Stabilization of the pesticide Fenitrothion toward O and N nucleophiles in the presence of cyclodextrins

The reaction of Fenitrothion with O and N nucleophiles (H₂O₂, NH₂OH, n‐butylamine and piperidine) was studied at 25 °C in water containing 2% 1,4‐dioxane in the presence of native cyclodextrins (α‐, β‐, and γ‐CD). For all the nucleophiles, the presence of CD produces reaction inhibition with saturation kinetics. The greatest effect in all cases is observed with β‐CD, and the greatest inhibition was observed for the reaction of Fenitrothion with H₂O₂ (81%), which is the most efficient nucleophile in promoting Fenitrothion degradation in homogeneous media. In the absence of CD, competition between the S_N2(P) and the S_N2(C) pathways was observed with piperidine as was reported before for the reaction with NH₂OH and n‐butylamine. The presence of β‐CD does not modify product distribution in the case of the reaction with NH₂OH and n‐butylamine, whereas there is an increase in S_N2(C) pathway when the nucleophile is piperidine. Copyright © 2014 John Wiley & Sons, Ltd.     

Self‐assembly of a [2]pseudorotaxane complex by the threading of a nitrobenzimidazol‐based guest on dibenzo‐24‐crown ether host

A new derivative of the previously reported 1,2‐bis(benzimidazol‐2‐yl)ethane motif, cation [1H₂]²⁺, was synthesized under microwave irradiation and fully characterized by solution NMR, high‐resolution mass spectrometry, cyclic voltammetry and X‐ray crystallography. This cation presents a linear geometry and incorporates nitro substituents as electrochemical handles. In solution, cation [1H₂]²⁺, is capable of threading the cavity of dibenzo‐24‐crown‐8 ether host (DB24C8) giving rise to a [2]pseudorotaxane complex [1H₂?DB24C8]²⁺, regardless of the counterion, [CF₃SO₃]^? or [CF₃COO] ^?. The interpenetrated structure of [1H₂?DB24C8]²⁺ was proven by solution NMR and X‐ray crystallography. This host–guest complex is held together by several non‐covalent interactions, such as hydrogen bonding and ion‐dipole. An electrochemical study of [1H₂]²⁺ in the presence of variable amounts of DB24C8 was performed; due to the irreversible redox behavior of cation [1H₂]²⁺, it was not possible to electrochemically control the association/dissociation process with DB24C8. Copyright © 2014 John Wiley & Sons, Ltd.     

The effect of hydrogen bond on the vibrational dynamics of genistein free and complexed with β‐cyclodextrins

Inclusion, or host–guest, complexes are supramolecular assemblies in which two or more molecules hold together and organize by means of intermolecular noncovalent bonds. In the pharmaceutical field, inclusion complexation of drugs with unsubstituted and derivative β‐cyclodextrins (β‐CDs) has proven to be a successful method to improve the dissolution of water insoluble drugs. Genistein (Gen), an isoflavone constituent of Onodis spinosae radix, turned out to be a suitable guest molecule for the encapsulation into β‐CD, resulting in a significant improvement of its aqueous solubility. In the present study, the modifications of the vibrational spectrum of Gen caused by its inclusion into β‐CDs cavity have been characterized by means of Raman scattering experiments. These changes have been interpreted by comparing the experimental data with the vibrational wavenumbers and Raman intensities obtained by simulation for the free and complexed guest molecule. Following this strategy, we have obtained a deeper understanding of the host–guest interactions involved in the formation and stabilization of the complexes, with particular regard to the role played by the guest chemical groups, as well as to disentangle the effects directly related to the complexation process from those ascribed to other factors, such as formation of intra‐ and intermolecular hydrogen bonds. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical insights into the host–guest interactions between [6]cycloparaphenyleneacetylene and its anthracene‐containing derivative and fullerene C70

The host–guest complexes formed with [6]cycloparaphenyleneacetylene ([6]CPPA) and its anthracene‐containing derivative ([6]CPPAs) hosts and fullerene C₇₀ guest were explored by density functional calculations. Besides two previously reported configurations in which C₇₀ guest is standing or lying in the cavity of the host, we found a new kind of configuration in which C₇₀ guest is half‐lying in the cavity of the host. More interestingly, the calculated results revealed that the fine‐tuning deformations occur readily during the formations of the complexes, suggesting that both [6]CPPA and [6]CPPAs are highly elastic host molecules. The large host–guest binding energies indicate that both two host molecules, [6]CPPA and [6]CPPAs, have excellent encapsulation ability for C₇₀ guest, and the [6]CPPAs even has much better encapsulation ability for C₇₀ than [6]CPPA. Furthermore, the host–guest interactions regions were detected and visualized in real space based on the electron density and reduced density gradient. Additionally, ¹H NMR spectra of those three different kinds of configurations mentioned earlier have been calculated with gage‐independent atomic orbital method, which may be helpful for further experimental characterizations in future. Copyright © 2014 John Wiley & Sons, Ltd.     

