Substituent effects on the reactions of aromatic triflones with para‐X‐anilines in methanol: Low intrinsic reactivity and transition state imbalances

A kinetic study is reported for S_NA_r reactions of 2,4,6‐tris(trifluoromethanesulfonyl) anisole 1a with a series of para‐X‐substituted anilines 2a–e in a methanol solution at various temperatures. The substituent effects on free energy (Δ^≠G), enthalpy (Δ^≠H), and entropy (Δ^≠S) of activation are examined. Aniline addition to triflone 1a is characterized by a β_X=0.57, α_Z=0.31, and an imbalance of I = α_Z–β_X=?0.26. The imbalance shows that resonance development lags behind C? N bond formation at the transition state. Interestingly, analysis of the results in terms of Marcus theory reveals that these S_NAr are associated with some extremely low intrinsic reactivity (log k_o=?1.25& © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 255–262, 2011     

The α‐effect exhibited in gas‐phase SN2@N and SN2@C reactions

In order to explore the existence of α‐effect in gas‐phase S_N2@N reactions, and to compare its similarity and difference with its counterpart in S_N2@C reactions, we have carried out a theoretical study on the reactivity of six α‐oxy‐Nus (FO^?, ClO^?, BrO^?, HOO^?, HSO^?, H₂NO^?) in the S_N2 reactions toward NR₂Cl (R = H, Me) and RCl (R = Me, i‐Pr) using the G2(+)_M theory. An enhanced reactivity induced by the α‐atom is found in all examined systems. The magnitude of the α‐effect in the reactions of NR₂Cl (R = H, Me) is generally smaller than that in the corresponding S_N2 reaction, but their variation trend with the identity of α‐atom is very similar. The origin of the α‐effect of the S_N2@N reactions is discussed in terms of activation strain analysis and thermodynamic analysis, indicating that the α‐effect in the S_N2@N reactions largely arises from transition state stabilization, and the “hyper‐reactivity” of these α‐Nus is also accompanied by an enhanced thermodynamic stability of products from the n_(N) → σ*_(O?Y) negative hyperconjugation. Meanwhile, it is found that the reactivity of oxy‐Nus in the S_N2 reactions toward NMe₂Cl is lower than toward i‐PrCl, which is different from previous experiments, that is, the S_N2 reactions of NH₂Cl is more facile than MeCl. © 2013 Wiley Periodicals, Inc.     

A detailed theoretical study on the excited‐state hydrogen‐bonding dynamics and the proton transfer mechanism for a novel white‐light fluorophore

In this work, density functional theory (DFT) and time‐dependent DFT (TDDFT) methods were used to investigate the excited‐state dynamics of the excited‐state hydrogen‐bonding variations and proton transfer mechanism for a novel white‐light fluorophore 2‐(4‐[dimethylamino]phenyl)‐7‐hyroxy‐6‐(3‐phenylpropanoyl)‐4H‐chromen‐4‐one ( 1 ). The methods we adopted could successfully reproduce the experimental electronic spectra, which shows the appropriateness of the theoretical level in this work. Using molecular electrostatic potential (MEP) as well as the reduced density gradient (RDG) versus the product of the sign of the second largest eigenvalue of the electron density Hessian matrix and electron density (sign[λ₂]ρ), we demonstrate that an intramolecular hydrogen bond O1–H2···O3 should be formed spontaneously in the S₀ state. By analyzing the chemical structures, infrared vibrational spectra, and hydrogen‐bonding energies, we confirm that O1–H2·O3 should be strengthened in the S₁ state, which reveals the possibility of an excited‐state intramolecular proton transfer (ESIPT) process. On investigating the excitation process, we find the S₀ → S₁ transition corresponding to the charge transfer, which provides the driving force for ESIPT. By constructing the potential energy curves, we show that the ESIPT reaction results in a dynamic equilibrium in the S₁ state between the forward and backward processes, which facilitates the emission of white light.     

