Elemental Fluorine: Not Only for Fluoroorganic Chemistry!

Fluoride occurrence in geothermal water of fault zone area, Southeast China

Fluoride (F^-) is a common trace component in groundwater, and long-term exposure to high-F^- groundwater is harmful to human health. Fluoride concentrations that exceed the World Health Organization guideline for drinking water (1.5 mg/L) have been detected in thermal and non-thermal groundwater from Southeast China, where the causes of the high fluoride occurrence are lack of study. To study the formation and migration mechanism of high fluoride groundwater from Southeast China, we carried out a systemic sampling of water samples in the surrounding area of Heyuan deep fault zone and Zijin-Boluo fault zone, then a comprehensive discussion including water hydrogeochemical, stable isotope composition, as well as hydrogeology analysis was conducted. Fluoride concentrations in geothermal and non-thermal water samples range from 1.11 to 22.76 mg/L and 0.04-8.3 mg/L, respectively. High temperature, alkaline conditions, and the depleted Ca²⁺ by reverse cation exchange and calcite precipitation would promote the release of fluoride from host rock to geothermal water. The availability of Ca-bearing and F-bearing minerals in host rock causes significant differences in fluoride concentrations between carbonate reservoir and granite reservoir. Hydrochemical diagrams reveal that the composition of groundwater is affected by mixing and that fractures act as the mixing channels in our study area. The addition of cold groundwater limits the fluoride concentrations by lowing temperature and increasing Ca²⁺ levels of geothermal water. Additionally, the relationship between F^- and SiO₂ indicates that geothermal water promotes the fluoride enrichment in cold groundwater, especially in confined aquifers which are more susceptible to geothermal water. The direct input of geothermal fluoride and secondary enrichment caused by alkaline condition contribute to the formation of high F^- concentrations (7.2-8.3 mg/L) in confined groundwater. Our findings highlight that the natural evolution of geothermal systems in fault zone can result in the formation of geogenic contaminated groundwater.

Promotion Energy Analysis Predicts Reaction Modes: Nucleophilic and Electrophilic Aromatic Substitution Reactions

To develop an approach to pre-emptively predict the existence of major reaction modes associated with a chemical system, based on exclusive consideration of reactant properties, we build herein on the valence bond perspective of chemical reactivity. In this perspective, elementary chemical reactions are conceptualized as crossovers between individual diabatic/semilocalized states. As demonstrated, the spacings between the main diabatic states in the reactant geometries-the so-called promotion energies-contain predictive information about which types of crossings are likely to occur on a potential energy surface, facilitating the identification of potential transition states and products. As an added bonus, promotion energy analysis provides direct insight into the impact of environmental effects, e.g., the presence of (polar) solvents and/or (local) electric fields, on a mechanistic landscape. We illustrate the usefulness of our approach by focusing on model nucleophilic and electrophilic aromatic substitution reactions. Overall, we envision our analysis to be useful not only as a tool for conceptualizing individual mechanistic landscapes but also as a facilitator of systematic reaction-network exploration efforts. Because the emerging VB descriptors are computationally inexpensive (and can alternatively be inferred through machine learning), they could be evaluated on-the-fly as part of an exploration algorithm. The so-predicted reaction modes could subsequently be examined in detail through computationally more-demanding methods.

[All]-S,S-dioxide Oligo-Thienylenevinylenes: Synthesis and Structural/Electronic Shapes from Their Molecular Force Fields

Oligo-S,S-dioxothienylenevinylenes have been prepared by transferring oxygen atoms to the sulfur atoms using the HOF⋅CH₃ CN complex. Their photophysical properties are presented in comparison with their thiophenevinylene congeners. Together with their vibrational properties and molecular force fields, this study allows for the interpretation of the alteration of aromaticity and inter-ring exocyclic π-conjugation in this series.

Electrochemical Flash Fluorination and Radiofluorination

A new method for rapid late-stage fluorination using the cation pool technique is presented. Fluorination and no-carrier-added radiofluorination of methyl (phenylthio) acetate, methyl 2-(methylthio) acetate, and methyl 2-(ethylthio) acetate were performed. The carbocations formed through electrochemical oxidation were stabilized by using a divided electrochemical cell and 2,2,2-trifluoroethanol (TFE) as the solvent at -20 °C. At the end of electrolysis, either stable-isotope [¹⁹F]fluoride or no-carrier-added radioactive [¹⁸F]fluoride was added to the reaction mixture to form the fluorinated or radiofluorinated product.