Solvent effects in the reaction between (anthracen‐9‐yl)methyl sulfides and electron‐deficient acetylenes

Solvent‐dependent diverse reactivity of (anthracen‐9‐yl)methyl sulfides with a few electron‐deficient acetylenes is described. Diversity in reactivity is attributed to competition between one electron transfer, two electron transfer and Diels–Alder reaction of these sulfides with electron‐deficient acetylenes. We have proposed plausible mechanisms to account for various reactions observed by us. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigation of cation–anion interaction in 1‐(2‐hydroxyethyl)‐3‐methylimidazolium‐based ion pairs by density functional theory calculations and experiments

Gas‐phase structure, hydrogen bonding, and cation–anion interactions of a series of 1‐(2‐hydroxyethyl)‐3‐methylimidazolium ([HOEMIm]⁺)‐based ionic liquids (hereafter called hydroxyl ILs) with different anions (X = [NTf₂]^–, [PF₆]^–, [ClO₄]^–, [BF₄]^–, [DCA]^–, [NO₃]^–, [AC]^– and [Cl]^–), as well as 1‐ethyl‐3‐methylimizolium ([EMIm]⁺)‐based ionic liquids (hereafter called nonhydroxyl ILs), were investigated by density functional theory calculations and experiments. Electrostatic potential surfaces and optimized structures of isolated ions, and ion pairs of all ILs have been obtained through calculations at the Becke, three‐parameter, Lee–Yang–Parr/6‐31 + G(d,p) level and their hydrogen bonding behavior was further studied by the polarity and Kamlet–Taft Parameters, and ¹H‐NMR analysis. In [EMIm]⁺‐based nonhydroxyl ILs, hydrogen bonding preferred to be formed between anions and C2–H on the imidazolium ring, while in [HOEMIm]⁺‐based hydroxyl ILs, it was replaced by a much stronger one that preferably formed between anions and OH. The O–H···X hydrogen bonding is much more anion‐dependent than the C2–H···X, and it is weakened when the anion is changed from [AC]^– to [NTf₂]^–. The different interaction between [HOEMIm]⁺ and variable anion involving O–H···X hydrogen bonding resulted in significant effect on their bulk phase properties such as ¹H‐NMR shift, polarity and hydrogen‐bond donor ability (acidity, α). Copyright © 2011 John Wiley & Sons, Ltd.     

Carbon-13 and Fluorine-19 NMR Spectroscopy of the Supramolecular Solid p-tert-Butylcalix[4]arene ·α,α,α-trifluorotoluene

The supramolecular 1 : 1 host–guest inclusion compound, p-tert-butylcalix[4]arene ·α,α,α-trifluorotoluene, 1, is characterized by ¹⁹F and ¹³C solid-state NMR spectroscopy. Whereas the ¹³C NMR spectra are easily interpreted in the context of earlier work on similar host–guest compounds, the ¹⁹F NMR spectra of solid 1 are, initially, more difficult to understand. The ¹⁹F{¹H} NMR spectrum obtained under cross-polarization and magic-angle spinning conditions shows a single isotropic resonance with a significant spinning sideband manifold. The static ¹⁹F{¹H} CP NMR spectrum consists of a powder pattern dominated by the contributions of the anisotropic chemical shift and the homonuclear dipolar interactions. The ¹⁹F MREV-8 experiment, which minimizes the ¹⁹F–¹⁹F dipolar contribution, helps to identify the chemical shift contribution as an axial lineshape. The full static ¹⁹F{¹H} CP NMR spectrum is analysed using subspectral analysis and subsequently simulated as a function of the ¹⁹F–¹⁹F internuclear distance (D_FF = 2.25 ± 0.01 Å) of the rapidly rotating CF₃ group without including contributions from additional libration motions and the anisotropy in the scalar tensor. The shielding span is found to be 56 ppm. The width of the centerband in the ¹⁹F{¹H} sample-spinning CP NMR spectrum is very sensitive to the angle between the rotor and the magnetic field. Compound 1 is thus an attractive standard for setting the magic angle for NMR probes containing a fluorine channel with a proton-decoupling facility.     