Fast,Reversible Lithium Storage with a Sulfur/Long‐Chain‐Polysulfide Redox Couple

The cathodic reactions in Li–S batteries can be divided into two steps. Firstly, elemental sulfur is transformed into long‐chain polysulfides (S₈?Li₂S₄), which are highly soluble in the electrolyte. Next, long‐chain polysulfides undergo nucleation reaction and convert into solid‐state Li₂S₂ and Li₂S (Li₂S₄?Li₂S) by slow processes. As a result, the second‐step of the electrochemical reaction hinders the high‐rate application of Li–S batteries. In this report, the kinetics of the sulfur/long‐chain‐polysulfide redox couple (theoretical capacity=419 mA h g^?1) are experimentally demonstrated to be very fast in the Li–S system. A Li–S cell with a blended carbon interlayer retains excellent cycle stability and possesses a high percentage of active material utilization over 250 cycles at high C rates. The meso‐/micropores in the interlayer are responsible for accommodating the shuttling polysulfides and offering sufficient electrolyte accessibility. Therefore, utilizing the sulfur/long‐chain polysulfide redox couple with an efficient interlayer configuration in Li–S batteries may be a promising choice for high‐power applications.     

A DFT/TDDFT Investigation of Excited State Dynamical Mechanism of (E)‐1‐((2,2‐Diphenylhydrazono)methyl)naphthalen‐2‐ol

The excited‐state intramolecular proton transfer (ESIPT) mechanism of a new compound (E )‐1‐((2,2‐diphenylhydrazono)methyl)naphthalen‐2‐ol ( EDMN ) sensor, reported and synthesized by Mukherjee et al . [Sensors Actuat. B‐Chem . 2014, 202 , 1190], is investigated in detail theoretically. The calculations on primary bond lengths, bond angles, and the corresponding infrared (IR) vibrational spectra and hydrogen‐bond energy involved in intramolecular hydrogen bond between the S₀ and S₁ states confirm that the intramolecular hydrogen bond is strengthened in the S₁ state, which reveals the tendency of ESIPT reaction. The fact that the experimental absorption and emission spectra are well reproduced demonstrates the rationality and effectiveness of the time‐dependent density functional theory (TDDFT) level of theory we adopt here. Furthermore, intramolecular charge transfer based on the frontier molecular orbitals (MOs) gives indication of the ESIPT reaction. The constructed potential energy curves of both the S₀ and S₁ states while keeping the O─H distance of EDMN fixed at a series of values are used to illustrate the ESIPT process. The lower barrier of ~3.934 kcal/mol in the S₁ state potential energy curve (lower than the 8.254 kcal/mol in the S₀ state) provides the transfer mechanism.     

Bis‐tert‐Alcohol‐Functionalized Crown‐6‐Calix[4]arene: An Organic Promoter for Nucleophilic Fluorination

A bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for S_N2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially “free” F^?. The synergistic actions of the crown‐6‐calix[4]arene subunit (whose O atoms coordinate the counter‐cation Cs⁺) and the terminal tert‐alcohol OH groups (forming controlled hydrogen bonds with F^?) of BACCA led to tremendous efficiency in S_N2 fluorination of base‐sensitive substrates.     

Quantum‐chemical,IR, NMR,and X‐ray diffraction studies on 2‐(4‐chlorophenyl)‐1‐methyl‐ 1H‐benzo[d]imidazole

The title molecule, 2‐(4‐chlorophenyl)‐1‐methyl‐1H‐benzo[d]imidazole (C₁₄H₁₁ClN₂), was prepared and characterized by ¹H NMR, ¹³C NMR, IR, and single‐crystal X‐ray diffraction. The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) ¹H and ¹³C NMR chemical shift values of the title compound in the ground state have been calculated by using the Hartree‐Fock (HF) and density functional theory (DFT/B3LYP) method with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters, and the theoretical vibrational frequencies and GIAO ¹H and ¹³C NMR chemical shifts show good agreement with experimental values. The energetic behavior of the title compound in solvent media has been examined using B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). Besides, molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, and nonlinear optical (NLO) properties of the title compound were investigated by theoretical calculations. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Chiral Heterospirocyclic 2H‐Azirin‐3‐amines as Synthons for 3‐Amino‐2,3,4,5‐tetrahydrofuran‐3‐carboxylic Acid and Their Use in Peptide Synthesis

