EPR and DFT Study of the Polycyclic Aromatic Radical Cations from Friedel-Crafts Alkylation Reactions

Electron paramagnetic resonance and electron-nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel-Crafts alkylating sys- tem, with m-xylene, or p-xylene and alkyl chloride. The results indicate that the observed electron paramagnetic resonance spectra are due to polycyclic aromatic radicals formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel-Crafts alkylation reactions undergo Scholl self-condensation to give polycyclic aromatic hydrocar- bons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl3. The identification of observed two radicals 2,6-dimethylanthracene and 1,4,5,8-tetramethylanthraeene were supported by density functional theory calculations using the B3LYP/6-31G（d,p）//B3LYP/6-31G（d） approach. The theoretical coupling constants support the experimental assignment of the observed radicals.     

ESR Studies on a Friedel‐Crafts Alkylation Reaction in [bmim]Cl‐AlCl3 Ionic Liquid

The paramagnetic complexes formed in Friedel‐Crafts alkylation reaction systems are invistigated by electron spin resonance (ESR) spectroscopy, in room temperature ionic liquids system 1‐butyl‐3‐methyl‐limidazolium chloride‐aluminium chloride ([bmim]Cl‐AlCl₃). The results indicate that ESR spectra observed are due to polycyclic aromatic radical cations formed from their parent hydrocarbons. ESR spectrum of spin adduct is obtained in an ionic liquid system composed of [bmim]Cl‐AlCl₃. In acidic solution the ¹⁴N hyperfine coupling constant of 4‐oxo‐TEMPO, 2.15 mT, is appreciably larger due to an adduct formed with AlCl₃.     

付氏反应中自由基的鉴定及其生成机理

本文通过一系列卤代烃与苯的付氏反应的ESR谱,研究了自由基物种的生成机理:它们起因于Lewis酸对由Scholl反应所生成的9,10-二取代蒽进行单电子氧化,并建立了一种生成9,10-二取代蒽自由基正离子的简单方法。首次报道了一个新自由基正离子1,2,3,4,5,6,7,8-八氘蒽的ESR谱。     

Computational studies on the cyclization of polycyclic aromatic hydrocarbons in the synthesis of curved aromatic derivatives.

Computational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six- or five-membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial C-C bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic C-H cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest-energy mechanism. The formation of five-membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic C-H dissociation to afford the corresponding cations is the most favourable process. These cations react with H* without barrier to give H2* Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors.     

Electrochemical and ESR studies of the methane C-H bond activation in the presence of radical cations of pyrazine-di-N-oxide and its substituted derivatives

Activation of the methane C-H bond in the presence of electrochemically generated radical cations of pyrazine-di-N-oxide and also of 2,5-dimethyl- and 2,3,5,6-tetramethyl-pyrazine-di-N-oxides is studied by methods of cyclic voltammetry (CVA), quantum chemical simulations, and ESR electrolysis. The studies are carried out on glassy carbon (GC) and Pt electrodes in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of aromatic di-N-oxides in the absence and in the presence of methane are recorded. The changes in the shape CVA curves and the intensity of ESR signals of di-N-oxide radical cations observed in the presence of methane point to the activation of the methane C-H bond followed by its oxidation. The reaction of pyrazinedi-N-oxide at the methane C-H bond is simulated by quantum chemical methods. The obtained results are explained within the framework of the mechanism of overall two-electron oxidation of methane within its complex with an aromatic di-N-oxide radical cation.     

Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion-molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH(3)C≡CCH(3)) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH(3) loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP+2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C(4) side-chain, followed by cyclization and/or low-energy H atom β-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph˙)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH(3) loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid molecular weight growth reactions to yield polycyclic aromatic nitrogen hydrocarbons (PANHs).     

Unsymmetric Twofold Scholl Cyclization of a 5,11‐Dinaphthyltetracene: Selective Formation of Pentagonal and Hexagonal Rings via Dicationic Intermediates

Even though the Scholl reaction is one of the most powerful processes for the synthesis of polycyclic aromatic hydrocarbons (PAHs), its mechanism still remains a subject of discussion. Herein, we report a unique twofold Scholl cyclization of a 5,11‐dinaphthyltetracene. Single‐crystal X‐ray diffraction analysis of the cyclization product revealed that unsymmetric cyclizations of the two naphthyl groups resulted in the formation of fully unsaturated pentagonal and hexagonal rings. The thus obtained product exhibits a twisted π‐surface and an absorption band that reaches up to 950 nm. A combined experimental and theoretical study showed that such unsymmetric Scholl cyclizations can be rationalized in terms of a mechanism that involves dicationic intermediates, which stands in contrast to previously reported pathways based on radical cations and arenium ions.     

Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyrene

Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H(2) formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H(2) is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H(2). The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion-molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H(2) formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H(2) formation because deuteration lowers the energies of polyatomic PAH cations appreciably.     

Electronic spectra of radical cations and their correlation with photoelectron spectra—IV. IP1-EA correlation

In an extension of earlier work on the correlation between optical spectra of radical cations and the photoelectron spectra of the neutral molecules, a new relationship is shown to hold between the first ionization potentials of alternant polycyclic aromatic hydrocarbons (PAHs) and the optical “A” bands of their radical cations which correspond to the transition from the singly occupied to the lowest unoccupied π level. From the regression obtained, the first IPs of the molecules can be reasonably estimated. It is further suggested that this approach may be used to good advantage to identify the A bands in complex radical cation spectra solely from the knowledge of the first ionization potentials.     

Early events in radiation chemistry and in photoionization

Real-time studies of aliphatic and aromatic hydrocarbons by pulse radiolysis and laser photoionization reveal the chemistry of the ionic species in the condensed phase. The occurrence of radical cation reactions with solvent molecules provides the core mechanism capable of explaining a wide range of observations in photoionization and radiation chemistry. The study of products and transients in photoionization of aromatic solutes in hydrocarbon and alcohol solvents illustrates several details of this “high-energy” chemistry. A reaction pathway involving ion-molecule reaction of excited ions is indicated for a series of polycyclic aromatic hydrocarbons photoionized using intense excimer laser (248 and 308 nm) pulses in hydrocarbon and alcohol solutions. We have found that condensed-phase ion-molecule reactions in radiolysis are ubiquitous and we speculate on their overall role in hydrocarbon radiolysis.     

磺酰化修饰聚苯乙烯的合成及芳香烃萃取性能

利用自由基聚合反应合成了低分子量聚苯乙烯, 经过端基氧化和磺酰化反应, 制备出一系列极性砜基修饰的低分子量聚苯乙烯. 通过红外光谱(FTIR)、 核磁共振谱(NMR)、 差示扫描量热分析(DSC)以及热重分析(TGA)等手段对聚合物的结构和性能进行了表征, 并通过混合烃萃取分离实验对其芳香烃选择性进行了测试. 结果表明, 随着磺化比例的增加, 甲苯的选择系数和分布系数均显著提高, 表明极性修饰聚苯乙烯对多种芳香烃/链烷烃混合物均具有明显的芳香烃选择性.     

磺酰化修饰聚苯乙烯的合成及芳香烃萃取性能简

利用自由基聚合反应合成了低分子量聚苯乙烯,经过端基氧化和磺酰化反应,制备出一系列极性砜基修饰的低分子量聚苯乙烯. 通过红外光谱(FTIR)、核磁共振谱(NMR)、差示扫描量热分析(DSC)以及热重分析(TGA)等手段对聚合物的结构和性能进行了表征,并通过混合烃萃取分离实验对其芳香烃选择性进行了测试. 结果表明,随着磺化比例的增加,甲苯的选择系数和分布系数均显著提高,表明极性修饰聚苯乙烯对多种芳香烃/链烷烃混合物均具有明显的芳香烃选择性.     

羟基取代的多环芳烃与一氧化氮的反应研究

内源性的一氧化氮(NO)是多种生理过程中必不可少的信使分子,它在神经系统递质传导、神经发育、脑血流调节以及免疫调节等过程中具有十分重要的作用。为揭示NO生理功能的化学本质,NO成为化学家近年来研究的重要课题之一。     

Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A fluid-solution ESR study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i.e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobicyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1–3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a a remarkable dependence of the ¹⁴N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A Fluid-Solution ESR Study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i. e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobi-cyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1 – 3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a remarkable dependence of the ^l4N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

Montmorillonite K10 Clay Catalysed Friedel-Crafts Aralkylation in Room Temperature: Synthesis of Functionalised Linear Aryl-Alkyl-Aryl Systems

The Montmorillonite K10 clay catalysed Friedel-Crafts aralkylation of compound 2 with a number of aromatic hydrocarbons furnished functionalised linear Aryl-Alkyl-Aryl systems in good yield. A comparative study of this reaction was carried out with mont-K10 and Fe³⁺-Mont-K10.     

