Defluorination Reactions Research Articles

Sulfidized Nanoscale Zero-Valent Iron: Tuning the Properties of This Complex Material for Efficient Groundwater Remediation

ConspectusGroundwater contamination by halogenated organic compounds, especially chlorinated and fluorinated ones, threatens freshwater sources globally. Nanoscale zero-valent iron (NZVI) has been extensively studied (>5000 publications) and deployed for in situ groundwater remediation, but NZVI selectivity for contaminants is poor, reactive lifetimes are short, and it cannot promote defluorination reactions. Recently, sulfidized NZVI (SNZVI) has emerged, and has revitalized academic and industrial interests in this material for remediation. Sulfidation broadens the range of reactive contaminants, and significantly increases the selectivity and reactive lifetime of NZVI by 2 orders of magnitude, while inhibiting the undesirable H2 evolution reaction between Fe0 and water. This Account provides a state-of-the-art understanding of the chemical properties controlling the reactivity and selectivity of SNZVI and will advance the field toward the rational design of efficient groundwater remediation materials.SNZVI is a complex mixture of reactive body-centered cubic (BCC) metallic iron and unspecified iron sulfides. Most published SNZVI research has aimed at exploring the breadth of its reactivity toward various environmental contaminants rather than understanding the factors that influence the reactivity of this complex mixture of materials. Recent works from our laboratory have aimed at tuning the synthesis conditions to control the amount and speciation of sulfur in the SNZVI structure, and elucidating how these structural changes result in physicochemical properties (e.g., hydrophobicity, electron-transfer resistance, and H adsorption sites) that provide desirable reactivity and selectivity for important groundwater contaminants.This Account explains the reasons for the more desirable properties of SNZVI compared to NZVI. The degradation pathways, and reactive sites (Fe or S sites) and species (direct electron transfer or atomic H) of SNZVI for the dechlorination of trichloroethene and defluorination of florfenicol are determined from batch experiments, theoretical calculations, and analysis of degradation products. A better understanding of why SNZVI is reactive with C–F bonds under ambient conditions may also promote the use of SNZVI and its derivatives for the defluorination of emerging groundwater contaminants.Finally, this Account provides guidance for measuring and reporting the complex material properties of SNZVI. This will enable comparisons between future studies to elucidate the reasons for differences in the reactivity of SNZVI synthesized by different research groups. Overall, this Account unveils the structure–property–performance relationships of SNZVI, makes strides toward the controlled synthesis and rational design of robust SNZVI with properties tailored for specific application scenarios, and provides mechanistic insights into SNZVI materials for in situ groundwater remediation of chlorinated and fluorinated contaminants.

Base-Catalyzed Intramolecular Defluorination/O-Arylation Reaction for the Synthesis of 3-Fluoro-1,4-oxathiine 4,4-Dioxide

AbstractA novel process involving base-catalyzed intramolecular defluorination/O-arylation of readily available α-fluoro-β-one-sulfones was realized and provided a series of 3-fluoro-1,4-oxathiine 4,4-dioxide derivatives in good to excellent yields. Unlike traditional defluorination reactions with stoichiometric base as the deacid reagent, this process is triggered by a catalytic amount of base (TMG: tetramethylguanidine) and molecular sieves serve as both an adsorbent to remove HF acid and an activator to assist C–F bond cleavage.

Low-Cost Fluoride Source for Organic Liquid Electrolyte-Based Fluoride Shuttle Battery

The effects of using low-cost inorganic fluoride salts (i.e., KF or NaF) as fluoride sources in fluoride shuttle batteries (FSBs) on the electrochemical compatibility of BiF3 electrodes are investigated herein. The preparation of electrolytes containing saturated KF or NaF and 0.5 M 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (DiOB-Py) in G4 is described. For Py/NaF/G4, the discharge and charge reactions of BiF3 were hindered because of the low solubility of NaF as well as the low ionic conductivity of the electrolyte. However, inductively coupled plasma mass spectrometry (ICP-MS) analysis revealed that the solubility of KF in Py/KF/G4 was moderate and the ionic conductivity of Py/KF/G4 was promising. Higher oxidation and reduction peaks observed in the cyclic voltammograms of Py/KF/G4 than those of Py/G4 and Py/NaF/G4 are attributed to the enhanced electrochemical activity of the former. Consequently, the BiF3/C nanocomposite electrode exhibits good cycling capability in Py/KF/G4, with initial discharge/charge capacities of 316/218 mAh g−1, respectively. Moreover, the ICP-MS and Raman spectroscopy analyses revealed that defluorination reactions of BiF3 occur via a direct desorption mechanism. Py/KF/G4 is the first effective electrolyte based on a low-cost inorganic salt. FSBs exhibit improved performance in Py/KF/G4 compared with CsF salt systems, which warrants further investigation.

