Ion-Pair SN 2 Reaction of OH- and CH3 Cl: Activation Strain Analyses of Counterion and Solvent Effects

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.

Quasi-classical trajectory study of the OH- + CH3I reaction: theory meets experiment

Regarding OH^- + CH₃I, several studies have focused on the dynamics of the reaction. Here, high-level quasi-classical trajectory simulations are carried out at four different collision energies on our recently developed potential energy surface. In all, more than half a million trajectories are performed, and for the first time, the detailed quasi-classical trajectory results are compared with the reanalysed crossed-beam ion imaging experiments. Concerning the previously reported direct dynamics study of OH^- + CH₃I, a better agreement can be obtained between the revised experiment and our novel theoretical results. Furthermore, in the present work, the benchmark geometries, frequencies and relative energies of the stationary points are also determined for the OH^- + CH₃I proton-abstraction channel along with the earlier characterized S_N2 channel.

Diastereoselectivity on Intramolecular Alder-ene Reaction of 1,6-Dienes

A detailed computational study of the intramolecular Alder-ene reaction of different 1,6-dienes at M06-2X(PCM)/TZ2P level of theory has been performed. We want to understand the influence of enophile-geminal substitution pattern in the cis : trans selectivity of the cyclization process. Our analysis of the reaction coordinate by means of activation strain model of chemical reactivity (ASM-distortion interaction model) reveals that the cis-selectivity observed for unactivated reagents is related with high stabilizing orbital interaction and lower strain energy, consequence of an early transition structure. On the other hand, the presence of activating groups increases the asynchronicity of the transition structures and reduces the activation barrier due to more stabilizing orbital and electrostatic interactions, favoring trans-selectivity.

Theoretical study of a derivative of chlorophosphine with aliphatic and aromatic Grignard reagents: SN2@P or the novel SN2@Cl followed by SN2@C?

The proposed SN2 reactions of a hindered organophosphorus reactant with aliphatic and aromatic nucleophiles [Ye et al., Org. Lett., 2017, 19, 5384–5387] were studied theoretically in order to explain the observed stereochemistry of the products.

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

Insights into the nucleophilic substitution of pyridine at an unsaturated carbon center

Bimolecular nucleophilic substitution (S_N2) is a fundamental reaction that has been widely studied. So far, the nucleophiles are mainly anionic species in S_N2 reactions. In this study, we use density functional theory calculations to assess the mechanisms of substitution of carbonyl, imidoyl, and vinyl compounds with a neutral nucleophile, pyridine. Charge decomposition analysis is performed to explore the main components of the transition state's LUMO. For reactions of imidoyl or carbonyl compounds with pyridine or Cl⁻, the LUMOs of the transition states are composed of mixed orbitals originating from the nucleophile and the substrate. Considering the unique mixed nature of the orbitals, the reaction mode is termed S_Nm (m means mix). Moreover, the main components of the transition state's LUMO are pure σ*_C–Cl MO in the reactions of H₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCl with pyridine or Cl⁻. Computations were also performed for RYCHX substrates with different X and Y groups (X= Cl⁻, Br⁻, or F⁻; Y = O, N, or C).

The nucleophilic substitution of carbonyl, imidoyl, and vinyl carbon centers with pyridine or halides is investigated in this paper.

Inter- and Intra-Molecular Organocatalysis of SN2 Fluorination by Crown Ether: Kinetics and Quantum Chemical Analysis

We present the intra- and inter-molecular organocatalysis of S_N2 fluorination using CsF by crown ether to estimate the efficacy of the promoter and to elucidate the reaction mechanism. The yields of intramolecular S_N2 fluorination of the veratrole substrates are measured to be very small (<1% in 12 h) in the absence of crown ether promoters, whereas the S_N2 fluorination of the substrate possessing a crown ether unit proceeds to near completion (~99%) in 12 h. We also studied the efficacy of intermolecular rate acceleration by an independent promoter 18-crown-6 for comparison. We find that the fluorinating yield of a veratrole substrate (leaving group = −OMs) in the presence of 18-crown-6 follows the almost identical kinetic course as that of intramolecular S_N2 fluorination, indicating the mechanistic similarity of intra- and inter-molecular organocatalysis of the crown ether for S_N2 fluorination. The calculated relative Gibbs free energies of activation for these reactions, in which the crown ether units act as Lewis base promoters for S_N2 fluorination, are in excellent agreement with the experimentally measured yields of fluorination. The role of the metal salt CsF is briefly discussed in terms of whether it reacts as a contact ion pair or as a “free” nucleophile F⁻.

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN 2 Reaction of NH 2 - and CH3 Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N 2) reactions of M_n NH₂ ^(n-1) + CH₃ Cl (M⁺  = Li⁺ , Na⁺ , K⁺ , and MgCl⁺ ; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N 2-b) versus frontside attack (S_N 2-f). The results were compared to the corresponding ion-pair S_N 2 reactions involving F^- and OH^- nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N 2 reactivity along the nucleophiles F^- , OH^- , and NH 2 - , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.

Symmetry-Adapted Perturbation Theory Decomposition of the Reaction Force: Insights into Substituent Effects Involved in Hemiacetal Formation Mechanisms

The decomposition of the reaction force based on symmetry-adapted perturbation theory (SAPT) has been proposed. This approach was used to investigate the substituent effects along the reaction coordinate pathway for the hemiacetal formation mechanism between methanol and substituted aldehydes of the form CX₃CHO (X = H, F, Cl, and Br), providing a quantitative evaluation of the reaction-driving and reaction-retarding force components. Our results highlight the importance of more favorable electrostatic and induction effects in the reactions involving halogenated aldehydes that leads to lower activation energy barriers. These substituent effects are further elucidated by applying the functional-group partition of symmetry-adapted perturbation theory (F-SAPT). The results show that the reaction is largely driven by favorable direct noncovalent interactions between the CX₃ group on the aldehyde and the OH group on methanol.

Effect of an α-Methyl Substituent on the Dienophile on Diels-Alder endo:exo Selectivity

A detailed computational study of the Diels‐Alder reaction of cyclopentadiene with acrylonitrile, methylacrylate and their α‐methylated counterparts methacrylonitrile and methyl methacrylate at the M06‐2X(PCM)/TZVP level of theory has been performed. We want to understand the excellent exo‐selectivities observed experimentally due the presence of this substituent. To this end, analysis of the reaction coordinate by means of activation strain model of chemical reactivity (ASM‐distortion interaction model) including solvation effects and NBO second order perturbation energy have been carried out.