The heterospirocyclic N‐methyl‐N‐phenyl‐5‐oxa‐1‐azaspiro[2.4]hept‐1‐e n‐2‐amine (6 ) and N‐(5‐oxa‐1‐azaspiro[2.4]hept‐1‐en‐2‐yl)‐(S)‐proline methyl ester ( 7 ) were synthesized from the corresponding heterocyclic thiocarboxamides 12 and 10 , respectively, by consecutive treatment with COCl₂, 1,4‐diazabicyclo[2.2.2]octane, and NaN₃ (Schemes 1 and 2). The reaction of these 2H‐azirin‐3‐amines with thiobenzoic and benzoic acid gave the racemic benzamides 13 and 14 , and the diastereoisomeric mixtures of the N‐benzoyl dipeptides 15 and 16 , respectively (Scheme 3). The latter were separated chromatographically. The configurations and solid‐state conformations of all six benzamides were determined by X‐ray crystallography. With the aim of examining the use of the new synthons in peptide synthesis, the reactions of 7 with Z‐Leu‐Aib‐OH to yield a tetrapeptide 17 (Scheme 4), and of 6 with Z‐Ala‐OH to give a dipeptide 18 (Scheme 5) were performed. The resulting diastereoisomers were separated by means of MPLC or HPLC. NMR Studies of the solvent dependence of the chemical shifts of the NH resonances indicate the presence of an intramolecular H‐bond in 17 . The dipeptides (S,R)‐ 18 and (S,S)‐ 18 were deprotected at the N‐terminus and were converted to the crystalline derivatives (S,R)‐ 19 and (S,S)‐ 19 , respectively, by reaction with 4‐bromobenzoyl chloride (Scheme 5). Selective hydrolysis of (S,R)‐ 18 and (S,S)‐ 18 gave the dipeptide acids (R,S)‐ 20 and (S,S)‐ 20 , respectively. Coupling of a diastereoisomeric mixture of 20 with H‐Phe‐O^tBu led to the tripeptides 21 (Scheme 5). X‐Ray crystal‐structure determinations of (S,R)‐ 19 and (S,S)‐ 19 allowed the determination of the absolute configurations of all diastereoisomers isolated in this series.     

Stereoselective Preparation of 3‐Amino‐2‐fluoro Carboxylic Acid Derivatives,and Their Incorporation in Tetrahydropyrimidin‐4(1H)‐ones,and in Open‐Chain and Cyclic β‐Peptides

The preparation of (2S,3S)‐ and (2R,3S)‐2‐fluoro and of (3S)‐2,2‐difluoro‐3‐amino carboxylic acid derivatives, 1 – 3 , from alanine, valine, leucine, threonine, and β³h‐alanine (Schemes 1 and 2, Table) is described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N‐dibenzyl‐2‐amino‐3‐hydroxy and 3‐amino‐2‐hydroxy carboxylic acid esters is discussed (Fig. 1). The fluoro‐β‐amino acid residues have been incorporated into pyrimidinones ( 11 – 13 ; Fig. 2) and into cyclic β‐tri‐ and β‐tetrapeptides 17 – 19 and 21 – 23 (Scheme 3) with rigid skeletons, so that reliable structural data (bond lengths, bond angles, and Karplus parameters) can be obtained. β‐Hexapeptides Boc[(2S)‐β³hXaa(αF)]₆OBn and Boc[β³hXaa(α,αF₂)]₆‐OBn, 24 – 26 , with the side chains of Ala, Val, and Leu, have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and many intermediates are fully characterized by IR‐ and ¹H‐, ¹³C‐ and ¹⁹F‐NMR spectroscopy, by MS spectrometry, and by elemental analyses, [α]_D and melting‐point values.     