Copper-Catalyzed Selective Defluorinative Sulfuration of Trifluoropropanamides Leading to α-Fluorothioacrylamides.

A practical and efficient method for the synthesis of α-fluorothioacrylamide was developed from selective defluorinative sulfuration of trifluoropropanamides with disulfides. The N-chelation-assisted copper-catalyzed defluorination and sulfurization reactions feature excellent functional group tolerance and incorporation of both sulfur atoms of disulfides into acrylamides.

Reductive Defluorination and Mechanochemical Decomposition of Per- and Polyfluoroalkyl Substances (PFASs): From Present Knowledge to Future Remediation Concepts.

Over the past two decades, per- and polyfluoroalkyl substances (PFASs) have emerged as worldwide environmental contaminants, calling out for sophisticated treatment, decomposition and remediation strategies. In order to mineralize PFAS pollutants, the incineration of contaminated material is a state-of-the-art process, but more cost-effective and sustainable technologies are inevitable for the future. Within this review, various methods for the reductive defluorination of PFASs were inspected. In addition to this, the role of mechanochemistry is highlighted with regard to its major potential in reductive defluorination reactions and degradation of pollutants. In order to get a comprehensive understanding of the involved reactions, their mechanistic pathways are pointed out. Comparisons between existing PFAS decomposition reactions and reductive approaches are discussed in detail, regarding their applicability in possible remediation processes. This article provides a solid overview of the most recent research methods and offers guidelines for future research directions.

First-Principles Study for Fluorination/Defluorination Reaction Mechanism in Fluoride-Shuttle Batteries

INTRODUCTIONRechargeable batteries with energy densities higher than those of the present Li-ion batteries (LIBs) are required for electric vehicles. We focus on fluoride-ion (F−) transfer batteries whose electrodes are composed of metal/metal fluoride (M|MF n ) combinations. Electrochemical charge-transfer reactions occur during the discharge/charge processes at the two electrodes. Defluorination of the metal fluoride MF n and fluorination of another metal M’ occurs at the positive and negative electrodes, respectively, during the discharging process asM1/n F + e− → M1/n + F− (1),M’1/m + F− → M’1/m F + e− (2).This type of battery is called a fluoride-shuttle battery (FSB), which is expected to produce high voltage and have large capacity.1 In 2017, Okazaki et al. fabricated a Bi|BiF3 electrode with a liquid electrolyte prepared by dissolving an organic fluoride in an ionic liquids (ILs) at room temperature. 2 In 2018, Davis et al. reported another type of liquid electrolyte composed of a simple organic solvents (OSs) with F−conductivity greater than 1 mS/cm.3 From the development of these liquid electrolytes, the F− conductivity has been dramatically increased in recent years. After improvement of the electrolyte conductivity, the fluorination and defluorination reactions at the electrodes are considered to be the rate-determining steps in FSBs. However, the mechanism of the fluorination and defluorination reactions in FSBs has not been clarified in contrast to the well-investigated Li-intercalation reaction in LIBs.METHODSIn this study, we classify the fluorination and defluorination reactions into two possible reaction pathways from a thermodynamic point of view: reaction through the solid-solution state and reaction through the two-phase state. It is possible to find which reaction pathway is realized from the free energy difference. Selection of these reaction pathways can be determined from the enthalpies of all of the intermediate states, since the entropy term in the free energy is fairly cancelled when we consider the difference of free energies. Selection rule for the fluorination and defluorination reactions in FSBs is schematically illustrated in Figure 1. The solid-solution states of M1/n F x for x ≈ 0 and for x ≈ 1 are assumed to be formed by introducing an interstitial and a vacancy defect, respectively. In this case, the free energy relation for the selection can be described as G ss(x) < G tp(x) → H I(M1/n ) < H(M1/n F) or H(M1/n F) < −H V(M1/n F) (3),where G ss(x), G tp(x), H (M1/n F), H I(M1/n F) , H V(M1/n F) are the free energy of the solid-solution state of M1/n F x , that of two-phase state, the formation enthalpy of M1/n F, that of interstitial defect in M1/n , and that of vacancy defect in M1/n F, respectively. We use the energies from first-principles calculations for the enthalpy differences, whose accuracy is validated by comparing them with experimentally obtained thermodynamic enthalpy data.4 The first-principles calculations were performed with the configurations of the bulk crystal systems using Quantum Espresso package.5 The crystal data were taken from the AtomWork database.6 RESULTSThe relative formation enthalpies of typical, transition, and relativistic metal fluorides are well reproduced by first-principles calculations within 0.1, 0.2, and 0.4 eV, respectively. By comparing the formation enthalpies of H, H V, and H I, the relation H I > H > −H V is always satisfied in all of the calculated metal and metal-fluoride combinations (M = Li, Mg, Ca, Al, Co, Ni, Cu, Pb, Bi). Thus, the selection rule of eq. (3) strongly suggests that fluorination and defluorination in FSB electrodes occur by two-phase reaction.7 This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.AcknowledgmentsThis work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 Project (RISING2) administrated by the New Energy and Industrial Technology Development Organization (NEDO).REFERENCES1) F. Gschwind, G. Rodriguez-Garcia, D. Sandbeck, A. Gross, M. Weil, M. Fichtner, N. Hörmann, J. Fluorine Chem. 182, 76 (2016).2) K.-I. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett. 2, 1460 (2017).3) V. K. Davis et al., Science 362, 1144 (2018).4) Handbook of Chemistry: Pure Chemistry, 5th ed.5) P. Giannozzi et al., J. Phys.: Condens. Matter. 29, 465901 (2017).6) National Institute for Materials Science (NIMS) AtomWork <http://crystdb.nims.go.jp/>7) J. Haruyama, K.-I. Okazaki, Y. Morita, H. Nakamoto, E. Matsubara, T. Ikeshoji, M. Otani, “Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study” in revision. Figure 1. Two reaction pathways considered in this study. In the fluorination and defluorination reactions, the solid-solution (M1/n F x , 0 < x< 1) and two-phase (M1/n and M1/n F) states are considered to be possible intermediate states. Figure 1

