N-Dimethoxyphenylation of highly basic pyrazoles during undivided electrolysis

The reactions of pyrazole, 3,5-dimethylpyrazole, and its 4-nitro derivatives with 1,4-dimethoxybenzene during undivided amperostatic electrolysis in MeCN (CH₂Cl₂) were studied. The basicity of the medium, which depends on the solvent nature, the nature and concentration of pyrazole and the acid-base properties of additives, and the amount of electricity passed determine the yield and relative content of the target products, viz., 1,4-dimethoxy-2-(pyrazol-1-yl)benzenes (1) and 1,4-dimethoxy-1,4-di(pyrazol-1-yl)cyclohexa-2,5-dienes (2). The process occurs mainly through the interaction of the nonionized solvato complex of pyrazole with the 1,4-dimethoxybenzene radical cation and affords radical intermediates structurally similar to compounds 1 and 2. The key stage of the process determining the 1 : 2 ratio is the rearrangement of the intermediately produced 1,4-dimethoxy-1-(pyrazol-1-yl)arenonium cation to the 1-(pyrazol-1-yl)-2,5-dimethoxyarenonium cation.     

Electrosynthesis of N-Arylazoles by Electrolyzing a Mixture of Azole and 1,4-Dimethoxybenzene in a Diaphragmless Cell

An analysis is performed and data are compared on the electrosynthesis of N-arylazoles and regularities of this process in conditions of a diaphragmless galvanostatic electrolysis (Pt, MeCN, Bu₄NClO₄) of a mixture of 1,4-dimethoxybenzene (DMB) with azoles (pyrazole, triazole, their derivatives, tetrazole). Electrolysis of an azole/DMB mixture leads to the formation of products of an ortho-substitution—1,4-dimethoxy-2-(azolyl-1)benzenes—and, simultaneously, hydrolytically unstable products of an ipso-bis-attachment—1,4-dimethoxy-1,4-di-(azolyl-1)cyclohexa-2,5-dienes. The overall yield of these compounds increases upon adding a base (collidine) or an acid (AcOH) into the initial mixture, and the basicity of initial azoles substantially affects the electrosynthesis results. New notions on the nature of nucleophilic species interacting with radical cation of DMB are considered. The species in question are complexes of azoles with one another or with collidine generated at the expense of the hydrogen bond, rather than azolate ions. Furthermore, the cathodic process is largely connected not with the generation of azolate ions (as a result of the reduction of initial azoles) but with the deprotonation of onium compounds (BH⁺)—products of the interaction of azoles or collidine with protons. The mechanism of electrosynthesis of N-arylazoles is discussed. The key stages of the synthesis are the attack of a nucleophile on the ipso- and, possibly, ortho-positions of the benzene ring of radical cation of DMB, as well as the rearrangement of the intermediate cation of 1,4-dimethoxy-1-(azolyl-1)arenonium into the cation of 1-(azolyl-1)-2,5-dimethoxyarenonium, which affects both the yield and ratio of final products of the reaction mixture.     

Reactions of some arenes and pyrazoles in MeCN under conditions of undivided-cell electrolysis

As exemplified for the first time by pyrazole and its 4-nitro and 3,5-dimethyl derivatives, N-arylation of pyrazoles can be performed under conditions of undivided-cell amperostatic electrolysis (Pt electrodes, MeCN) of systems containing the pyrazolate anion and (or) pyrazole, arene (benzene, 1,4-dimethoxybenzene, or xylene), and a supporting electrolyte. In the case of electrolysis involving 1,4-dimethoxybenzene as arene, N-arylation followed simultaneously three routes to form an ortho-substitution product (1,4-dimethoxy-2-(pyrazol-1-yl)benzene), an ipso-substitution product (4-methoxy-1-(pyrazol-1-yl)benzene), and an ipso-bisaddition product (1,4-dimethoxy-1,4-di(pyrazol-1-yl)cyclohexa-2,5-diene) in a total current yield of up to 50%. The acid-base properties of the pyrazoles under study affect the ratio of the N-arylation products and govern the required composition of the starting reaction mixture. In the case of a stronger base, such as 3,5-dimethylpyrazole, N-arylation with 1,4-dimethoxybenzene occurred even in the pyrazole—arene—tetraalkylammonium perchlorate system, whereas N-arylation of 4-nitropyrazole (a weaker base) proceeded only in the presence of the pyrazolate anion or another base, viz., sym-collidine. Oxidation of arene to the radical cation is the key anodic reaction. Not only the pyrazolate anion, but also highly basic pyrazole or a solvate complex of weakly basic pyrazole with collidine can serve as a nucleophilic partner in subsequent transformations of these radical cations.     