Electrochemical Fluorination and Radiofluorination of Methyl(phenylthio)acetate Using Tetrabutylammonium Fluoride (TBAF)

Electrochemical fluorination of methyl(phenylthio)acetate was achieved using tetrabutylammonium fluoride (TBAF). Electrochemical fluorination was performed under potentiostatic anodic oxidation using an undivided cell in acetonitrile containing TBAF and triflic acid. The influence of several parameters including: oxidation potential, time, temperature, sonication, TBAF concentration and triflic acid concentration on fluorination efficiency were studied. It was found that the triflic acid to TBAF concentration ratio plays a key role in the fluorination efficiency. Electrochemical fluorination resulted in formation of mono-fluorinated methyl 2-fluoro-2-(phenylthio)acetate verified by gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) Spectroscopy. Under optimum conditions 44 ± 3% mono fluorination yield was obtained after a 30 min electrolysis. Electrochemical radiofluorination for the synthesis of methyl 2-[¹⁸F]fluoro-2-(phenothio) acetate was also achieved with the same optimized electrochemical cell parameters where TBAF was first passed through an anion exchange resin containing fluorine-18. A radiochemical fluorination efficiency of 7 ± 1% was achieved after 30 min of electrolysis.

Tuning the polarity of charge carriers using electron deficient thiophenes

Thiophene-1,1-dioxide (TDO) oligomers have fascinating electronic properties. We previously used thermopower measurements to show that a change in charge carrier from hole to electron occurs with increasing length of TDO oligomers when single-molecule junctions are formed between gold electrodes. In this article, we show for the first time that the dominant conducting orbitals for thiophene/TDO oligomers of fixed length can be tuned by altering the strength of the electron acceptors incorporated into the backbone. We use the scanning tunneling microscope break-junction (STM-BJ) technique and apply a recently developed method to determine the dominant transport channel in single-molecule junctions formed with these systems. Through these measurements, we find that increasing the electron affinity of thiophene derivatives, within a family of pentamers, changes the polarity of the charge carriers systematically from holes to electrons, with some systems even showing mid-gap transport characteristics.

HOF·CH3CN: probably the best oxygen transfer agent organic chemistry has to offer

The complex HOF·CH3CN is readily obtained by bubbling dilute fluorine into aqueous acetonitrile solution. It does not have to be purified or isolated, and its solution can react as is, after the concentration has been establish by any iodometric method. It is the only reagent possessing a distinctive positive oxygen species. This enables electrophilic oxygen transfer with results no other reagent can match. HOF·CH3CN demonstrates its ability in epoxidations that either could not be performed before or could only obtained 5 orders of magnitude slower. This complex is also an excellent tool for oxygenation of compounds at the α position of a carbonyl, including the synthesis of some hard-to-come-by indanediones, which are important for fingerprint visualization on paper. HOF·CH3CN proves itself as a very efficient reagent for oxygenating tertiary nitrogen atoms both in aliphatic (including azides) and in aromatic amines, which could not be accomplished despite many attempts in the last 50 years. Oxygenation of two tertiary nitrogen atoms in the same molecule also becomes feasible as demonstrated for various phenanthrolines, bipyridines, diazafluorenones, and quinoxalines. It was also used to oxygenate primary amines, and because of the exceptionally mild conditions, it could transform vicinal aliphatic diamines to vicinal dinitro derivatives as well as amino acids to the corresponding nitro ones, practically unknown transformations before. Its ability to react with azines and hydrazones and convert them to the original carbonyls helped to establish these groups as good protecting tools for a variety of carbonyls. HOF·CH3CN excels in oxygenation of various sulfur and selenium compounds that could not be oxygenated in the past. The selectivity of the oxidation is quite good, and if there are alcohols, double bonds, and sulfides in the same molecule, usually the sulfur atom will be attacked first. Of special interest is the reaction with oligothiophenes resulting at will in either [all]-S,S-dioxooligothiophenes or in partially oxygenated ones. Some of these last derivatives have the narrowest HOMO-LUMO gap of all oligothiophenes tested, a very desirable feature. This reagent can also oxidize thiols or disulfides to either sulfonic or sulfinic acids at will, all in seconds and in very high yields. Since the oxygen atom of HOF·CH3CN originates in water, it is very easy and relatively inexpensive to introduce the heavy oxygen isotope in many sites of a variety of molecules, some of them quite important. The (18)O tirapazamine and any desirable alcohol, R(Ar)(18)OH, are two examples.

