An experimental investigation of substituent effects on the formation of 2,3‐dihydroquinazolin‐4(1H)‐ones: a kinetic study

The effect of different substituents on the kinetics of the reactions between 2‐amino‐benzamide and some of benzaldehyde derivatives have been spectrally investigated in the presence of formic acid. The proposed mechanism were challenged due to the determination of rate‐determining step (RDS) and also, to obtain the general rate law of the reaction. For all substituents, the reactions followed the second‐order kinetics and the partial orders of reactions were recognized with respect to each reactant. Electron withdrawing substituents on benzaldehyde ring increased the rate of reaction. Kinetic values (k and E_a) and associated activation parameters (ΔH^?, ΔG^? and ΔS^?) of the reactions were determined. Both the Arrhenius and the Eyring equations were used to calculate activation energy. Comparison of magnitude of and T showed that the reactions were enthalpy controlled. Isokinetic plots for the reactions were plotted and linear relationship between and recognized that relative contribution of enthalpy and entropy to the overall free energy was the same in the reactions.     

新颖的酸催化迈克尔型巯基荧光探针的机理

迈克尔加成反应通常为碱催化的反应,但是在本工作体系当中,2-(2-乙烯基,7-羟基喹啉)丙二酸(QMA)能够被酸催化检测巯基. 从实验和理论计算证明了QMA探测巯基是,且溶液pH值越低,活 性越强. 在碱性溶液中,它的负离子形式QMA[-2H⁺]2^-不能够与电离前或电离后的巯基反应. 与QMA相比,它的酯QME显示出相反的pH效应,这与一般的基于迈克尔加成的巯基荧光探针类似. DFT计算得到的活化焓支持上述两种探针的pH效应. 在酸性溶液中,质子化的QMA与中性和碱性溶液中的负离子形式相比更容易与亲核试剂发生加成,因此能够与低活性的未电离硫醇反应. 相反地,QME与未电离硫醇反应的能垒很高,与电离后的硫醇反应活化能很低. 理论计算研究显示QMA与未电离硫醇的反应为1,4-加成而QME与电离后的硫醇的反应为1,2-加成.     

Kinetics and thermochemistry of the unusual [2π + 2σ + 2σ]‐cycloaddition of quadricyclane with some dienophiles

Kinetic parameters of the unusual [2π + 2σ + 2σ]‐cycloaddition reactions of quadricyclane ( 1 ) with tetracyanoethylene ( 2 ), 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione ( 3 ), N‐phenylmaleimide ( 4 ), and diethyl azodicarboxylate ( 5 ) are determined experimentally. Additionally, the enthalpies of 1  +  2 reaction in 1,4‐dioxane solution (?236.6 ± 1.0 kJ mol^?1) and 1  +  3 reaction in toluene (?255.0 ± 2.8 kJ mol^?1) are determined calorimetrically and shown to be the largest in absolute magnitude among all known cycloaddition reactions involving these dienophiles. Solvent effect on the rate of 1 + 3 reaction in 11 solvents is studied and found to be moderate and similar to that of the conventional Diels‐Alder and ene reactions. The difference in the reaction rate constants of 1 with different dienophiles can be up to 9 orders of magnitude and is mainly caused by the difference in activation enthalpies. This difference is not correlated with the standard enthalpies of reactions and is likely the result of high sensitivity of the [2π + 2σ + 2σ] reaction rates to the energy of donor‐acceptor interactions between the reactants.     

Formation of pyrazol‐1,3,4‐thiadiazoles through 1,3‐dipolar cycloadditions of 3‐thioxo‐[1,2,4]‐triazepin‐5‐one with nitrilimines: an experimental and computational study

In this work the experimental results and the computational study of the title compounds and some ancillary compounds are reported. Two bicyclic pyrazol‐1,3,4‐thiadiazole derivatives were synthezised by reaction between 6‐dimethylaminomethylene‐3‐thioxo‐[1,2,4]‐triazepin‐5‐one 1 and several nitrilimines 2a–f to give corresponding spirocycloadducts 3a–f , which undergo a rapid rearrangement leading to the new bicyclic compounds, 4a–f and 5a–f . These obtained bicyclic products were characterized by ¹H and ¹³C NMR spectroscopy and finally by X‐ray crystallography. Theoretical calculations have been carried out using DFT methods to rationalize the formation of the two new bicyclic compounds. Two reaction types are involved in the formation of the compounds 4a–f and 5a–f . The first one is a 1,3‐dipolar cycloaddition (13DC) reaction between 1 acting as dipolarophile and 2a–f as dipoles. The results indicate that the cycloaddition between 1 and 2g , as model of 2a–c , takes place via a high asynchronous bond‐formation process. The regioselectivity obtained from the calculations is in complete agreement with the formation of the unique spirocycloadducts 3a–f . The second reaction leading to the formation of the final products is a domino process that is initiated by the quick and irreversible cleavage in a catalytic acid environment of triazepenic ring. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical Study on Mechanism of Cycloadditional Reaction Between Dichloro-Germylidene and Formaldehyde