Lactone-Based Li+/F- Hybrid Liquid Electrolytes for Room-Temperature Fluoride Shuttle Batteries

The fluoride shuttle batteries (FSBs) represent one of the potentially promising beyond-lithium-ion innovative battery systems, operating with a redox-active fluoride-ion transporting electrolyte. The low-weight, anion-based, multi-electron-transfer reactions at the anode and cathode, with a wide range of options on metals and their fluoride counterparts as the active materials, offer the opportunity to hopefully breakthrough the energy density limitation of the state-of-the-art lithium ion batteries.Of the two options of FSBs, all-solid type and liquid-type cells, we have focused on the latter format that would probably enable a much easier room-temperature operation than does the all-solid based cell, if only we could succeed in the development of a redox-active fluoride-ion (or its complexed forms) transporting liquid electrolyte that concomitantly ensures a high electrochemical stability over a widest possible potential window. To this end, we have recently established a novel method to fabricate a plain fluoride-ion electrolyte in a high boiling lactone solvent (γ-butyl lactone; GBL), denoted as CsF(KF)/GBL, where either CsF or KF is dissociated without the help of any extra addenda. This is believed to be by virtue of the acidic α-hydrogen atoms next to the carbonyl group of GBL. As shown in Figure 1, F- ions in GBL may be stabilized in the configuration where they are coordinated by two GBL molecules via the interactions with α-hydrogens on both sides. This structure prevents the formation of HF (by the α-hydrogen abstraction by the F- ion), thus resulting in the sufficient stability and the long (many months or even years) shelf-life of CsF(KF)/GBL as the liquid electrolyte for room-temperature FSBs.Nevertheless, we subsequently realized that while the high reactivity of the GBL-solvated F- ions brought about good reversible fluorination and defluorination reactions for almost arbitrary metals from Ag down to Zn in the redox series, it also caused a highly irreversible reductive sub-reaction that could never be traced back to the simple electrochemical reduction of GBL on its own. What we then came up with is the Li+/F- hybrid liquid electrolyte, where the Li+ ions mixed in a large excess of an equimolar composition with respect to the F- ions form some complexes, Li x F( x -1)+, that are soluble in GBL, and thus prevent the precipitation of LiF.The lactone-based Li+/F- hybrid liquid electrolyte prepared in the above-noted manner largely extended, especially, the high negative potential window to or beyond the standard redox potential of Li+/Li. The Li+/F- hybrid liquid electrolyte was also accompanied by a unique function as an AA (anion acceptor) that facilitates a very minor but yet substantial dissolution of various metal fluorides as the active materials in the proximity of the electrodes under the respective redox operations. This, together with the expanded potential window, greatly improved the charge/discharge capacities and reversibility of both positive and negative electrodes in the practical composite forms employing nano-scale or sub-nanoscale particles of the relevant active materials. We are demonstrating the charge/discharge behaviors of some typical electrodes in the Li+/F- hybrid electrolyte, such as Cu/CuF2, Bi/BiF3, Al in combination with a highly disordered phase of AlF3, and so forth. Acknowledgement:This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery 2 (RISING2), financially supported by New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Topochemical Fluorination of n = 2 Ruddlesden-Popper Type Sr3Ti2O7 to Sr3Ti2O5F4 and Its Reductive Defluorination.

