Facile SN2 Reaction in Protic Solvent: Quantum Chemical Analysis

Nucleophilic Reactions Using Alkali Metal Fluorides Activated by Crown Ethers and Derivatives

We review crown ether-facilitated nucleophilic reactions using metal salts, presenting the studies using kinetic measurements and quantum chemical methods. We focus on the mechanistic features, specifically on the contact ion-pair (CIP) mechanism of metal salts for nucleophilic processes promoted by crown ethers and derivatives. Experimental verification of the CIP form of the metal salt CsF complexed with [18-Crown-6] by H-NMR spectroscopy is described. The use of chiral crown ethers and derivatives for enantioselective nucleophilic processes is also discussed.

Ionic Liquids as Organocatalysts for Nucleophilic Fluorination: Concepts and Perspectives

Besides their extremely useful properties as solvent, ionic liquids (ILs) are now considered to be highly instructive tools for enhancing the rates of chemical reactions. The ionic nature of the IL anion and cation seems to be the origin of this fascinating function of ILs as organocatalyst/promoter through their strong Coulombic forces on other ionic species in the reaction and also through the formation of hydrogen bonds with various functional groups in substrates. It is now possible to tailor-make ILs for specific purposes as solvent/promoters in a variety of situations by carefully monitoring these interactions. Despite the enormous potentiality, it seems that the application of ILs as organocatalysts/promoters for chemical reactions have not been fully achieved so far. Herein, we review recent developments of ILs for promoting the nucleophilic reactions, focusing on fluorination. Various aspects of the processes, such as organocatalytic capability, reaction mechanisms and salt effects, are discussed.

Microsolvated Ion-Molecule SN2 Reactions with Dual Nucleophiles Induced by Solvent Molecules

Singly-hydrated HOO - anion was found to induce alternative nucleophile HO - via proton transfer from water molecule as react with CH 3 Cl recently. To investigate the generality of this effect, the competition between the solvent-induced HO - -S N 2 pathway and the normal HOO - -S N 2 pathway is studied for the microsolvated HOO - (H 2 O) n=1,2,3 + CH 3 X (X = F, Cl, Br, I) reaction by quantum chemistry calculation. Incremental hydration increases the barrier heights of both pathways and enlarges the barrier difference between them, which favors the HOO - -S N 2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculation shows the water induced HO - -S N 2 pathway is highly suppressed as the degree of hydration increases beyond two. The differential barrier under incremental hydration can be explained by solvent molecules stabilizing the HOMO level of HO - (HOOH)(H 2 O) n-1 nucleophile more than that of HOO - (H 2 O) n nucleophile. Comparison between these HO - -nucleophiles and HOO - -nucleophiles suggests that α-effect exists. Activation strain analysis attributes the barrier differences to the stronger distortion of the TS of HO - -S N 2 pathway than the counterparts of HOO - -S N 2 pathway. This work adds our understanding of the role of individual solvent molecules to induce new nucleophiles of the fundamental organic reaction.

Origin of Salt Effects in SN2 Fluorination Using KF Promoted by Ionic Liquids: Quantum Chemical Analysis

Quantum chemical analysis is presented, motivated by Grée and co-workers’ observation of salt effects [Adv. Synth. Catal. 2006, 348, 1149–1153] for S_N2 fluorination of KF in ionic liquids (ILs). We examine the relative promoting capacity of KF in [bmim]PF₆ vs. [bmim]Cl by comparing the activation barriers of the reaction in the two ILs. We also elucidate the origin of the experimentally observed additional rate acceleration in IL [bmim]PF₆ achieved by adding KPF₆. We find that the anion PF₆⁻ in the added salt acts as an extra Lewis base binding to the counter-cation K⁺ to alleviate the strong Coulomb attractive force on the nucleophile F⁻, decreasing the Gibbs free energy of activation as compared with that in its absence, which is in good agreement with experimental observations of rate enhancement. We also predict that using 2 eq. KF together with an eq. KPF₆ would further activate S_N2 fluorination