用MP2/6-31G^*方法研究了单线态二氯亚甲基锗烯与甲醛环加成反应的反应机理,该反应有两条相互竞争的主反应通道,同时伴随着两中间体(INT3和INT4)副产物的生成. 第一条主反应通道所生成的物种为三员环中间体(INT1)和Ge-O顺位的四员环产物(P1);第二条主反应通道所生成的物种为Ge-O对位的扭曲四员环中间体(INT2)和氯迁移产物(P2);P1和INT2分别与甲醛(R2)的进一步作用而导致了两副产物的生成.     

The UV absorption spectrum of the phenyl radical isolated in solid argon

Phenyl radicals have been generated by photodecomposition of nitrosobenzene isolated in solid argon at 12 K. In this medium the origin of the first UV absorption band of nitrosobenzene corresponding to theS ₀ S ₂ transition is found at 30260 cm^–1. Excitation with an excess energy of 2200 cm^–1 results in very efficient photodissociation. The fragments NO and phenyl are stable for many hours. Annealing of the sample above 35 K led to partial recombination and recovery of the absorption spectrum of nitrosobenzene. The UV spectrum of the phenyl radical was obtained in the wavenumber range 25000–45000 cm^–1, apparently without contamination by other species. It shows sharp lines at 25220, 33880, and 34820 cm^–1 and two broader maxima at 38360 and 41060 cm^–1. Of these only the line at 34820 cm^–1 had been assigned to the phenyl radical in earlier work. These data are compared to recent molecular orbital calculations.Dedicated to Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Density functional study of the chemisorption of O2 on the armchair surface of graphite

The reaction between O₂ and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory. Both equilibrium and transition state geometries were optimized to provide a fundamental understanding of the energetics and kinetics of the chemisorption, desorption, rearrangement, and migration reactions that contribute to carbon gasification. A small barrier of 18 kJ mol⁻¹ was found for the chemisorption reaction, which is 578 kJ mol⁻¹ exothermic overall, producing a stable quinone. A number of reaction pathways with barriers below 578 kJ mol⁻¹ were characterized. Gasification of carbon occurs as CO, with barriers of 296 and 435 kJ mol⁻¹ for the first and second CO loss, respectively. The stable quinone can also undergo a rearrangement reaction to form two ketene groups, with a barrier of 260 kJ mol⁻¹. If the armchair edge is extended to include an adjacent aromatic ring, the oxide can migrate along the surface. This initially forms a furan-like bridge structure, with a barrier of just 89 kJ mol⁻¹. A further barrier of 383 kJ mol⁻¹ leads to CO desorption from the furan. The furan can also rearrange further with a barrier of 212 kJ mol⁻¹ to form a five-membered lactone, the most stable structure identified on the potential energy surface. Rearrangement and migration reactions, which have not generally been incorporated into carbon gasification models, are shown to be potentially important pathways in carbon oxidation reactions.     

Computational study of cycloaddition reactions on the SiC(1 0 0)-c(2 × 2) surface

Cycloaddition reactions between 1,3-butadiene and the C-terminated SiC(1 0 0)-c(2 × 2) surface have been addressed using quantum-chemical methods. The c(2 × 2) structure consists of ---CC--- bridges between underlayer Si atoms which themselves form Si---Si bonds. Of various possible reaction products, the one formed by a [2 + 4] reaction with the ---CC--- bridge (giving a species resembling 1,4-cyclohexadiene) is the lowest in energy. Density functional calculations for the bare c(2 × 2) surface, using a cluster model with mechanical embedding, gave good agreement with structural parameters obtained in previous fully ab initio studies. Similar calculations for the cycloaddition product and for the transition state gave a reaction energy of −50.3 kcal/mol and an activation energy of +6.1 kcal/mol to form a planar ring structure lying normal to the surface. Detailed results for the frequency and infrared polarization behavior of adsorbate vibrational modes have also been obtained.     