Within this study, we show that a sequence of substitutive topochemical fluorination of the n = 2 Ruddlesden-Popper type compounds Sr3Ti2O7 to Sr3Ti2O5F4 followed by reductive topochemical defluorination reactions between the oxyfluoride and the reducing agent sodium hydride allows for a substantial reduction of the oxidation state of Ti due to selective extraction and hydride substitution of fluoride ions. The oxyfluoride Sr3Ti2O5F4 has been synthesized and characterized structurally for the first time. The defluorination experiments have been conducted at temperatures as low as 300 °C, enabling also the reduction of this metastable compound. The evolution of phase fractions and unit cell volumes of various reduced phases as well as of side products has been monitored by an X-ray diffraction study as a function of the amount of sodium hydride used. Strong structural changes within the reduced phases, involving considerable decreases in the c lattice parameters partly accompanied by symmetry, lowering have been observed. To gain a deeper understanding of the structural changes, selected reduction reaction products have been further investigated by coupled analysis of X-ray and neutron powder diffraction data. Moreover, changes in the oxidation state of Ti have been studied using magnetic measurements and X-ray photoelectron spectroscopy examining differences between the bulk and the surface properties. Additionally, similarities and differences between previously published results on the topochemical defluorination of the n = 1 Ruddlesden-Popper type compound Sr2TiO3F2 are discussed.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study.

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy HV, and an interstitial fluorine defect HI from the simple selection rule. The enthalpy relation of HI > H > -HV observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity?

The biodegradation of compounds with C-F bonds is challenging due to the fact that these bonds are stronger than the C-H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N-bridged diiron-phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H2 O2 effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C6 F6 activation by an iron(IV)-oxo active species of the N-bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate-determining electrophilic C-O addition reaction followed by a 1,2-fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C-F bond is the aliphatic C-F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C-H hydroxylation by iron(IV)-oxo intermediates such as cytochrome P450 Compound I.

Influence between Desulfurization and Defluorination Reactions Using Ca(OH)&lt;sub&gt;2&lt;/sub&gt; in the Simulated Coal-briquettes

Influence between Desulfurization and Defluorination Reactions Using Ca(OH)&lt;sub&gt;2&lt;/sub&gt; in the Simulated Coal-briquettes

Working toward fluoride batteries

Batteries Owing to the low atomic weight of fluorine, rechargeable fluoride-based batteries could offer very high energy density. However, current batteries need to operate at high temperatures that are required for the molten salt electrolytes. Davis et al. push toward batteries that can operate at room temperature, through two advances. One is the development of a room-temperature liquid electrolyte based on a stable tetraalkylammonium salt–fluorinated ether combination. The second is a copper–lanthanum trifluoride core-shell cathode material that demonstrates reversible partial fluorination and defluorination reactions. Science , this issue p. [1144][1] [1]: /lookup/doi/10.1126/science.aat7070

Room-temperature cycling of metal fluoride electrodes: Liquid electrolytes for high-energy fluoride ion cells

Fluoride ion batteries are potential "next-generation" electrochemical storage devices that offer high energy density. At present, such batteries are limited to operation at high temperatures because suitable fluoride ion-conducting electrolytes are known only in the solid state. We report a liquid fluoride ion-conducting electrolyte with high ionic conductivity, wide operating voltage, and robust chemical stability based on dry tetraalkylammonium fluoride salts in ether solvents. Pairing this liquid electrolyte with a copper-lanthanum trifluoride (Cu@LaF3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in a fluoride ion electrochemical cell cycled at room temperature. Fluoride ion-mediated electrochemistry offers a pathway toward developing capacities beyond that of lithium ion technology.