The role of intermolecular forces in ionic reactions: the solvent effect, ion-pairing, aggregates and structured environment

The environment enclosing an ionic species has a critical effect on its reactivity. In a more general sense, medium effects are not limited to the solvent, but involve the counter ion effect (ion pairing), formation of larger aggregates and structured environment as provided by the host in the case of host-guest complexes. In this review, a general view of the medium effect on anion-molecule reactions is presented. Nucleophilic substitution reactions of aliphatic (SN2) and aromatic (SNAr) systems, as well as elimination reactions (E2), are the focus of the discussion. In particular, nucleophilic fluorination with KF, CsF and tetraalkylammonium fluoride was used as the main model, because of the importance of this kind of reaction and the recent advances in the study of these systems. The solvent effect, ion pairing, formation of aggregates and formation of complexes with crown ethers, cryptands and calixarenes are discussed. For a deeper insight into the medium effect, many results of reliable theoretical calculations in close agreement with experiments were chosen as examples.

Nucleophilic Fluorination with KF Catalyzed by 18-Crown-6 and Bulky Diols: A Theoretical and Experimental Study

The activation of potassium fluoride for nucleophilic fluorination of alkyl halides is an important challenge because of the high lattice energy of this salt and its low solubility in many polar aprotic solvents. Crown ethers have been used for increasing the solubilization of KF during several decades. Nevertheless, these macrocycles are not enough to produce a high reaction rate. In this work, theoretical methods were used for designing a synergic combination of bulky diols with crown ethers able to accelerate this kind of reaction. The calculations have predicted that the bulky diol 1,4-Bis(2-hydroxy-2-propyl)benzene, which has distant hydroxyl groups, is able to catalyze nucleophilic fluorination in combination with 18-crown-6 via two hydrogen bonds to the S_N2 transition state. Experimental studies following the theoretical predictions have confirmed the catalytic effect and the estimated kinetic data point out that the bulky diol at 1 mol L^-1 in combination with 18-crown-6 is able to produce an 18-fold increase in the reaction rate in relation to crown ether catalysis only. The reaction produces 46% yield of fluorination after 24 h at moderate temperature of 82 °C, with minimal formation of the side elimination product. Thus, this work presents an improved method for fluorination with KF salt.

Potential of Mean Force Calculations for an SN2 Fluorination Reaction in Five Different Imidazolium Ionic Liquid Solvents Using Quantum Mechanics/Molecular Mechanics Molecular Dynamics Simulations

The use of ionic liquids (ILs) as both catalysts and solvents in a wide range of chemical reactions has received considerable attention over the last few years due to their positive effects in enhancing reaction rates and selectivities. In this work, hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations were carried out in conjunction with umbrella-sampling techniques to study the bimolecular nucleophilic substitution (S_N2) fluorination reaction between propyl-mesylate and potassium fluoride using five ILs as solvents, specifically, 1-butyl-3-methylimidazolium mesylate ([C₄mim][OMs]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]), 1-butyl-3-methylimidazolium trifluoroacetate ([C₄mim][CF₃COO]), 1-butyl-3-methylimidazolium bromide ([C₄mim][Br]), and 1-butyl-3-methylimidazolium chloride ([C₄mim][Cl]) at 373.15 K. The QM region (reactive part) in all QM/MM systems was simulated using the Parametric Method 6 (PM6) semiempirical methods, and for the MM region (IL solvent), classical force fields (FF) were employed, with the FF developed within the group. The calculated activation free energy barriers (ΔG^‡) for the S_N2 reaction in the presence of [C₄mim][OMs] and [C₄mim][BF₄] ILs were in agreement with the experimental values reported in the literature. On the other hand, only predicted values were obtained for the activation energies for the [C₄mim][CF₃COO], [C₄mim][Br], and [C₄mim][Cl] ILs. These activation energies indicated that the S_N2 reaction would be more facile to proceed using the [C₄mim][Cl] and [C₄mim][OMs] ILs, in contrast with the use of [C₄mim][Br] IL, which presented the highest activation energy. Energy-pair distributions, radial distribution functions, and noncovalent interactions (NCI) were also calculated to elucidate the molecular interactions between the reactive QM region and the solvents or reaction media. From these calculations, it was found that not only the reactivity can be enhanced by selecting a specific anion to increase the K-F separation but also the cation plays a relevant role, producing a synergetic effect by forming hydrogen bonds with the fluorine atom from KF and with the oxygen atoms within the mesylate leaving group. Three interactions are significant for the IL catalytic behavior, F_QM-HX, K_QM-anion, and O_QM-HX interactions, where the F_QM and K_QM labels correspond to fluorine and potassium atoms from the KF salt, O_QM corresponds to oxygen atoms within the mesylate leaving group (reactant), and HX refers to hydrogen atoms within the IL cation. The NCI analysis revealed that K_QM-anion interactions are of weak type, indicating the importance of hydrogen bond interactions from the cation such as F_QM-HX and O_QM-HX for the catalytic behavior of ILs.