Radical chemistry of glucosamine naphthalene acetic acid and naphthalene acetic acid: a pulse radiolysis study

Free radical‐induced oxidation reactions of glucosamine naphthalene acetic acid (GNaa) and naphthalene acetic acid (Naa) have been studied using pulse radiolysis. GNaa was synthesized by covalently attaching Naa on glucosamine. Hydroxyl adduct (from the reaction of hydroxyl radicals (^●OH) at the naphthalene ring) was identified as the major transient intermediate (suggesting that the ^●OH reaction is on the naphthalene ring) and is characterized by its absorption maxima of 340 and 400 nm. Both GNaa and Naa undergo similar reaction pattern. The bimolecular rate constants determined for the reactions are 4.8 × 10⁹ and 8.9 × 10⁹ dm³ mol^?1 s^?1 for GNaa and Naa respectively. The mechanism of reaction of ^●OH with GNaa was further confirmed using steady‐state method. Radical cation of GNaa was detected as an intermediate during the reaction of sulfate radical (SO₄^●?) with GNaa (k₂ = 4.52 × 10⁹ dm³ mol^?1 s^?1). This radical cation transforms to a ^●OH adduct at higher pH. The radical cation of GNaa is comparatively long lived, and a cyclic transition state by neighboring group participation accounts for its stability. The oxy radical anion (O^●?) reacts with GNaa (k₂ = 1.12 × 10⁹ dm³ mol^?1 s^?1) mainly by one‐electron transfer mechanism. The reduction potential values of Naa and GNaa were determined using cyclic voltammetric technique, and these are 1.39 V versus NHE for Naa and 1.60 V versus NHE for GNaa. Copyright © 2014 John Wiley & Sons, Ltd.     

Is corannulene a better diene or dienophile? A DFT analysis

Diels Alder reactivity of corannulene has been probed using density functional theory (DFT) at B3LYP/6‐31G^* level by employing it both as a diene and a dienophile in cycloaddition with ethylene and 1,3‐butadiene as typical partners. Computations reveal that corannulene acts better as a dienophile than as a diene and as a dienophile it undergoes normal electron demand type addition with 1,3‐butadiene, and as a diene corannulene undergoes inverse electron demand type addition with ethylene. When employed as a dienophile the addition takes place preferentially in the rim position than in the spoke position due to strong steric and electronic reasons. Further in the rim addition rim exo approach is favored kinetically and thermodynamically. As a diene, corannulene shows regioselectivity for rim–spoke addition over spoke–spoke addition. Concerted type cycloadditions have been studied and the reactions are seen to take place preferentially on the convex face. The effect of substituents in butadiene on the reactivity and the reaction of butadiene–pentaindenocorannulene (an extended corannulene) system has been investigated for the most favorable rim exo positions. Copyright © 2007 John Wiley & Sons, Ltd.     

Photochromism of dihydroindolizines: part XIV. Synthesis and photophysical behavior of photochromic dihydroindolizine‐tripodal linkers toward anchoring sensitizers to semiconductor nanoparticles

Photochromic dihydroindolizines (DHIs) 4a,5‐dihydropyrrolo[1,2‐b]pyridazine based tripodal‐linker systems with adamantane core and ethyl benzoate tripods as anchoring groups have been successfully synthesized. In addition, new spirocyclopropene precursors have been prepared through both chemical and photochemical processes. The photochromic properties of the newly synthesized DHIs derivatives have been optimized and fine‐tuned by the incorporation of various substituents on the fluorene (region A) and pyridazine (region C) moieties. Several alternative routes for the synthesis of the DHIs under investigation have been established. The Sonogashira crosscoupling reaction was utilized for fragment coupling between DHIs and the phenylacetylene tether of the adamantane core. Several reaction conditions of this key reaction were surveyed to obtain optimal yields of a new series of coupling products targeted for anchoring to semiconductor nanoparticles. The chemical structures of the newly synthesized materials were elucidated by both analytical and spectroscopic tools. Irradiation of the photochromic DHIs with polychromatic light resulted in ring opened colored betaines which underwent cycloreversion reactions via thermal 1,5‐electrocyclization processes. The kinetic of the thermal 1,5‐electrocyclization was studied by using a UV/VIS/NIR spectrophotometer. The kinetic measurements showed the half‐lives of the colored betaines to be in the second domain. A pronounced increase in the half‐lives of betaines bearing dimethyl‐substituted pyridazine was noted compared with non‐substituted pyridazine betaines. A strong effect of solvent polarity on the λ_max and half‐lives of the betaines was observed. The further adjustment of the absorption maxima and the kinetic properties via the manipulation of substituents on the fluorene (region A) and pyridazine moieties (region C) should yield more refined systems for application as supports onto metal‐oxide surfaces which remains an active area of our ongoing research. Copyright © 2010 John Wiley & Sons, Ltd.     

