to assess the average molecular weight of various humic and fulvic acid isolates as model DOM, using literature HO^• quenching constants, <i>k</i>_C,_DOM. The results obtained by this method were compared with previous estimates of average molecular weight. The average molecular weight (<i>M</i>_n) values obtained with this approach are lower than the <i>M</i>_n measured by other techniques such as size exclusion chromatography (SEC), vapor pressure osmometry (VPO), and flow field fractionation (FFF). This suggests that DOM is an especially good quencher for HO^•, reacting at rates close to the diffusion-control limit. It was further observed that humic acids generally react faster than fulvic acids. The high reactivity of humic acids toward HO^• is in line with the antioxidant properties of DOM. The benefit of this method is that it provides a firm upper bound on the average molecular weight of DOM, based on the kinetic limits of the HO^• reaction. The results indicate low average molecular weight values, which is most consistent with the recent understanding of DOM. A possible DOM size distribution is discussed to reconcile the small nature of DOM with the large-molecule behavior observed in other studies

Synthesis and Reactivity of an Isolable Cobalt(I) Complex Containing a β-Diketiminate-Based Acyclic Tetradentate Ligand

A model for cobalamin was synthesized using a new monoanionic tetradentate nitrogen donor ligand; 2-(4-tolyl)-1,3-bis­(2-isopropylpyridyl)­propenediimine (Tol-BDI^(2‑pp)2H) (<b>1</b>), which utilizes isopropylpyridines as pendant arms on a β-diketiminate (BDI) backbone. During the synthesis of <b>1</b>, the rearrangement product, Tol-BDI^{(2‑pp)(4‑pp)}H (<b>2</b>) was observed. Metalation of <b>1</b> with zinc iodide and cobalt chloride yielded the corresponding Tol-BDI^(2‑pp)2ZnI (<b>3</b>) and Tol-BDI^(2‑pp)2CoCl (<b>4</b>) complexes. The redox properties of <b>4</b> in comparison to cobalamin were examined through electrochemical studies. Electrochemical and bulk reduction of complex <b>4</b> gave a diamagnetic cobalt­(I) complex, Tol-BDI^(2‑pp)2Co (<b>5</b>). Reactivity of <b>5</b> toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)­acetylene, yielding Tol-BDI^(2‑pp)2Co­(CH₃)I and Tol-BDI^(2‑pp)2Co­(C₂Si­(CH₃)₃)I respectively. Synthesis and characterization details for these complexes, including the crystal structure of <b>3</b>, are reported

Synthesis and Reactivity of an Isolable Cobalt(I) Complex Containing a β-Diketiminate-Based Acyclic Tetradentate Ligand

A model for cobalamin was synthesized using a new monoanionic tetradentate nitrogen donor ligand; 2-(4-tolyl)-1,3-bis­(2-isopropylpyridyl)­propenediimine (Tol-BDI^(2‑pp)2H) (<b>1</b>), which utilizes isopropylpyridines as pendant arms on a β-diketiminate (BDI) backbone. During the synthesis of <b>1</b>, the rearrangement product, Tol-BDI^{(2‑pp)(4‑pp)}H (<b>2</b>) was observed. Metalation of <b>1</b> with zinc iodide and cobalt chloride yielded the corresponding Tol-BDI^(2‑pp)2ZnI (<b>3</b>) and Tol-BDI^(2‑pp)2CoCl (<b>4</b>) complexes. The redox properties of <b>4</b> in comparison to cobalamin were examined through electrochemical studies. Electrochemical and bulk reduction of complex <b>4</b> gave a diamagnetic cobalt­(I) complex, Tol-BDI^(2‑pp)2Co (<b>5</b>). Reactivity of <b>5</b> toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)­acetylene, yielding Tol-BDI^(2‑pp)2Co­(CH₃)I and Tol-BDI^(2‑pp)2Co­(C₂Si­(CH₃)₃)I respectively. Synthesis and characterization details for these complexes, including the crystal structure of <b>3</b>, are reported