Comparative Computational Study of Hydrogen Abstraction Reactions of CY3H + XO− (X,Y = F,Cl, and Br)

The effects of halogen substituents on the reactivity are characterized by the hybrid B3LYP and BHandHLYP functionals of density functional theory using the aug‐cc‐pVDZ basis set. The species XO⁻ and CY₃H, where X, Y = F, Cl, and Br, have been chosen as model reactants in this work. Also, the mechanism of the hydrogen abstraction (HAT) reaction has been used to study the chemical reactivity of these anionic reactions. Our theoretical findings suggest that the relative reactivity of the CY₃H + XO⁻ reactions increases as Y goes from F to Br and decreases as X goes from F to Br. Moreover, among all reactions investigated in this study, the special role of the Y has very dominant effect on activation of the C–H bond in CY₃H when XO⁻ attacks the CY₃H. Again, through the transition state theory the rate constants at 298–1000 K are also evaluated for the HAT reactions, indicating the lower the temperature the faster is the chemical reaction.     

Stereochemistry of Reaction Paths as Determined from Crystal Structure Data—A Relationship between Structure and Energy

The three-dimensional structure of frequently occurring molecular fragments has been studied systematically. Molecular subunits which were examined include hydrogen bridges (O—H?O), triiodide anions (I^?₃), other linear triatomic fragments (Cl—Sb?Cl, S—S?S, Mo—O?Mo), tetrahedral ions (SO^2?₄, PO^3?₄, AlCl^?₄) and related species (MSO₃, etc.), molecules containing both keto- and amino-groups (mostly alkaloids), substituted [10]annulenes and cycloheptatrienes, organic five-membered rings, and five-coordinated metal and nonmetal atoms. The bond distances and angles describing the structure of a giving fragment cover a range that is many times larger than the range of the experimental standard deviations. The changes of the various structural parameters of a fragment are correlated with each other. The observed mutual dependence (structural correlation) may be described by means of Pauling's equation relating bond length r to bond number n:r = r₀ - clogn. The bond numbers n are expressed in terms of bond angles. The sum of bond numbers at a given atom is roughly constant and does not depend on the environment. “Standard bond lengths” of a fragment are determined by a least-squares procedure based on all available data. They are supplemented by curves that describe the observed distortions. The shape of these correlation curves is reminiscent of the structural changes occurring along the pathways of chemical reactions, e.g. nucleophilic substitution at tetrahedrally coordinated atoms (S_N1 and S_N2), nucleophilic addition to carbonyl groups, electrocyclic ring closure of polyenes, pseudo-rotation of five-membered rings or Berry pseudo-rotation. For many of these reactions approximate energy hypersurfaces have been obtained from quantum mechanical calculations, model force fields and from spectroscopic information (IR, NMR). Comparison between reaction pathways determined from structural correlations with those obtained from models of the energy surfaces shows fair agreement.     

Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

A theoretical kinetic study of the thermal decomposition of 1‐chlorohexane in gas phase between 600 and 1000 K was performed. Transition‐state theory and unimolecular reaction rate theory were combined with molecular information provided by quantum chemical calculations. Particularly, the B3LYP, BMK, M05–2X, and M06–2X formulations of the density functional theory (DFT) and the high‐level ab initio methods G3B3 and G4 were employed. The possible reaction channels for the thermal decomposition of 1‐chlorohexane were investigated, and the reaction takes place through the elimination of HCl with the formation of 1‐hexene. The derived high‐pressure limit rate coefficients are k _∞(600–1000 K) = (8 ± 5) × 10¹³ exp[‐((56.7 ± 0.4) kcal mol⁻¹/RT )] s⁻¹. The pressure effect over the reaction was analyzed from the calculation of the low‐pressure limit rate coefficients and the falloff curves. In addition, the standard enthalpies of formation at 298 K of −46.9 ± 1.5 kcal mol⁻¹ for 1‐chlorohexane and 5.8 ± 1.5 kcal mol⁻¹ for C₆H₁₃ radical were derived from isodesmic and isogiric reactions at high levels of theory.     