A thorough understanding of the Diels–Alder reaction of 1,3‐butadiene and ethylene

The Diels–Alder (DA) reaction is one of the most important reactions in organic chemistry. The controversy surrounding this reaction as to whether it follows a concerted or stepwise mechanism has existed for a long time. The reaction of 1,3‐butadiene and ethylene is the paradigmatic example of the DA reaction. We have reinvestigated the mechanism of this reaction using density functional theory. The theoretical study considered all types of possible pathways for the reaction of 1,3‐butadiene and ethylene using six functionals at different rungs of Jacob's ladder. Therefore, a complete picture is given for a thorough understanding of the iconic DA reaction, and a new stationary point during the reaction processes has been reported for the first time. The calculated results indicated that three functionals, ωB97X‐D, M06‐2X, and B2‐PLYP, of the fourth and fifth rungs of Jacob's ladder performed well in the investigation of the mechanism of this reaction and that the reliable basis set should be larger than 6‐311+G(2d,p). The cis‐1,3‐butadiene more easily reacted with ethylene compared with 1,3‐butadiene in the trans conformation. The concerted mechanism was found to be energetically favorable, whose energy barrier is around 10 kcal/mol lower than that of the stepwise mechanism. Two investigated solvents, toluene and CH₃CN, had little impact on this simple DA reaction. Copyright © 2014 John Wiley & Sons, Ltd.     

A remarkable difference in the deprotonation steps of the Friedel–Crafts acylation and alkylation reactions

Friedel–Crafts acylation and alkylation reactions were investigated using density functional theory calculations. The reaction systems studied were (benzene + acetyl chloride + Al₂Cl₆ (or AlCl₃)) and (benzene + 2‐chloropropane + Al₂Cl₆). In the acylation reaction, the acylium ion intermediate is reached either via a Me? C(Cl)?O? Al₂Cl₆ complex or via direct Cl transfer: Me? C(?O)Cl? Al₂Cl₆ → Me? C?O^?+? Al₂Cl. The ion adds to benzene electrophilically to form a Wheland intermediate containing a strong C? H? Cl hydrogen bond, which leads to deprotonation and the subsequent formation of acetophenone. The resulting H? Cl? Al₂Cl₆ fragment is subjected to a nucleophilic attack by the carbonyl oxygen of the acetophenone, and recovery of the Al₂Cl₆ bridge is unlikely. Attack of the Al₂Cl₆ moiety by Me? C(Cl)?O gives the complex Me? C(Cl)?O–AlCl₃, whose reactivity toward acylation is similar to that of the Me? C(Cl)?O–Al₂Cl₆ complex. In the alkylation reaction, deprotonation does not take place, but rather a [1,2] H‐shift from the Wheland intermediate. The resulting α‐protonated cumene undergoes deprotonation, with subsequent recovery of the Al₂Cl₆ bridge. In addition, the Al₂Cl₆‐catalyzed isomerization of the n‐propyl to the isopropyl cation was found to be a dyotropic shift. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural and energetic properties of alkylfluoride–BF3 complexes in the gas phase and condensed‐phase media: computations and matrix infrared spectroscopy

We have undertaken an experimental and computational study of the structural properties of a few alkylfluoride–BF₃ complexes (RF′–BF₃), which are proposed intermediates in a certain class of Friedel–Crafts reactions. Using density functional theory and second‐order Møller–Plesset calculations, we have obtained gas‐phase structures, frequencies, and B–F′ bond potentials for CH₃F–BF₃, (CH₃)₂CHF–BF₃, and (CH₃)₃CF–BF₃. All the complexes are weakly‐bonded in the gas phase, with B–F′ distances (X3LYP/aug‐cc‐pVTZ) of about 2.4 Å and binding energies (MP2/aug‐cc‐pVTZ) ranging from 5.4 and 6.7 kcal/mol. Accordingly, gas‐phase bond potentials are relatively shallow and flat for these complexes. However, even though the inner walls of the potentials are rather soft (the energies rise by only about 5 to 10 kcal/mol between 2.4 and 1.6 Å), we observe no global or local minima at short B–F′ distances. For the (CH₃)₂CHF–BF₃ and (CH₃)₃CF–BF₃ potentials in dielectric media, we do observe a distinct flattening along the inner wall, which results in shelf‐like region near 1.7 Å, but this feature is not a true local minimum. We have also obtained low‐temperature infrared spectra of the (CH₃)₂CHF–BF₃ complex in solid neon, and the frequencies agree quite favorably with those obtained via computations, which validates the computational assessment of the gas‐phase complexes. Copyright © 2011 John Wiley & Sons, Ltd.     

