Nucleofugality and nucleophilicity of fluoride in protic solvents

Co-Catalyzed Hydrofluorination of Alkenes: Photocatalytic Method Development and Electroanalytical Mechanistic Investigation

The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)-F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and the poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a photoredox-mediated polar-radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radiopharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calculations, unveiled the involvement of both carbocation and CoIV-alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.

Solvent Polarity under Vibrational Strong Coupling

Vibrational strong coupling (VSC) occurs when molecular vibrations hybridize with the modes of an optical cavity, an interaction mediated by vacuum fluctuations. VSC has been shown to influence the rates and selectivity of chemical reactions. However, a clear understanding of the mechanism at play remains elusive. Here, we show that VSC affects the polarity of solvents, which is a parameter well-known to influence reactivity. The strong solvatochromic response of Reichardt's dye (RD) was used to quantify the polarity of a series of alcohol solvents at visible wavelengths. We observed that, by simultaneously coupling the OH and CH vibrational bands of the alcohols, the absorption maximum of Reichardt's dye redshifted by up to ∼15.1 nm, corresponding to an energy change of 5.1 kJ·mol^-1. With aliphatic alcohols, the magnitude of the absorption change of RD was observed to be related to the length of the alkyl chain, the molecular surface area, and the polarizability, indicating that dispersion forces are impacted by strong coupling. Therefore, we propose that dispersion interactions, which themselves originate from vacuum fluctuations, are impacted under strong coupling and are therefore critical to understanding how VSC influences chemistry.

Hydrolytically Stable and Thermo-Mechanically Tunable Poly(Urethane) Thermoset Networks that Selectively Degrade and Generate Reusable Molecules

Cross-linked polymeric networks that possess tunable properties and degrade on-demand have broad applications in today's society. Herein, we report on silyl-containing poly(urethane) (silyl-PU) thermoset networks, which are highly cross-linked stimuli-responsive materials with hydrolytic stability at 37.7 °C and 95% relative humidity, thermal stability of 280-311.2 °C, tensile properties of 0.38-51.7 MPa strength and 73.7-256.4% elongation, including storage modulus of 2268-3499 MPa (in the glassy state). However, unlike traditional (i.e., nondegradable) PU thermosets, these silyl-PUs selectively activate with fluoride ion under mild and static conditions to completely degrade, via cascading bond cleavages, and generate recoverable and reusable molecules. Silyl-PUs, as thin films, also demonstrated complete removal (within 30 min) from a strongly adhered epoxy thermoset network without altering the structure of the latter. Silyl-PU thermosets have potential applications in composite parts, vehicle and industrial coatings, and rigid plastics for personal devices, and may reduce environmental waste compared to nondegradable, single-use materials.

Electrochemical C-C bond cleavage of cyclopropanes towards the synthesis of 1,3-difunctionalized molecules

Electrochemistry has a lot of inherent advantages in organic synthesis and many redox reactions have been achieved under electrochemical condition. However, the electrochemical C-C bond cleavage and functionalization reactions are less studied. Here we develop electrochemical C-C bond cleavage and 1,3-difuntionalization of arylcyclopropanes under catalyst-free and external-oxidant-free conditions. 1,3-difluorination, 1,3-oxyfluorination and 1,3-dioxygenation of arylcyclopropanes are achieved with a high chemo- and regioselectivity by the strategic choice of nucleophiles. This protocol has good functional groups tolerance and can be scaled up. Mechanistic studies demonstrate that arylcyclopropane radical cation obtained from the anode oxidation and the subsequently generated benzyl carbonium are the key intermediates in this transformation. This development provides a scenario for constructing 1,3-difunctionalized molecules.

Intercepting the Banert cascade with nucleophilic fluorine: direct access to α-fluorinated NH-1,2,3-triazoles

The treatment of propargylic azides with silver(i) fluoride in acetonitrile was found to yield α-fluorinated NH-1,2,3-triazoles via the Banert cascade. The reaction was regioselective and the products result from an initial [3,3] rearrangement. The reaction is demonstrated on >15 examples with yields ranging from 37% to 86%.

Silver-Promoted Fluorination Reactions of α-Bromoamides

Silver-promoted C-F bond formation in α-bromoamides by using AgF under mild conditions is reported. This simple method enables access to tertiary, secondary, and primary alkyl fluorides involving biomolecular scaffolds. This transformation is applicable to primary and secondary amides and shows broad functional-group tolerance. Kinetics experiments revealed that the reaction rate increased in the order of 3°>2°>1° α-carbon atom. In addition, it was found that the acidic amide proton plays an important role in accelerating the reaction. Mechanistic studies suggested generation of an aziridinone intermediate that undergoes subsequent nucleophilic addition to form the C-F bond with stereospecificity (i.e., retention of configuration). The synthesis of sterically hindered alcohols and ethers by using Ag^I is also demonstrated. Examples of reactions of α-bromoamides with O nucleophiles are presented.