Integrated Chemical Process: One-Pot Aromatization of Cyclic Enones by the Double Elimination Methodology

A variety of aromatic hydrocarbons bearing multiple alkyl substituents are accessible with perfect regiocontrol in a one-pot reaction starting from cyclohexenones and their aromatic analogues [Eq. (1)]. The present methodology can be further extended to the synthesis of polycyclic aromatic hydrocarbons. The drawbacks encountered in the Friedel–Crafts reaction are resolved since the reaction proceeds under basic conditions.     

Optimization of solid‐phase microextraction using Taguchi design to quantify trace level polycyclic aromatic hydrocarbons in water

This article introduces a simple, rapid, and reliable solid‐phase microextraction (SPME) method coupled with GC‐MS for the quantitative determination of 16 polycyclic aromatic hydrocarbons in water. In this study, the Taguchi experimental design was used to optimize extraction conditions of polycyclic aromatic hydrocarbons using SPME method to obtain highly enriched analytes. Consequently, quantitative determination of polycyclic aromatic hydrocarbons in water was achieved by GC‐MS technique. The selected parameters affecting enrichment of polycyclic aromatic hydrocarbons were sample extraction time, stirring speed, temperature, ionic strength, and pH. The study revealed that optimal operating conditions were found to be 90‐min extraction time, 1400 rpm stirring speed, and 60°C sample temperature. The effect of ionic strength and pH were shown to be insignificant. Optimized conditions were also reevaluated by placing the 16 polycyclic aromatic hydrocarbons into several subgroups based on their molecular weight. The extraction efficiency of polycyclic aromatic hydrocarbons with low molecular weight was shown to be a function of only the extracting temperature. Satisfactory results were obtained for linearity (0.983–0.999), detection limits (2.67–18.02 ng/L), accuracy (71.2–99.3%), and precision (4.3–13.5%). The optimum conditions reported by other design approaches were evaluated and generalized optimum conditions were suggested.     

Determination of 15 polycyclic aromatic hydrocarbons in aquatic products by solid‐phase extraction and GC–MS

We propose a method for the simultaneous determination of 15 kinds of polycyclic aromatic hydrocarbons in marine samples (muscle) employing gas chromatography with mass spectrometry after saponification with ultrasound‐assisted extraction and solid‐phase extraction. The experimental conditions were optimized by the response surface method. In addition, the effects of different lyes and extractants on polycyclic aromatic hydrocarbons extraction were discussed, and saturated sodium carbonate was first used as the primary saponification reaction and extracted with 10 mL of ethyl acetate and secondly 1 mol/L of sodium hydroxide and 10 mL of n‐hexane were used to achieve better results. The average recovery was 67–112%. Satisfactory data showed that the method has good reproducibility with a relative standard deviation of <13%. The detection limits of polycyclic aromatic hydrocarbons were 0.02–0.13 ng/g. Compared with other methods, this method has the advantages of simple pretreatment, low solvent consumption, maximum polycyclic aromatic hydrocarbons extraction, the fast separation speed, and the high extraction efficiency. It is concluded that this method meets the batch processing requirements of the sample and can also be used to determine polycyclic aromatic hydrocarbons in other high‐fat (fish, shrimp, crab, shellfish) biological samples.     

芳烃酰基化后还原制备芳香醇的工艺进展

芳烃制备高附加值精细化学品芳香醇(9-芴甲醇),一直以来存在产物选择性低以及合成成本高等问题。基于此,本文主要综述了芳烃酰基化后还原合成芳香醇的工艺,包括第一步采用Friedel-Crafts酰基化反应、Vilsmeier-Haack反应、Reimer-Tiemann反应、Duff反应等过程将芳烃酰基化合成芳香醛/酮;第二步通过金属氢化物还原、催化加氢还原、活泼金属还原、Cannizarro反应、Meerwein-Ponndorf-Verley还原反应等过程将芳香醛/酮还原合成芳香醇。在总结和归纳各种工艺过程优缺点的基础上,提出了合理的芳香醇制备工艺,为9-芴甲醇产业化制备技术的开发提供帮助。