Theoretical studies on the hydrogen abstraction reactions of methyl esters with HO2 radical and the following β‐scission reactions

Unsaturated fatty acid methyl esters are ubiquitous in biodiesel fuels. The C = C double bond greatly affects the combustion characteristics of biodiesel, especially its ignition behavior at low temperatures. In this work, we report detailed theoretical study on the mechanism and kinetics of the hydrogen abstraction reactions of linear unsaturated C6 methyl esters with hydroperoxy radical (HO₂), which play a critical role in the low‐temperature combustion of biodiesel. Reaction profiles are obtained via intrinsic reaction coordinate (IRC) analysis including the formation of reactant complexes and product complexes at the entrance and exit channels, respectively. The potential energy surfaces are explored at the CBS‐QB3 level. The following β‐scission reactions of the forming radicals are also investigated at the same level of theory. The high‐pressure limit rate constants for all the reactions in the temperature range from 500 to 2000 K are calculated via conventional transition‐state theory with quantum tunneling effect and fitted to the modified Arrhenius expression.     

Understanding the stereo‐ and regioselectivities of the polar Diels–Alder reactions between 2‐acetyl‐1,4‐benzoquinone and methyl substituted 1,3‐butadienes: a DFT study

The polar Diels–Alder (DA) reactions of 2‐acetyl‐1,4‐benzoquinone (acBQ) with methyl substituted 1,3‐butadienes have been studied using DFT methods at the B3LYP/6‐31G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3‐dienes to the β conjugated position of the acBQ followed by ring‐closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions. Copyright © 2008 John Wiley & Sons, Ltd.     

Homo‐Diels–Alder reaction of a very inactive diene,bicyclo[2,2,1]hepta‐2,5‐diene,with the most active dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione. Solvent,temperature, and high pressure influence on the reaction rate

Solvent, temperature, and high pressure influence on the rate constant of homo‐Diels–Alder cycloaddition reactions of the very active hetero‐dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta‐2,5‐diene (2), and of 1 with some substituted anthracenes have been studied. The rate constants change amounts to about seven orders of magnitude: from 3.95^.10^?3 for reaction (1+2) to 12200 L mol^?1 s^?1 for reaction of 1 with 9,10‐dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity (ln k₂) and the heat of reactions (?_r‐nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has been performed. The heat of reaction (1+2) is ?218 ± 2 kJ mol^?1, of 1 with 9,10‐dimethylanthracene ?117.8 ± 0.7 kJ mol^?1, and of 1 with 9,10‐dimethoxyanthracene ?91.6 ±0.2 kJ mol^?1. From these data, it follows that the exothermicity of reaction (1+2) is higher than that with 1,3‐butadiene. However, the heat of reaction of 9,10‐dimethylanthracene with 1 (?117.8 kJ mol^?1) is nearly the same as that found for the reaction with the structural C=C counterpart, N‐phenylmaleimide (?117.0 kJ mol^?1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond (611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of activation (?V^≠) and the volume of reaction (?V_r‐n) of the homo‐Diels–Alder reaction (1+2) is considered as “normal”: ?V^≠/?V_r‐n = ?25.1/?30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd.     

A computational study of the thermolysis of β‐hydroxy ketones in gas phase and in m‐xylene solution

Theoretical calculations at the M05‐2X/6‐31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the β‐hydroxy ketones, 4‐hydroxy‐2‐butanone, 4‐hydroxy‐2‐pentanone, and 4‐hydroxy‐2‐methyl‐2‐pentanone in gas phase and in m‐xylene solution. The mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state. A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 5.9:4.7:1.0 in m‐xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively. The progress of the thermal decomposition reactions of β‐hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O₁–C₂ bond with the initial migration of the H₆ atom from O₅ to O₁ can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are “advanced”, nearer to the products than to the reactants. Copyright © 2012 John Wiley & Sons, Ltd.