Highly accurate first-principles benchmark data sets for the parametrization and validation of density functional and other approximate methods. Derivation of a robust, generally applicable, double-hybrid functional for thermochemistry and thermochemical kinetics

We present a number of near-exact, nonrelativistic, Born-Oppenheimer reference data sets for the parametrization of more approximate methods (such as DFT functionals). The data were obtained by means of the W4 ab initio computational thermochemistry protocol, which has a 95% confidence interval well below 1 kJ/mol. Our data sets include W4-08, which are total atomization energies of over 100 small molecules that cover varying degrees of nondynamical correlations, and DBH24-W4, which are W4 theory values for Truhlar's set of 24 representative barrier heights. The usual procedure of comparing calculated DFT values with experimental atomization energies is hampered by comparatively large experimental uncertainties in many experimental values and compounds errors due to deficiencies in the DFT functional with those resulting from neglect of relativity and finite nuclear mass. Comparison with accurate, explicitly nonrelativistic, ab initio data avoids these issues. We then proceed to explore the performance of B2x-PLYP-type double hybrid functionals for atomization energies and barrier heights. We find that the optimum hybrids for hydrogen-transfer reactions, heavy-atoms transfers, nucleophilic substitutions, and unimolecular and recombination reactions are quite different from one another: out of these subsets, the heavy-atom transfer reactions are by far the most sensitive to the percentages of Hartree-Fock-type exchange y and MP2-type correlation x in an (x, y) double hybrid. The (42,72) hybrid B2K-PLYP, as reported in a preliminary communication, represents the best compromise between thermochemistry and hydrogen-transfer barriers, while also yielding excellent performance for nucleophilic substitutions. By optimizing for best overall performance on both thermochemistry and the DBH24-W4 data set, however, we find a new (36,65) hybrid which we term B2GP-PLYP. At a slight expense in performance for hydrogen-transfer barrier heights and nucleophilic substitutions, we obtain substantially better performance for the other reaction types. Although both B2K-PLYP and B2GP-PLYP are capable of 2 kcal/mol quality thermochemistry, B2GP-PLYP appears to be the more robust toward nondynamical correlation and strongly polar character. We additionally find that double-hybrid functionals display excellent performance for such problems as hydrogen bonding, prototype late transition metal reactions, pericyclic reactions, prototype cumulene-polyacetylene system, and weak interactions.

Facile stereoselective synthesis of fluorinated flavanone derivatives via a one-pot tandem reaction

A series of fluorinated flavanones were synthesized in moderate to good yields with excellent diastereoselectivities under mild reaction conditions via a one-pot tandem procedure involving a proline-catalyzed Knoevenagel condensation, a Michael addition, and an electrophilic fluorination by NFSI.

Reaction mechanism of oxidation, hydroxylation, and epoxidation by hypofluorous acid: a theoretical study of unusual H-bond-assisted catalysis

The oxidation of organic molecules by hypofluorous acid (HOF) was studied extensively and systematically by Rozen et al. Therefore, it seems appropriate to refer to the process as Rozen oxidation. An entire set of model molecules was selected for quantum chemical investigation of the oxidation mechanism: a C=C double bond in ethylene, sulfur and selenium in dimethyl derivatives, nitrogen and phosphorus in trimethyl derivatives, as well as methyl azides. In the gas phase, van der Waals complexes between HOF and the previously mentioned species easily are formed, but these complexes are reluctant to undergo oxidation. The addition of another HOF molecule connected with the formation of a cyclic complex (i.e., substrate and two molecules of HOF) seems to be decisive for the oxidation process. The attempt to substitute the second HOF molecule with H2O demonstrated the superiority of HOF. Complexes of this kind decompose along the reaction path smoothly (i.e., with a low activation energy) to the respective oxidation product. A potential role of the hydroxyl cation (HO+) in the oxidation step is mentioned. Besides an oxidation product, one HOF molecule is released (an essential feature of catalysis), and furthermore, hydrogen fluoride is formed. It was suggested by Sertchook et al. (J. Phys. Chem. A 2006, 110, 8275) that the interaction between the substrate to be oxidized and HOF is catalytically influenced by the HF molecule. The mechanism suggested here is more feasible and, particularly at the early stages of the oxidation process, decisive. Also, the role of acetonitrile, used as a solvent by Rozen et al., is discussed in terms of a continuum model. Moreover, passing from potential energies to Gibbs energies is considered.

Transforming Natural Amino Acids into α-Alkyl-Substituted Amino Acids with the Help of the HOF·CH3CN Complex

Alpha-alkyl amino acids can be efficiently prepared in high yields from the respective amino acids themselves. The key step is the oxidation of the amine function to create the corresponding alpha-nitro acid in a fast and very high yield reaction followed by phase-transfer alkylation and finally reduction to the desired alpha-alkyl amino acid. Several such acids containing aromatic rings or additional carboxylic groups and acids with steric hindrance at the alpha-position are suitable substrates. Several alkyl halides were examined as alkylating agents.