Anion coordination chemistry using O-H groups

This review covers significant advances in the use of O-H groups in anion coordination chemistry. The review focuses on the use of these groups in synthetic anion receptors, as well as more recent developments in transport, self-assembly and catalysis.

Competition of F/OH-Induced SN2 and Proton-Transfer Reactions with Increased Solvation

The potential energy profiles of F/OH-induced nucleophilic substitution (S_N2) and proton-transfer (PT) channels evolving with solvation for reactions of F^-(H₂O) _n=1-2 + CH₃I were characterized using B3LYP/ECP/d method. The hydrogen-bonded F^-(H₂O) _n---HCH₂I prereaction complex at the entrance of potential energy surface (PES) has a significant role on the reaction dynamics for each channel. Among the above three channels, the F-S_N2 channel is the most preferred and OH-S_N2 could be competitive. In contrast, the PT channel will occur at much higher collision energy. Importantly, for each channel, the central barrier is gradually increased with the addition of water molecules. This phenomenon indicates that the reactivity will decrease with degrees of solvation and this has been confirmed by experiment and direct dynamics simulations. Moreover, compared with the previous trajectory simulations, a non-IRC behavior has been uncovered. The water delivering process from fluorine to iodine side as illustrated on PES is barely observed, and instead, the reaction tends to dehydrate before passing through the S_N2 barrier and proceeds with the less hydrated pathway in order to weaken the steric effect. The work presented here shows the comprehensive potential energy surfaces and structures information on the F-S_N2, PT, and OH-S_N2 channels, and predict their competitive relationship, which would be helpful for better understanding the dynamics behavior of the title and analogous reactions.

Potassium fluoride activation for the nucleophilic fluorination reaction using 18-crown-6, [2.2.2]-cryptand, pentaethylene glycol and comparison with the new hydro-crown scaffold: a theoretical analysis

Ion-pair binding in the hydro-crown ether leads to more effective catalytic cycle for nucleophilic fluorination.

Revisiting the Dielectric Constant Effect on the Nucleophile and Leaving Group of Prototypical Backside SN2 Reactions: A Reaction Force and Atomic Contribution Analysis

The solvent effect on the nucleophile and leaving group atoms of the prototypical F^- + CH₃Cl → CH₃F + Cl^- backside bimolecular nucleophilic substitution reaction (S_N2) is analyzed employing the reaction force and the atomic contributions methods on the intrinsic reaction coordinate (IRC). Solvent effects were accounted for using the polarizable continuum solvent model. Calculations were performed employing 11 dielectric constants, ε, ranging from 1.0 to 78.5, to cover a wide spectrum of solvents. The reaction force data reveal that the solvent mainly influences the region of the IRC preceding the energy barrier, where the structural rearrangement to reach the transition state occurs. A detailed analysis of the atomic role in the reaction as a function of ε reveals that the nucleophile and the carbon atom are the ones that contribute the most to the energy barrier. In addition, we investigated the effect of the choice of nucleophile and leaving group on the ΔE₀ and ΔE^‡ of Y^- + CH₃X → YCH₃ + X^- (X, Y = F, Cl, Br, I) in aqueous solution. Our analysis allowed us to find relationships between the atomic contributions to the activation energy and leaving group ability and nucleophilicity.