Bulky Diamine Ligand Promotes Cross‐Coupling of Difluoroalkyl Bromides by Iron Catalysis

AbstractAlthough iron‐catalyzed cross‐coupling of Grignard reagents with alkyl halides has been well established, the adoption of the reaction for fluoroalkylations has not been reported because traditional catalytic systems often lead to defluorination reactions. Described herein is the investigation of an iron‐catalyzed cross‐coupling between arylmagnesium bromides and difluoroalkyl bromides with modified N,N,N′,N′‐tetramethyl‐ethane‐1,2‐diamine (TMEDA) as a ligand. The use of this bulky diamine, in which a butylene is substituted at one carbon atom of the ethylene backbone in TMEDA, enables the iron‐catalyzed difluoroalkylation under mild reaction conditions with a wide range of difluoroalkyl bromides, including vulnerable bromodifluoromethane, thus providing a general and cost‐efficient route for applications in medicinal chemistry.

Bulky Diamine Ligand Promotes Cross-Coupling of Difluoroalkyl Bromides by Iron Catalysis.

Although iron-catalyzed cross-coupling of Grignard reagents with alkyl halides has been well established, the adoption of the reaction for fluoroalkylations has not been reported because traditional catalytic systems often lead to defluorination reactions. Described herein is the investigation of an iron-catalyzed cross-coupling between arylmagnesium bromides and difluoroalkyl bromides with modified N,N,N',N'-tetramethyl-ethane-1,2-diamine (TMEDA) as a ligand. The use of this bulky diamine, in which a butylene is substituted at one carbon atom of the ethylene backbone in TMEDA, enables the iron-catalyzed difluoroalkylation under mild reaction conditions with a wide range of difluoroalkyl bromides, including vulnerable bromodifluoromethane, thus providing a general and cost-efficient route for applications in medicinal chemistry.

Reductive Defluorination of Branched Per- and Polyfluoroalkyl Substances with Cobalt Complex Catalysts

This study investigates structure–reactivity relationships within branched per- and polyfluoroalkyl substances (PFASs) undergoing cobalt-catalyzed reductive defluorination reactions. Experimental results and theoretical calculations reveal correlations among the extent of PFAS defluorination, the local C–F bonding environment, and calculated bond dissociation energies (BDEs). In general, BDEs increase in the following order: tertiary C–F bonds < secondary C–F bonds < primary C–F bonds. A tertiary C–F bond adjacent to three fluorinated carbons (or two fluorinated carbons and one carboxyl group) has a relatively low BDE that permits an initial defluorination to occur. Both a biogenic cobalt–corrin complex (B12) and an artificial cobalt–porphyrin complex (Co-PP) are found to catalytically defluorinate multiple C–F bonds in selected PFASs. In general, Co-PP exhibits an initial rate of defluorination that is higher than that of B12. Neither complex induced significant defluorination in linear perfluorooctanoic...

Promiscuous Defluorinating Enoyl-CoA Hydratases/Hydrolases Allow for Complete Anaerobic Degradation of 2-Fluorobenzoate.

Biodegradation of the environmentally hazardous fluoroaromatics has mainly been associated with oxygenase-dependent defluorination reactions. Only very recently a novel mode of oxygen-independent defluorination was identified for the complete degradation of para-substituted fluoroaromatics in the denitrifying Thauera aromatica: a promiscuous class I benzoyl-coenzyme A (BzCoA) reductase (BCR) catalyzed the ATP-dependent defluorination of 4-F-BzCoA to BzCoA. Here, we studied the unknown enzymatic defluorination during the complete degradation of 2-F-benzoate to CO2 and HF. We demonstrate that after activation of 2-F-benzoate by a promiscuous AMP-forming benzoate-CoA ligase, the 2-F-BzCoA formed is subsequently dearomatized by BCR to a mixture of 2-F- and 6-F-cyclohexa-1,5-diene-1-carboxyl-CoA (2-F-/6-F-1,5-dienoyl-CoA). This finding indicates that BCR is not involved in C–F-bond cleavage during growth with 2-F-benzoate. Instead, we identified defluorination of the two isomers by enoyl-CoA hydratases/hydrolases involved in down-stream reactions of the BzCoA degradation pathway. (i) The 1,5-dienoyl-CoA hydratase hydrated the F-1,5-dienoyl-CoA isomers to a mixture of the stable 2-F-6-OH-1-enoyl-CoA and the unstable α-fluorohydrin 6-F-6-OH-1-enoyl-CoA; the latter spontaneously decomposed to HF and 6-oxo-cyclohex-1-enoyl-CoA (6-oxo-1-enoyl-CoA), a common intermediate of the BzCoA degradation pathway. (ii) 6-Oxo-1-enoyl-CoA hydrolase/hydratase catalyzed the defluorination of 2-F-6-OH-1-enoyl-CoA to 2-oxo-6-OH-1-enoyl-CoA and HF again via water addition to an F-enoyl-CoA functionality. Based on these in vitro results, we demonstrate a previously overseen capability of 2-F-benzoate degradation for many but not all tested facultatively and obligately anaerobic bacteria that degrade aromatic compounds via the BzCoA degradation pathway. In conclusion, the newly identified enzymatic defluorination by enoyl-CoA hydratases via α-fluorohydrin formation represents an abundant, physiologically relevant principle of enzymatic defluorination.