Predicting Facile Epoxidation of the Diamond (100) Surface by Dioxiranes and Subsequent Ring-Opening Reactions with Nucleophiles

By means of density functional theory coupled with effective cluster models, we have theoretically predicted the viability of epoxidation of the diamond (100) surface by organic dioxiranes. In addition, subsequent ring-opening reactions of the as-formed epoxide surface species with some nucleophiles, including water, ammonia, and alcohol, have also been explored. The facile epoxidation of diamond (100) by dioxiranes presents a new alternative for oxidation of the diamond (100) surface. More importantly, the as-formed epoxide-like surface species would be a useful springboard for further functionalizations of the diamond surface given the well-known abundant chemistry of organic epoxides. Therefore, this approach provides another new route to chemical functionalization of the diamond surface, which is potentially useful for leading to the improvement of diamond behavior and constructing new hybrid diamond-based materials for wide potential applications in many fields. In perspective, implications for other theoretical work are also discussed.

Rozen's Epoxidation Reagent, CH3CN·HOF: A Theoretical Study of Its Structure, Vibrational Spectroscopy, and Reaction Mechanism

Rozen's epoxidation reagent, CH(3)CN.HOF, and a prototype epoxidation reaction employing it, have been subjected to an extensive ab initio and density functional study. Its anharmonic force field reveals a very strong red shift for the OH stretch and a strong blue shift for the HOF bend, in semiquantitative agreement with experiment. The very strong hydrogen bond (8.20 kcal/mol at the W1 level) not only serves to stabilize the reactant but also considerably lowers the barrier height for epoxidation of ethylene. Moreover, the reaction byproduct HF is found to act autocatalytically. The OH moiety acquires HO(+) character in the transition state. Our W1 benchmark data for the reaction profile allow the performance of various DFT functionals to be assessed. In general, "kinetics" functionals overestimate barrier heights, the BMK functional less so than the others. The B1B95 and TPSS33B95 meta-GGA functionals both perform very well, whereas general-purpose hybrid GGAs underestimate barrier heights. The simple PBE0 functional does reasonably well.

Attaching the fluorine atom to organic molecules using BrF3 and other reagents directly derived from F2

Elemental fluorine is a starting point for nucleophilic fluorinations (e.g., BrF3), radical fluorinations (e.g., F2 under irradiation), and electrophilic fluorinations (e.g., AcOF). All three categories are represented in this Account. Bromine trifluoride, although commercially available, can be readily made from the elements and is a very good source for naked nucleophilic fluoride ions. To minimize radical reactions, an anchor has to be installed in the molecules with which it reacts. Such an anchor is constituted of a soft base such as nitrogen and especially sulfur atoms. This reagent was used for constructing compounds with a CF2, CF3, CHF2, or CF2COOH group in specific sites. F2 itself was used for completing perfluorination of various polyfluoroethers, while the electrophilic acetyl hypofluorite is an excellent tool for introducing a single fluorine atom into organic molecules such as carboxylic acids and nitro compounds.

From Azides to Nitriles. A Novel Fast Transformation Made Possible by BrF3

[reaction: see text]. Various alkyl and aryl azides, readily obtained from halides or alcohols, were transformed into the corresponding nitriles using bromine trifluoride in moderate to good yields. The reaction is general and gives positive results with aliphatic, aromatic, cyclic, and functionalized azides. It can also be applied to the synthesis of optically active nitriles.

Synthesis of 1,10-N,N‘-Phenanthroline Dioxides Using HOF·CH3CN Complex

[reaction: see text] HOF.CH(3)CN, a very efficient oxygen-transfer agent, made readily from F(2), H(2)O, and CH(3)CN, was reacted with various 1,10-phenanthroline derivatives to form the corresponding N,N'-dioxides in good yields and short reaction times.

A Fast, High-Yield Preparation of Vicinal Dinitro Compounds Using HOF·CH3CN

HOF.CH3CN, a very efficient oxygen-transfer agent, was reacted with various aliphatic and aromatic vicinal diamino compounds. The products were the rare, vicinal dinitro derivatives formed in excellent yields and short reaction times. This is in contrast to other oxygen-transfer agents which tend to break the central C-C bond of the diamino precursor. This reaction was also used for making dinitro compounds with all four oxygens, being the [18]O isotope.

Sidewall epoxidation of single-walled carbon nanotubes: a theoretical prediction

[reaction: see text] By means of a two-layered ONIOM approach, we predict that sidewall epoxidation of single-walled carbon nanotubes (SWNTs) with dioxiranes is viable. The SWNT epoxides thus produced could be precursors for further chemical modification of SWNTs, given the abundant and well-established chemistry of organic epoxides. This opens the door for routine chemical manipulation of SWNTs.