Ion‐Pair SN2 Substitution: Activation Strain Analyses of Counter‐Ion and Solvent Effects

The ion‐pair S_N2 reactions of model systems M_nF^n?1+CH₃Cl (M⁺=Li⁺, Na⁺, K⁺, and MgCl⁺; n=0, 1) have been quantum chemically explored by using DFT at the OLYP/6‐31++G(d,p) level. The purpose of this study is threefold: 1) to elucidate how the counterion M⁺ modifies ion‐pair S_N2 reactivity relative to the parent reaction F^?+CH₃Cl; 2) to determine how this influences stereochemical competition between the backside and frontside attacks; and 3) to examine the effect of solvation on these ion‐pair S_N2 pathways. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. The ASM has been extended to treat reactivity in solution. These findings contribute to a more rational design of tailor‐made substitution reactions.     

Formation of a Thymine‐HgII‐Thymine Metal‐Mediated DNA Base Pair: Proposal and Theoretical Calculation of the Reaction Pathway

A reaction mechanism that describes the substitution of two imino protons in a thymine:thymine (T:T) mismatched DNA base pair with a Hg^II ion, which results in the formation of a (T)N3‐Hg^II‐N3(T) metal‐mediated base pair was proposed and calculated. The mechanism assumes two key steps: The formation of the first Hg^II? N3(T) bond is triggered by deprotonation of the imino N3 atom in thymine with a hydroxo ligand on the Hg^II ion. The formation of the second Hg^II? N3(T) bond proceeds through water‐assisted tautomerization of the remaining, metal‐nonbonded thymine base or through thymine deprotonation with a hydroxo ligand of the Hg^II ion already coordinated to the thymine base. The thermodynamic parameters ΔG_R=?9.5 kcal mol^?1, ΔH_R=?4.7 kcal mol^?1, and ΔS_R=16.0 cal mol^?1 K^?1 calculated with the ONIOM (B3LYP:BP86) method for the reaction agreed well with the isothermal titration calorimetric (ITC) measurements by Torigoe et al. [H. Torigoe, A. Ono, T. Kozasa, Chem. Eur. J. 2010 , 16, 13218–13225]. The peculiar positive reaction entropy measured previously was due to both dehydration of the metal and the change in chemical bonding. The mercury reactant in the theoretical model contained one hydroxo ligand in accord with the experimental pK_a value of 3.6 known for an aqua ligand of a Hg^II center. The chemical modification of T:T mismatched to the T‐Hg^II‐T metal‐mediated base pair was modeled for the middle base pair within a trinucleotide B‐DNA duplex, which ensured complete dehydration of the Hg^II ion during the reaction.     

Chemical and thermal stability of N‐heterocyclic ionic liquids in catalytic C–H activation reactions

¹H‐NMR spectrum analyses are applied to study the chemical and thermal stability of selected N‐heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C–H bond of methane and convert it into the C–O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt‐based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH₄⁺). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

2-氢吡喃分子光激发开环反应机理的理论研究

用完全活性空间多组态(CASSCF)方法对2-氢吡喃分子光激发开环反应机理进行了研究。利用价键理论(VB)和自然键道分析(NBO)探究了沿能量最低反应途径电子的重新分布情况。计算结果表明从S0-Min p®p*垂直激发到Franck-Condon点后很容易弛豫到S1-Min,经较低的势垒到达圆锥交叉点S1/S0。而S1/S0与S1-Min相比能量低0.63eV。这样体系沿非绝热最低反应途径从激发单重态经交叉点S1/S0很容易得到产物S0-Prod。     