Regularities of anodic acetoxylation of 1,4-dimethoxybenzene in protic and aprotic media

Electrochemical acetoxylation of 1,4-dimethoxybenzene during amperostatic electrolysis in an undivided cell at Pt electrodes in MeCN or MeOH solutions containing Et₄NOAc gives 2,5-dimethoxyphenyl acetate if AcOH or CH₂Cl₂ co-solvent has been added in a concentration of ≥50%. The reaction mechanism includes a nucleophilic attack of AcO⁻ ion on the ipso-position of 1,4-dimethoxybenzene radical cation. The process efficiency depends on factors that determine the stability and reactivity of the intermediate 1,4-dimethoxy-1-acetoxyarenonium cation. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1534–1538, July, 2005.     

Electrochemical <Emphasis Type="Italic">N</Emphasis>-arylation of azoles in MeOH using undivided electrolysis of their mixtures with 1,4-dimethoxybenzene

The reactions of 1,4-dimethoxybenzene with azoles (pyrazole, triazole, and their derivatives, as well as tetrazole) were studied by undivided amperostatic electrolysis at Pt electrodes in MeOH. The process proceeds via the formation of a 1,1,4-trimethoxyarenonium cation as the key intermediate and affords 1,1,4,4-tetramethoxycyclohexa-2,5-diene, 1,1,4-trimethoxy-4-(azol-1-yl)cyclohexa-2,5-diene, and 1,4-dimethoxy-2-(azol-1-yl)benzene as the main products. Azole and solvent molecules compete as nucleophiles during electrolysis. A fine mechanism of the process was considered. Dedicated to Academician N. K. Kochetkov on the occasion of his 90th birthday. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1166–1171, May, 2005.     

Cycloalumination of allylbenzenes with triethylaluminum in the presence of Cp<Subscript>2</Subscript>ZrCl<Subscript>2</Subscript>. One-pot synthesis of 2-benzylbutane-1,4-diols as precursors of dibenzylbutane lignans

One-pot synthesis of 2-[(hydroxy- and methoxyphenyl)methyl]butane-1,4-diols in an overall yield of 60–65% by cycloalumination of allylbenzenes (4-allyl-1-methoxybenzene, 4-allyl-1,2-dimethoxybenzene, 5-allyl-2-methoxyphenol, and 5-allyl-1,2,3-trimethoxybenzene) with triethylaluminum in the presence of Cp₂ZrCl₂ is reported for the first time. The developed procedure opens a new synthetic route to practically important β-substituted butane-1,4-diols that are precursors to dibenzylbutane lignans.     

Arenium cation as the key intermediate of the electrosynthesis of <Emphasis Type="Italic">N</Emphasis>-(2,5-dimethoxyphenyl)azoles. A new approach to the synthesis of <Emphasis Type="Italic">N</Emphasis>-(dimethoxyphenyl)azoles

Data on the effect of the acid-base properties of the medium on the yield and composition of the products of N-dimethoxyphenylation of azoles (pyrazole, triazole, their substituted derivatives, and tetrazole) upon galvanostatic electrolysis of azole—1,4-dimethoxybenzene mixtures in nucleophilic (MeOH) and neutral (MeCN) media were considered and the trends of this process were discussed. The generation of arenium cations (1,4-dimethoxy-1-azolylbenzenium in MeCN and 1,1,4-trimethoxybenzenium in MeOH) as the key intermediates of electrosynthesis of N-(dimethoxyphenyl)azoles, was proved experimentally. A new approach to the synthesis of N-(dimethoxyphenyl)azoles through electrosynthesis of 1,1,4,4-tetramethoxycyclohexa-2,5-diene by electrooxidation of 1,4-dimethoxybenzene in MeOH as the first step and the reaction of this quinone diketal with azoles as the second step was suggested. The efficiency of this route to N-(dimethoxyphenyl)azoles is comparable with the efficiency of the purely electrochemical one-step process. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2101–2109, November, 2007.     