Intramolecular Diels–Alder reaction in enyne–allenes: a computational investigation and comparison with the Myers–Saito and Schmittel reactions

The reaction mechanisms as well as substituted effect and solvent effect of the enyne–allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers–Saito and Schmittel reactions. The Myers–Saito reaction of non‐substituted enyne–allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C₂? C₇ cyclization and more exothermic (Δ_RE = ?69.38 kcal/mol). For R₁ = CH₃ and t‐Bu, the increasing barrier of the C₂? C₇ cyclization is higher than that for the C₂? C₆ cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron‐releasing group. Moreover, the strong steric effect of R₁ = t‐Bu would shift the C₂? C₇ cyclization to the [4 + 2] cycloaddition. On the other hand, for R₁ = Ph, NH₂, O^?, NO₂, and CN substituents, the barrier of the C₂? C₆ cyclization would be more diminished than the C₂? C₇ cyclization due to strong mesomeric effect; the reaction path of C₂? C₇ cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential‐energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation of the possibility of functionalization of C20 fullerene by benzene via Diels–Alder reaction

C₂₀ fullerene, this novel species with all its pentagonal faces has displayed some unique operations in making fast pericyclic reactions. As an example, the high dienophile character of the C₂₀ fullerene and the ability of this species in making an ultra-fast Diels–Alder reaction with 1,3-butadiene, has been recently reported. Moreover, new experimental reports claim that the C₆₀ fullerene, one of the fullerene family, could make a Diels–Alder reaction with the central ring of anthracene and make the ring non-aromatic. These reports may encourage researchers to do more studies on the properties of this small carbon cage.To address this question, the present research has discussed all the reaction channels of the Diels–Alder cycloaddition of benzene molecule as a 1,3-diene with the C₂₀ fullerene in order to answer this question: “Is C₂₀ fullerene able to make a Diels–Alder reaction with this molecule?”.     

Catalytic effects of Lewis acids on 1,3-DC reaction: A luminescent study

Competitive coordination by the nitrone and the dipolarophile to the Lewis acid catalysts like metal triflates controls the acceleration and deceleration of the catalyzed 1,3-dipolar cycloaddition reactions as compared to the uncatalyzed one. This fact has been established through variation in steady state fluorescence of the substrates in presence of catalyst. Li⁺ found to catalyze both the cycloaddition between α-p-chlorophenyl-N-phenyl nitrone (N1) and Benzylidene acetophenone (D1) as well as between N1 and 3-Phenyl-1-pyridin-2-yl-propenone (D2) efficiently. While Yb³⁺ can catalyze only the cycloaddition between N1 and D2. The experimental findings have been rationalized on the basis of DFT calculations and analysis based on the frontier molecular orbital energies and relative electrophilicity of the reactants.     

Kinetics and related studies of the formation of 2,3‐dihydroquinazolin‐4(1H)‐ones in the presence of different benzaldehyde derivatives

The kinetics and activation parameters for the reaction between 2‐amino‐benzamide and some benzaldehyde derivatives in the presence of formic acid have been reported and discussed. A linear plot of lnk vs l/T showed that the reactions obey the Arrhenius equation. Both the Arrhenius and the Eyring equations were used to calculate the activation energy. The effect of nitro groups was studied on different positions of benzaldehyde. For all substituents, the reactions followed second‐order kinetics, and the partial orders of reactions were recognized with respect to each reactant. Comparisons between the magnitudes of ΔH^? and TΔS^?showed that the reactions were enthalpy controlled. The validity of the isokinetic relationship and the compensation effect was tested, and the isokinetic temperature (β) was obtained. A linear enthalpy‐entropy plot (ΔH^?versusΔS^?) showed that the compensation effect is established, and this process occurs via a same mechanism across a series of reactions. From the Van't Hoff and Exner's plots, the isokinetic temperature was obtained.     