Titania-catalyzed radiofluorination of tosylated precursors in highly aqueous medium

Nucleophilic radiofluorination is an efficient synthetic route to many positron-emission tomography (PET) probes, but removal of water to activate the cyclotron-produced [(18)F]fluoride has to be performed prior to reaction, which significantly increases overall radiolabeling time and causes radioactivity loss. In this report, we demonstrate the possibility of (18)F-radiofluorination in highly aqueous medium. The method utilizes titania nanoparticles, 1:1 (v/v) acetonitrile-thexyl alcohol solvent mixture, and tetra-n-butylammonium bicarbonate as a phase-transfer agent. Efficient radiolabeling is directly performed with aqueous [(18)F]fluoride without the need for a drying/azeotroping step to significantly reduce radiosynthesis time. High radiochemical purity of the target compound is also achieved. The substrate scope of the synthetic strategy is demonstrated with a range of aromatic, aliphatic, and cycloaliphatic tosylated precursors.

Origin of Enhanced Reactivity of a Microsolvated Nucleophile in Ion Pair SN2 Reactions: The Cases of Sodium p-Nitrophenoxide with Halomethanes in Acetone

In a kinetic experiment on the SN2 reaction of sodium p-nitrophenoxide with iodomethane in acetone-water mixed solvent, Humeres et al. (J. Org. Chem. 2001, 66, 1163) found that the reaction depends strongly on the medium, and the fastest rate constant was observed in pure acetone. The present work tries to explore why acetone can enhance the reactivity of the title reactions. Accordingly, we make a mechanistic study on the reactions of sodium p-nitrophenoxide with halomethanes (CH3X, X = Cl, Br, I) in acetone by using a supramolecular/continuum model at the PCM-MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) level, in which the ion pair nucleophile is microsolvated by one to three acetone molecules. We compared the reactivity of the microsolvated ion pair nucleophiles with solvent-free ion pair and anionic ones. Our results clearly reveal that the microsolvated ion pair nucleophile is favorable for the SN2 reactions; meanwhile, the origin of the enhanced reactivity induced by microsolvation of the nucleophile is discussed in terms of the geometries of transition state (TS) structures and activation strain model, suggesting that lower deformation energies and stronger interaction energies between the deformed reactants in the TS lead to the lower overall reaction barriers for the SN2 reaction of microsolvated sodium p-nitrophenoxide toward halomethanes in acetone.

Palladium-catalyzed allylic C-H fluorination

The first catalytic allylic C-H fluorination reaction using a nucleophilic fluoride source is reported. Under the influence of a Pd/Cr cocatalyst system, simple olefin substrates undergo fluorination with Et3N·3HF in good yields with high branched:linear regioselectivity. The mild conditions and broad scope make this reaction a powerful alternative to established methods for the preparation of allylic fluorides from prefunctionalized substrates.

Iridium-catalyzed allylic fluorination of trichloroacetimidates

A rapid allylic fluorination method utilizing trichloroacetimidates in conjunction with an iridium catalyst has been developed. The reaction is conducted at room temperature under ambient air and relies on Et(3)N·3HF reagent to provide branched allylic fluorides with complete regioselectivity. This high-yielding reaction can be conducted on a multigram scale and shows considerable functional group tolerance. The use of [(18)F]KF·Kryptofix allowed (18)F(-) incorporation in 10 min.