Time-Resolved Spectroscopic Study of the Defluorination and Cyclization Reactions of Lomefloxacin in Water.

The mechanism of the defluorination reaction(s) of lomefloxacin (LF) upon light illumination was investigated by using ultrafast laser flash photolysis combined with transient resonance Raman spectroscopy in near neutral water solution. The zwitterionic configuration of LF was determined to be the main species present in the near neutral water solution and was the species that was photoexcited to initiate the photochemical reaction. Femtosecond transient absorption revealed that the first excited singlet state (S1) of LF did not appreciably undergo intersystem crossing (ISC), and instead partially decayed to the ground state via fluorescence emission, and there was partial cleavage of the carbon-fluorine bond at position 8 to produce a singlet LF aryl cation intermediate. The transient resonance Raman results provided a direct observation and vibrational spectral characterization of the singlet LF aryl cation species. Subsequently, the transformation from the singlet LF aryl cation to a triplet carbene via an ISC process was seen in nanosecond transient absorption spectra. Finally, the triplet carbene experienced a cyclization reaction with the N-ethyl chain to form a tricyclic product.

Photodegradation of Polyfluorinated Dibenzo-p-Dioxins in Organic Solvents: Experimental and Theoretical Studies.

Eighteen polyfluorinated dibenzo-p-dioxins (PFDDs) were synthesized by pyrolysis of fluorophenols. Using a 500 W Xe lamp as the light source, the PFDDs photodegradation kinetics in n-hexane were investigated. The photolysis reactions obeyed the pseudo-first-order rate equation, and higher fluorinated PFDDs tended to photolyze more slowly. Theoretically calculated parameters reflecting the molecular structural properties were used to develop a new model of PFDDs photolysis rates. The results indicated that the substitution pattern for fluorine atoms and the C-O bond length were major factors in the photolysis of PFDDs. We selected octafluorinated dibenzo-p-dioxin (OFDD) as a representative PFDDs to explore the influence of solvent on the photolysis rate of PFDDs, and the results indicated that neither the polarity nor donor hydrogen of organic solvents are independent influencing factors. Mechanistic pathways for the photolysis of OFDD in n-hexane were first studied. The results indicated that photodegradation of OFDD produces octafluorinated dihydroxybiphenyls, octafluorinated phenoxyphenols, and fluorinated phenols. The major pathway for photodegradation of OFDD was C-O bond cleavage. Defluorination reactions did not occur during the photolysis process.

The biodegradation vs. biotransformation of fluorosubstituted aromatics.

Fluoroaromatics are widely and--in recent years--increasingly used as agrochemicals, starting materials for chemical syntheses and especially pharmaceuticals. This originates from the special properties the carbon-fluorine bond is imposing on organic molecules. Hence, fluoro-substituted compounds more and more are considered to be important potential environmental contaminants. On the other hand, the microbial potentials for their transformation and mineralization have received less attention in comparison to other haloaromatics. Due to the high electronegativity of the fluorine atom, its small size, and the extraordinary strength of the C-F bond, enzymes and mechanisms known to facilitate the degradation of chloro- or bromoarenes are not necessarily equally active with fluoroaromatics. Here, we review the literature on the microbial degradation of ring and side-chain fluorinated aromatic compounds under aerobic and anaerobic conditions, with particular emphasis being placed on the mechanisms of defluorination reactions.