19F- and 18F-Arene Deoxyfluorination via Organic Photoredox-Catalysed Polarity-Reversed Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution (SNAr) is routinely used to install 19F- and 18F- in aromatic molecules, but is typically limited to electron-deficient arenes due to kinetic barriers associated with C-F bond formation. Here we demonstrate that a polarity-reversed photoredox-catalysed arene deoxyfluorination operating via cation radical-accelerated nucleophilic aromatic substitution (CRA-SNAr) enables the fluorination of electron-rich arenes with 19F- and 18F- under mild conditions, thus complementing the traditional arene polarity requirements necessary for SNAr-based fluorination. The utility of our radiofluorination strategy is highlighted by short reaction times, compatibility with multiple nucleofuges, and high radiofluorination yields, especially that of an important cancer positron emission tomography (PET) agent [18F]5-fluorouracil ([18F]FU). Taken together, our fluorination approach enables the development of fluorinated and radiofluorinated compounds that can be difficult to access by classical SNAr strategies, with the potential for use in the synthesis and discovery of PET radiopharmaceuticals.

Influence of solvent mixture on nucleophilicity parameters: the case of pyrrolidine in methanol-acetonitrile

The course of organic chemical reactions is efficiently modelled through the concepts of “electrophiles” and “nucleophiles” (meaning electron-seeking and nucleus-seeking reactive species). On the one hand, an advanced approach of the correlation of the nucleophilicity parameters N and electrophilicity E has been delivered from the linear free energy relationship log k (20 °C) = s(N + E). On the other hand, the general influence of the solvent mixtures, which are very often employed in preparative synthetic chemistry, has been poorly explored theoretically and experimentally, to date. Herein, we combined experimental and theoretical studies of the solvent influence on pyrrolidine nucleophilicity. We determined the nucleophilicity parameters N and s of pyrrolidine at 20 °C in CH₃OH/CH₃CN mixtures containing 0, 20, 40, 60, 80 and 100% CH₃CN by kinetic investigations of their nucleophilic substitution reactions to a series of 2-methoxy-3-X-5-nitrothiophenes 1a–e (X = NO₂, CN, COCH₃, CO₂CH₃, CONH₂). Depending on the resulting solvation medium, the N parameters range from 15.72 to 18.32 on the empirical nucleophilicity scale of Mayr. The nucleophilicity parameters N first evolve linearly with the content of acetonitrile up to 60% CH₃CN by volume, but is non linear for higher amounts. We designed a general computation protocol to investigate the solvent effect at the atomistic scale. The nucleophilicity in solvent mixtures was evaluated by combining classical molecular dynamic (MD) simulations of solvated pyrrolidine and a few density functional theory (DFT) calculations of Parr nucleophilicity. The pyrrolidine theoretical nucleophilicity 1/ω obtained in various CH₃OH/CH₃CN mixtures are in excellent agreement with Mayr's nucleophilicity (N) parameters measured. Analyses of the molecular dynamic trajectories reveal that the decrease of the nucleophilicity in methanol rich mixtures arises predominantly from the solvation of the pyrrolidine by methanol molecules through strong hydrogen bonds. Last, we proposed a simple model to predict and accurately reproduce the experimentally obtained nucleophilicity values.

Combined experiments and modelling rationalize the large influence of solvent composition on pyrrolidine nucleophilicity.

Selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes from unactivated gem-difluoroalkanes controlled by aluminum-based Lewis acids

The highly selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes via the cleavage of inert C(sp³)–F bonds in unactivated gem-difluoroalkanes using readily available and inexpensive aluminum-based Lewis acids of low toxicity is reported. The selectivity of this reaction can be controlled by modifying the substituents on the central aluminum atom of the promoter. An intramolecular cascade Friedel-Crafts alkylation of unactivated gem-difluorocarbons can be achieved using a stoichiometric amount of AlCl₃. The subsequent synthesis of alkenyl chlorides via F/Cl exchange followed by an elimination can be accomplished using AlEt₂Cl as a fluoride scavenger and halogen source. The defluorinative elimination of acyclic and cyclic gem-difluorocarbons to give monofluoroalkenes can be achieved using AlEt₃.

Gold-Catalyzed Hydrofluorination of Internal Alkynes Using Aqueous HF

The gold-catalyzed hydrofluorination reaction of internal alkynes using hydrofluoric acid is reported. Notably, those conditions use one of the most economical sources of HF and are free of additional additives. Both symmetrical and unsymmetrical internal alkynes can be utilized, and the use of alkynes bearing a fluorinated group at the propargylic position as substrates allowed for a regioselective hydrofluorination reaction.