A comparative study on the effect of solvent on nucleophilic fluorination with [18F]fluoride: protic solvents as co-solvents in SN2 and SNAr reactions

The effect of solvent on nucleophilic substitution with cyclotron-produced [18F]fluoride was studied in polar aprotic (CH3CN and DMF) and protic solvent (t-BuOH and t-amyl alcohol) mixtures (CH3CN/co-solvent, 2:8) in a series of model compounds, 4-(R1-methyl)benzyl R2-benzoates, using a K2.2.2/[ 18F]KF phase transfer system (R1=–Cl, –OMs or –OH; R2=–Cl, –I or –NO2). 18F-fluorination of compounds 1–3, with chloride or mesylate as a leaving group in the benzylic position (R1), afforded the desired 4-([ 18F]fluoromethyl)benzyl analogues in all solvents during 15ߙmin reaction time. The highest radiochemical yields (RCY) in all the studied reaction temperatures (80, 120 and 160ºC) were achieved in CH3CN. Radiochemical yields in protic solvents were comparable to RCY in CH3CN only with the sulfonate ester 3 as a starting material. 18F-Fluorination of the benzylic halides 1 and 2 was not promoted in the same extent; in addition, labelled side-products were detected at higher reaction temperatures. Radiofluorination in tert-alcohols was also studied using [18F]CsF with and without added phase transfer catalyst, resulting in both conditions lower RCY when compared to K2.2.2/[18F]KF system. Protic solvents were not able to promote aromatic 18F-fluorination. 18F-Fluorination of compound 5, having para-activated nitro group in the aromatic position (R2), failed in tert-alcohols even at the highest temperature, but it was labelled successfully in DMF and to some extent in CH3CN.

SN2 fluorination reactions in ionic liquids: a mechanistic study towards solvent engineering

In the catalysis of S(N)2 fluorination reactions, the ionic liquid anion plays a key role as a Lewis base by binding to the counterion Cs(+) and thereby reducing the retarding Coulombic influence of Cs(+) on the nucleophile F(-). The reaction rates also depend critically on the structures of ionic liquid cation, for example, n-butyl imidazolium gives no S(N)2 products, whereas n-butylmethyl imidazolium works well. The origin of the observed phenomenal synergetic effects by the ionic liquid [mim-(t)OH][OMs], in which t-butanol is bonded covalently to the cation [mim], is that the t-butanol moiety binds to the leaving group of the substrate, moderating the retarding interactions between the acidic hydrogen and F(-). This work is a significant step toward designing and engineering solvents for promoting specific chemical reactions.

Thorpe-Ingold acceleration of oxirane formation is mostly a solvent effect

The Thorpe-Ingold hypothesis for the gem-dimethyl effect in the cyclization reactions of 2-chloroethoxide derivatives has been investigated computationally in the gas phase and in aqueous solution. Ab initio MP2/6-311+G(d,p) and CBS-Q calculations reveal little intrinsic difference in reactivity with increasing alpha-methylation for the series of reactants 1-3. However, inclusion of continuum hydration or of explicit hydration through mixed quantum and statistical mechanics (MC/FEP) simulations does reproduce the substantial, experimentally observed rate increases with increasing alpha-methylation. Analysis of the MC/FEP results provides clear evidence that the rate increases stem primarily from increased steric hindrance to hydration of the nucleophilic oxygen atom with increasing alpha-methylation. Thus, the gem-dimethyl acceleration of oxirane formation for 1-3 is found to be predominantly a solvent effect.

First solvation shell effects on ionic chemical reactions: new insights for supramolecular catalysis

This article presents a detailed discussion on medium effect on ionic S(N)2 reactions. Ion pairing and the first solvation shell play the main role on reactivity. Ab initio calculations are used to show that a combination of apolar molecular cavity with polar hydroxyl groups can have catalytic effects on these reactions. The present results open an avenue of development for supramolecular catalysts able to control bimolecular ionic reactions. In particular, this new catalytic concept could be useful in nucleophilic fluorinations.

Efficiency of bulky protic solvent for SN2 reaction

We calculate and compare the effects of aprotic vs protic solvent on the rate of SN2 reaction [F- + C3H7OMs--> C3H7F + OMs-]. We find that aprotic solvent acetonitrile is more efficient than a small protic solvent such as methanol. Bulky protic solvent (tert-butyl alcohol) is predicted to be quite efficient, giving the rate constant that is similar to that in CH3CN. Our calculated relative activation barriers of the SN2 reaction in methanol, tert-butyl alcohol, and CH3CN are in good agreement with experimental observations.