Synthesis of N‐alkoxybenzimidoyl azides and their reactions in electrophilic media

A new general route to N‐alkoxybenzimidoyl azides [ArC(N₃)=NOR] from a reaction of N‐alkoxybenzimidoyl bromide [ArC(Br)=NOR] with sodium azide in DMSO is described. These reactions result in the Z‐geometric configuration. These compounds show a moderate degree of thermal stability as assessed by differential scanning calorimetry, and lack reactivity in traditional 1,3‐dipolar cycloaddition ‘click’ reactions. Upon exposure to electrophilic compounds (trifluoroacetic acid or acetyl chloride), these azide compounds can react by two pathways: a Schmidt‐type rearrangement to form an N‐alkoxyurea or an isomerization–cyclization reaction pathway to form an N‐alkoxytetrazole. The route of the reaction has no dependence on solvent polarity and appears to depend upon the electrophile (H⁺ vs. CH₃CO⁺): reaction of the azide with trifluoroacetic acid results predominantly in the urea; reaction with acetyl chloride results solely in the tetrazole. Calculations indicate that the urea product is thermodynamically favored over the tetrazole product. They also indicate that both reaction conditions result in an equilibration between the starting azide and the tetrazole with the tetrazole being the major component in this equilibrium mixture. The fact that the azide also undergoes a Schmidt‐type rearrangement to form an N‐alkoxyurea when treated with trifluoroacetic acid appears to indicate that the barrier for aromatic ring migration is lower in the protonated azide produced on reaction with trifluoroacetic acid than in the acetylated azide produced on reaction with acetyl chloride. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2+ phosphors prepared by glycothermal methods

Manganese-doped Zn₂SiO₄ phosphors with different crystal structures and morphologies were synthesized by glycothermal reactions of zinc acetate dihydrate and manganese(II) acetate tetrahydrate with tetraethyl orthosilicate in various glycols at 315 °C. The reactions in 1,3-propanediol and 1,4-butanediol yielded α-Zn₂SiO₄:Mn²⁺, whereas the reactions in ethylene glycol and 1,5-pentanediol yielded β-Zn₂SiO₄:Mn²⁺ and ZnO, respectively. The samples obtained in 1,4-butanediol and 1,3-propanediol emitted green light (522 nm), and the sample prepared in 1,4-butanediol showed a higher emission intensity. The photoluminescence intensity of the Zn_1.96Mn_0.04SiO₄ phosphor prepared by a glycothermal reaction in 1,4-butanediol and subsequently calcined at 1100 °C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was obtained because the highly homogeneous distribution of Mn²⁺ in the α-Zn₂SiO₄ host synthesized by the glycothermal reaction was maintained during calcination treatment in air.     

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.     

Influence of the ortho effect in the solvolyses of dichlorobenzoyl chlorides

The ortho‐effect of substituents upon the kinetics of reactions taking place at a reaction center attached to an aromatic ring has long been a topic of interest. For benzoyl chloride solvolyses, it was shown by Bentley and coworkers that the 2,6‐dimethyl‐derivative followed an ionization pathway with characteristics very similar to those for the solvolyses of p‐methoxybenzoyl chloride. We have carried out a Grunwald–Winstein equation treatment of the solvolyses of 2,6‐dichlorobenzoyl chloride, with similar sized chlorines replacing the methyl groups but now with an overall electron‐withdrawing influence of the ortho‐substituents. In this way the reactivity is moderated and the study can be extended to the important fluoroalcohol‐containing solvents. For the 30 solvents studied, an ionization pathway with a moderate nucleophilic solvation component is indicated. For comparison purposes, the treatment has also been applied to the 2,4‐, 3,4‐, and 3,5‐dichloro‐ derivatives. For the 2,4‐dichloro‐derivative, the two reaction channels are clearly visible and the solvents included for each channel are consistent with their solvent properties. Copyright © 2011 John Wiley & Sons, Ltd.     

四氯苯醌与嘧啶类核酸碱基的光化学氢抽取和电子转移反应

采用纳秒时间分辨的激光闪光光解实验技术,研究了四氯苯醌(TCBQ)分子与两种嘧啶核酸碱基(胸腺嘧啶和尿嘧啶)分子在355nm激发下的光化学反应动力学. 355 nm将TCBQ分子布居到激发三重态³TCBQ^*,高反应活性的³TCBQ^*与碱基分子发生两个平行的反应,氢抽取反应和电子转移反应,分别对应生成瞬态光谱上观测到的两个光反应产物TCBQH·和TCBQ·^-. 这两个反应同时生成的碱基自由基和碱基阳