Transesterification Kinetics Investigation of R‐Substituted Phenyl Benzoates with 4‐Methoxyphenol in the Presence of K2CO3 in DMF

Transesterification of R‐substituted phenyl benzoates 1–5 with 4‐methoxyphenol 6 was kinetically investigated in the presence of K₂CO₃ in dimethylformamide (DMF) at various temperatures. The Hammett plots for the reactions of the 1–5 demonstrate good linear correlations with σ⁰ constants. Low magnitude of ρ_LG values indicate that the leaving group departure occurs after the rate‐determining step. The Brønsted coefficient values for the reactions (?0.2, ?0.16, ?0.13 at 15, 24, 36°C, respectively) demonstrate the weak effect of leaving group substituent on the reactivity of R‐substituted phenyl benzoates 1–5 for the reactions with 4‐methoxyphenol 6 in the presence of K₂CO₃ in DMF. The leaving group substituent effect on free energy (ΔG^≠), enthalpy (ΔH^≠), and entropy (ΔS^≠) of activation was examined. It was shown that the activation parameters obtained depend weakly on the leaving group substituent effect. The reaction is entropy controlled in case the leaving group substituent becomes electron withdrawing.     

Insights into the excited state intramolecular proton transfer process and mechanism of the novel 3‐hydroxythioflavone system: A theoretical study

It is well known that the molecular excited state dynamical process plays important roles in designing and developing novel applications. In this work, based on density functional theory and time‐dependent density functional theory methods, we theoretically explored a novel 3‐hydroxythioflavone (3HTF). Through calculating the electrostatic potential surface of the 3HTF structure, we confirm the formation of intramolecular hydrogen bonding O2‐H3···O4. Our theoretically obtained dominating bond lengths and bond angles involved in hydrogen bonds demonstrate that the intramolecular hydrogen bonds should be strengthened in the S₁ state. Coupling with the simulated infrared vibrational spectra, we further verify the enhanced hydrogen bonding O2‐H3···O4 in the S₁ state. Upon photoexcitation, we found that the charge transfer characteristics around hydrogen bonding moieties play important roles in facilitating the excited state intramolecular proton transfer (ESIPT) process. Via constructing potential energy curves in both S₀ and S₁ states, we confirm the almost nonbarrier ESIPT reaction should be an ultrafast process that further explains the previous experimental phenomenon. At last, we search the S₁‐state transition state (TS) structure along with ESIPT path, based on which we simulate the intrinsic reaction coordinate path that further confirms the ESIPT mechanism. We hope that our theoretical work could guide novel applications based on the 3HTF system in future.     

Structures,Energies, and Fundamental Vibrations of the Sulfonium Ions H3Sn+ (n = 1–4)

Using ab initio MO calculations at the MP2/6‐311G(2df,2pd) level of theory the most stable structures of the following seven ions were determined: H₃S⁺ (C_3v), H₂S–SH⁺ (C_s), H₂S–S–SH⁺ (C₁), HS–S(H)–SH⁺ (C₁), H₂S–S–S–SH⁺ (C₁), HS–S(H)–S–SH⁺ (C₁) and S(SH)₃⁺ (C₃). In the case of the isomeric H₃S₃⁺ cations the species protonated at the terminal sulfur atom is most stable while in the case of the H₃S₄⁺ ions the protonation at the β sulfur atom is energetically preferred. However, the energy differences between isomeric cations are rather small. At the same level of theory the wavenumbers of the harmonic fundamental vibrations were calculated and compared to the available experimental data leading to a support for the existing assignments in certain cases but in some cases to revisions. The reaction enthalpies and Gibbs free energies of the proton transfer reactions H₂S_n + H₂S_n+1 → H₃S_n⁺ + HS_n+1^– were calculated by the G2 method. For n = 1–3 the enthalpies are found in the range 639–731 kJ mol^–1.