Electron transfer reactions between pentafluorobenzoyl peroxide and dimethoxybenzenes

Pentafluorobenzoyl peroxide (FBP) reacted rapidly with dimethoxybenzenes in F₁₁₃ (CCl₂FCClF₂) with kinetics of first order in each component. High yields of ring-substituted esters of $\text{m}$-dimethoxybenzene ($\text{m}$-DMB) and $\text{p}$-dimethoxybenzene ($\text{p}$-DMB) were obtained, whereas for 2,5-di-$\text{t}$-butyl-1,4-dimethoxybenzene (DBDMB), the $\text{t}$-Bu group was simultaneously eliminated. For 2,5-dimethyl-1,4-dimethoxybenzene (DMDMB), the benzylic hydrogen was substituted. Rate and product studies both indicate a rate-determining electron transfer step leading to radical ion pairs which collapse to products.     

N-Arylation of azoles with low basicity by 1,4-dimethoxybenzene during undivided electrolysis

The reactions of 1,4-dimethoxybenzene with 4-nitropyrazole, 3,4-dinitro-5-methylpyrazole, 1,2,4-triazole, 3-nitro-1,2,4-triazole, and tetrazole were studied during undivided amperostatic electrolysis on a Pt electrode in MeCN, CH₂Cl₂, and MeOH. The main reaction products were 2-azolyl-1,4-dimethoxybenzenes and (or) 1,4-diazolyl-1,4-dimethoxycyclohexa-2,5-dienes. In all cases except 1,2,4-triazole, N-arylation occurs only in the presence of the Alk₄N⁺ salts of azoles or 2,4,6-trimethylpyridine as a base. The mechanism of the reactions is discussed.     

Iron(III)-catalyzed hydroarylation of propiolic acid with activated arenes

FeCl₃/AgOTf-catalyzed hydroarylation of propiolic acid with electron-rich arenes such as mesitylene, tetramethylbenzene, and pentamethylbenzene in trifluoroacetic acid proceeded to give 3-arylpropenoic acids in moderate to high yields. The same reactions with anisole and 1,4-dimethoxybenzene afforded double hydroarylation products, 3,3-diarylpropionic acids.     

How radical cations react? - Distonic radical cation mediated α(nucleophilic), β(radical)-dibenzoloxylation of donor ArCH = CHR

Rate determination and product studies have disclosed that the fragmentation pattern of radical cations 2-propenyl-1,4-dimethoxybenzene (1^{+ ·}) and 2-propenyl-1,4,5-trimethoxybenzene (2^+·) generated in one-electron oxidation of their parent substrates by 4-nitrobenzoyl peroxide (3) in CH₃CN is greatly affected by ring-substitution status of the donor molecules. While ringbenzoloxylation (product 5) predominated in the reaction of dimethoxylated substrate (1), the oxidation of trimethoxylated donor 2 ended up with distonic radical cation mediated ,-di-4-nitrobenzoloxylation as the major pathway.     

A highly soluble dimethoxybenzene derivative as a redox shuttle for overcharge protection of secondary lithium batteries

A compound 4-tertbutyl-1,2-dimethoxybenzene (TDB) was synthesized and tested as a redox shuttle for overcharge protection of Li–LiFePO₄ batteries. This isomer of tertbutyl-substituted dimethoxybenzene is miscible with the organic polar electrolytes and provides a solution for the poor solubility of ditertbutyl-substituted 1,4-dimethoxybenzenes as a redox shuttle additive. The experimental results demonstrated that the shuttle molecules added in the electrolyte cannot only provide feasible overcharge protection, but also have indiscernible detrimental influences on the charge–discharge behaviors of Li–LiFePO₄ cells, showing a great prospect for practical applications in commercial rechargeable lithium batteries.     