Activation of Saturated Fluorocarbons to Synthesize Spirobiindanes, Monofluoroalkenes, and Indane Derivatives

Fluorinated organic compounds are produced in abundance by the pharmaceutical and agrochemical industry, making such compounds attractive as building blocks for further functionalization. Unfortunately, activation of C(sp³)-F bond in saturated fluorocarbons, especially for aliphatic gem-difluoroalkanes, remains challenging. Here we describe the selective activation of inert C(sp³)-F bonds catalyzed by B(C₆F₅)₃. In hexafluoro-2-propanol (HFIP), chemically robust aliphatic gem-difluorides are converted in high yields to the corresponding substituted 2,2′,3,3′-tetrahydro-1,1′-spirobiindenes via a B(C₆F₅)₃-catalyzed intramolecular cascade Friedel-Crafts cyclization, not requiring a silicon-based trapping reagent. However, in the absence of a hydrogen-bonding donor solvent such as HFIP, the aliphatic gem-difluorides preferentially engage in a defluorination/elimination process that provides monofluorinated alkenes in good yields. Furthermore, a series of substituted 1-alkyl-2,3-dihydro-1H-indenes was obtained in high yield from the B(C₆F₅)₃-catalyzed defluorinative cyclization of aliphatic secondary monofluorides in HFIP. The protocol could inspire development of a new class of main-group Lewis acid-catalyzed C(sp³)-F bond activation in general unactivated fluorocarbons.

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C(sp³)-F bond activation in general unactivated fluorocarbons

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The activation of C(sp³)-F bonds in aliphatic gem-difluoroalkanes

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The selective activation of inert C(sp³)-F bonds catalyzed by B(C₆F₅)₃

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An intramolecular cascade defluorinative Friedel-Crafts cyclization

Philicity, fugality, and equilibrium constants: when do rate-equilibrium relationships break down?

Linear free energy relationships, in particular relationships between rate and equilibrium constants, are the basis for our rationalization of organic reactivity. Whereas relationships between the kinetic terms nucleophilicity and nucleofugality and the thermodynamic term basicity have been in the focus of interest for many decades, much less attention has been paid to the relationships between electrophilicity, electrofugality, and Lewis acidity. By using p- and m-substituted benzhydrylium ions (Aryl2CH+) as reference electrophiles, reference electrofuges, and reference Lewis acids of widely varying electron demand and constant steric surroundings of the reaction center, we have developed comprehensive reactivity scales which can be employed for classifying polar organic reactivity and for rationally designing synthetic transformations. It is a general rule that structural variations in electron-surplus species, which increase basicities, also increase nucleophilicities and decrease nucleofugalities, and that structural variations in electron-deficient species, which increase Lewis acidities also increase electrophilicities and decrease electrofugalities. Deviations from this behavior are analyzed, and it is shown that variations in intrinsic barriers are responsible for the counterintuitive observations that structural variation in one of the reactants alters the rates of forward and backward reactions in the same direction. A spectacular example of this phenomenon is found in vinyl cation chemistry: Vinyl cations are not only generated several orders of magnitude more slowly in SN1 reactions than benzhydrylium ions of the same Lewis acidity, but also react much more slowly with nucleophiles.

Exploring the Reducing Ability of Organic Dye (Acr+-Mes) for Fluorination and Oxidation of Benzylic C(sp3)-H Bonds under Visible Light Irradiation

The excellent oxidizing capability of acridinium-based organic dye (Acr⁺-Mes) is fully studied in photoredox catalysis. However, its reducing ability is always considered weak for organic transformation. The reducing ability of Acr⁺-Mes is developed by Selectfluor to achieve effective fluorination and oxidation of benzylic C(sp³)-H bonds under visible light irradiation, which is not available for the direct use of oxidizing ability of excited Acr⁺-Mes. Mechanistic insights provided strong evidence for the oxidative quenching of Acr⁺-Mes.

Mechanistic studies on the Pd-catalyzed vinylation of aryl halides with vinylalkoxysilanes in water: the effect of the solvent and NaOH promoter

The mechanism of the Pd-catalyzed vinylation of aryl halides with vinylalkoxysilanes in water has been studied using different catalytic precursors. The NaOH promoter converts the initial vinylalkoxysilane into a highly reactive water-soluble vinylsilanolate species. Similarly, deuterium-labeling experiments have shown that, irrespective of the catalytic precursor used, vinylation occurs exclusively at the CH vinylic functionality via a Heck reaction and not at the C-Si bond via a Hiyama cross-coupling. The involvement of a Heck mechanism is interpreted in terms of the reduced nucleophilicity of the base in water, which disfavors the transmetalation step. The Heck product (β-silylvinylarene) undergoes partial desilylation, with formation of a vinylarene, by three different routes: (a) hydrolytic desilylation by the aqueous solvent (only at high temperature); (b) transmetalation of the silyl olefin on the PdH Heck intermediate followed by reductive elimination of vinylarene; (c) reinsertion of the silyl olefin into the PdH bond of the Heck intermediate followed by β-Si syn-elimination. Both the Hiyama and Heck catalytic cycles and desilylation mechanisms b and c have been computationally evaluated for the [Pd(en)Cl2] precursor in water as solvent. The calculated Gibbs energy barriers support the reinsertion route proposed on the basis of the experimental results.

Photogeneration of benzhydryl cations by near-UV laser flash photolysis of pyridinium salts

Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV-vis spectroscopic measurements. The rate constants for the reaction of the 4,4'-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4-(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the second-order rate constants, we derived the nucleophilicity parameters N and s(N) for the substituted pyridines, as defined by the linear free energy relationship, log k(2) = s(N)(N + E).