Theoretic calculation for understanding the oxidation process of 1,4-dimethoxybenzene-based compounds as redox shuttles for overcharge protection of lithium ion batteries

The effect of substituents on the oxidation potential for the one-electron reaction of 1,4-dimethoxybenzene was understood with a theoretical calculation based on density functional theory (DFT) at the level of B3LYP/6-311+G(d). It is found that the oxidation potential for the one-electron reaction of 1,4-dimethoxybenzene is 4.13 V (vs Li/Li(+)) and can be changed from 3.8 to 5.9 V (vs Li/Li(+)) by substituting electron-donating or electron-withdrawing groups for the hydrogen atoms on the aromatic ring. These potentials are in the range of the limited potentials for the lithium ion batteries using different cathode materials, and thus the substituted compounds can be selected as the redox shuttles for the overcharge prevention of these batteries. The oxidation potential of 1,4-dimethoxybenzene decreases when the hydrogen atoms are replaced with electron-donating groups but increases when replaced with electron-withdrawing groups. The further oxidation of these substituted compounds was also analyzed on the basis of the theoretic calculation.     

Direct comparison of 2,5-di-tert-butyl-1,4-dimethoybenzene and 4-tert-butyl-1,2-dimethoxybenzene as redox shuttles in LiFePO4-based Li-ion cells

2,5-di-tert-butyl-1,4-dimethoxybenzene (DDB) and 4-tert-butyl-1,2-dimethoxybenzene (TDB) have recently been proposed by different research groups as effective redox shuttles for overcharge protection of LiFePO₄-based Li-ion cells. Different test methods used in the published accounts make direct comparison of the merits of DDB and TDB difficult. Here DDB and TDB are tested under the same conditions in Li/LiFePO₄, graphite/LiFePO₄ and Li_4/3Ti_5/3O₄/LiFePO₄ coin-type cells under conditions that approximate those found in practical cells. The results confirm that DDB can support over 200 shuttle-protected overcharge cycles each of 100% cell capacity for all three cell types while TDB can only support between 3 and 15 overcharge cycles. This highlights the importance of testing redox shuttles under conditions that mimic those found in commercial cells.     

Enantioselective total synthesis of (S)-(+)-lennoxamine through asymmetric hydrogenation mediated by l-proline-tetrazole ruthenium catalyst

A novel asymmetric synthetic strategy to prepare isoindolobenzazepine based lennoxamine alkaloid has been achieved in high ee% starting from 2-(benzo[d][1,3]dioxol-5-yl)ethanamine and 1-(chloromethyl)-2,3-dimethoxybenzene in 5 steps and with a 34% overall yield. The potentiality of this route involved the Bischler-Napieralsky cyclization that leads to tetracyclic indolinium skeleton, generation of chiral center through asymmetric hydrogen-transfer reaction employing l-proline-tetrazole as chiral ligand with Ru/Ir/Rh, and anodic oxidation as the key steps in the synthesis.     

Practical one-pot transformation of electron-rich aromatics into aromatic nitriles with molecular iodine and aq NH3 using Vilsmeier–Haack reaction

Various electron-rich aromatics could be efficiently transformed into the corresponding aromatic nitriles in good to moderate yields by treatment with DMF and POCl₃, followed by the reaction with molecular iodine or 1,3-diiodo-5,5-dimethylhydantoin (DIH) in aq NH₃. Some of less reactive aromatics, such as anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and mesityrene, could be also transformed into the corresponding aromatic nitriles in good to moderate yields using N-methylformanilide and O(POCl₂)₂, followed by the reaction with molecular iodine in aq NH₃. Moreover, propiophenone derivatives could be successfully transformed into the corresponding β-chlorocinnamonitriles by the reaction with DMF and POCl₃, followed by the reaction with molecular iodine and aq NH₃. These reactions are novel metal-free one-pot methods for the preparation of aromatic nitriles from electron-rich aromatics and β-chlorocinnamonitriles from propiophenones.     

The decomposition of substituted benzoyl peroxides in the presence of dimethoxybenzenes

Benzoyl peroxides, particularly those containing electron withdrawing substituents, undergo rapid decomposition in the presence of m-dimethoxybenzene, p-dimethoxybenzene, 2,5-di-t-butyl-1,4-dimethoxybenzene, and 2,5-dimethyl-1,4-dimethoxybenzene. Reactions are first order in peroxide and dimethoxy benzene, increasing in the order given. Identified products are the acids corresponding to the peroxide and esters involving ring substitution, ring-substitution with elimination of a t-Bu group, and benzylic substitution. It is proposed that reaction involves a rate-determining charge transfer transition state leading to radical ion pairs which collapse to products. No free radicals have been detected.     

Anodic oxidation of organic compounds based on the cation exchange reaction between KBF4 and solid-supported acids

We have developed a novel electrolytic system for anodic oxidation of organic compounds based on the cation exchange reaction between potassium tetrafluoroborate (KBF₄) and solid-supported acids. It was clarified by cyclic voltammetry as well as preparative electrolyses that hydrogen tetrafluoroborate (HBF₄) derived from the cation exchange reaction acts as a supporting electrolyte in MeCN. On the basis of the electrolytic system, anodic oxidation of 1,2,4-trimethoxybenzene was carried out to provide the corresponding homocoupling product in quantitative yield. Furthermore, anodic oxidation of benzyl alcohols having not only electron-donating but also electron-withdrawing groups at the para position was successfully achieved by optimizing the reaction conditions.     

Chemical trapping experiments support a cation-radical mechanism for the oxidative polymerization of aniline

The oxidative polymerization of aniline in aqueous acidic solution was carried out in the presence of a variety of organic compounds as potential traps for postulated intermediates. The polymerization was inhibited by hindered phenols and electron-rich alkenes, traps for cation-radicals. However, polyaniline was still obtained in the presence of electron-rich arenes, such as 1,3-dimethoxybenzene and 1,4-dimethoxybenzene, known as excellent receptors of nitrenium ions. Polymerization of N-phenyl-1,4-phenylenediamine was similarly carried out in the presence of potential traps. Polyaniline containing an N-phenyl group was obtained in the presence of 1,3-dimethoxybenzene and 1,4-dimethoxybenzene. Hindered phenols and 4-methoxystyrene only slightly inhibited polymerization of N-phenyl-1,4-phenylenediamine which most probably proceeded by way of the stable diarylamino radical. Copolymerization of aniline with 10 wt % of N-phenyl-1,4-phenylenediamine in the presence of these traps gave similar results to the polymerization of pure aniline. These results have led to the proposed cation-radical polymerization mechanism of aniline, in which the polymerization is a chain growth reaction through the combination of a polymeric cation-radical and an anilinium cation-radical. Step growth character is also present when a polymeric aminium cation-radical end combines with a diarylaminoended polymer. The copolymerization of N-phenyl-p-phenylenediamine can also occur by reaction of aniline cation-radical with a polyarylamine radical. The nitrenium mechanism was further rejected by the fact that attempted polymerization of N-phenylhydroxylamine, which forms authentic nitrenium ions in acid, failed to give polymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2569–2579, 1999     

Efficient Emission of Ultraviolet Light by Solid State Organic Fluorophores: Synthesis and Characterization of 1,4-Dialkeny-2,5-dioxybenzenes

The design and development of organic luminophores that exhibit efficient ultraviolet (UV) fluorescence in the solid state remains underexplored. Here, we report that 1,4-dialkenyl-2,5-dialkoxybenzenes and 1,4-dialkenyl-2,5-disiloxybenzenes act as such UV-emissive fluorophores. The dialkenyldioxybenzenes were readily prepared in three steps from 2,5-dimethoxy-1,4-diacetylbenzene or 2,5-dimethoxy-1,4-diformylbenzene via two to four steps from 1,4-bis(diethoxyphosphonylmethyl)-2,5-dimethoxybenzene. The dialkenyldioxybenzenes emit UV light in solution (λ_em=350–387 nm) and in the solid state (λ_em=328–388 nm). In addition, the quantum yields in the solid state were generally higher than those in solution. In particular, the adamantylidene-substituted benzenes fluoresced in the UV region with high quantum yields (Φ=0.37–0.55) in the solid state. Thin films of poly(methyl methacrylate) doped with the adamantylidene-substituted benzenes also exhibited UV emission with good efficiency (Φ=0.27–0.45). Density functional theory calculations revealed that the optical excitation of the dialkenyldimethoxybenzenes involves intramolecular charge-transfer from the ether oxygen atoms to the twisted alkenyl-benzene-alkenyl moiety, whereas the dialkenylbis(triphenylsiloxy)benzenes were optically excited through intramolecular charge-transfer from the oxygen atoms and twisted π-system to the phenyl-Si moieties of each triphenylsilyl group.