Arylboronic acids as versatile coupling partners in fast microwave promoted oxidative Heck chemistry

The useful and selective reactivity of arylboronic acids makes them favourite building blocks for many modern organic chemistry applications like the metal-mediated formation of C-C, C-O, C-N, and C-S bonds. This report describes oxidative Heck coupling reactions of arylboronic acids and olefins, which were conveniently and rapidly (5-30 min) carried out under air with temperature-controlled microwave heating. Different reaction conditions were investigated with regard to both microwave heating capability and chemical productivity. Copper(II) acetate was identified as a microwave compatible reoxidant of Pd(0). The scope and limitations of this high-speed chemistry protocol with diverse olefins and organoboronic acids are discussed.     

Carbonization of Chitosan in Palladium-Chitosan/ Montmorillonite Composites to Prepare Novel Heterogeneous Catalysts for Heck Reactions

Palladium-carbon/montmorillonite (Pd-C/MMT) heterogeneous catalysts were prepared by carbonization of chitosan in palladium-chitosan/montmorillonite (Pd-CS/MMT) composites under a N₂ atmosphere. The main purpose of this study was to determine the effects of the carbonization temperature (250, 450, and 650°C) on the material microstructure and its catalytic performance. X-ray diffraction patterns revealed that the formation of an intercalation of the carbonaceous material between the clay mineral layers which was also confirmed by Fourier transform infrared spectra results. The N content of the Pd-C/MMT catalyst decreased as the carbonization temperature was increased. Thermogravimetry curves revealed that the thermal stability of the Pd-C/MMT catalyst increased obviously as the carbonization temperature was increased. Pd nanoparticles, sized at about 20 nm, were mainly dispersed on the surface of the MMT, as observed by high-resolution transmission electron microscopy (HR-TEM). The prepared Pd-C/MMT catalysts had good catalytic activity when applied in Heck coupling reactions of aromatic halides and alkenes to produce aryl olefins. The relations between the recyclability and the material composition are discussed.     

Studies on high-temperature amination reactions of aromatic chlorides using discrete Palladium-N-Heterocyclic Carbene (NHC) complexes and in situ palladium/imidazolium salt protocols

The palladium catalysed coupling of aryl chlorides and amines can be readily achieved with short reaction times when carried out at high temperatures under thermal or microwave conditions. These coupling protocols are successful using two co-ordinate palladium-N-heterocyclic carbene complexes, or imidazolium salt protocols.     

Indium-mediated regioselective synthesis of ketones from arylstannanes under solvent-free ultrasound irradiation

The solvent-free indium-promoted reaction of alkanoyl chlorides with sterically and electronically diverse arylstannanes is a simple and direct method for the regioselective synthesis of primary, secondary and tertiary alkyl aryl ketones in good to excellent isolated yields (42–84%) under mild and neutral conditions. The protocol is also adequate for the synthesis of aryl vinyl ketones. Reaction times are drastically reduced (from 3–32 h to 10–70 min) under ultrasonic irradiation. Evidences for the involvement of a homolytic aromatic ipso-substitution mechanism, in which indium metal acts as radical initiator, are presented. It is possible the transference of two aryl groups from tin, thus improving effective mass yield, working with diarylstannanes as starting substrates.     

Combining the use of solid‐supported transition metal catalysis with microwave irradiation in solution‐phase parallel library synthesis

Microwave heating methods have been combined with the use of solid-supported catalysts to produce small solution-phase libraries of medicinally-relevant compounds. Palladium supported on charcoal (Pd/C) has been used to produce libraries of pyrazole compounds for screening in COX II studies via Suzuki cross coupling reactions, while the same catalyst has been used also to produce styryl-based nAChR compounds using analogous chemistry. Although the reaction substrates are very different (aryl vs. vinyl), this catalyst system provided consistently good and reliable results. The use of a polystyrene-supported Ru catalyst for ring-closing metathesis (RCM) reactions was also evaluated to prepare benzolactam structures for evaluation as factor Xa inhibitors.     

Combination of microwave reactions with fluorous separations in the palladium-catalyzed synthesis of aryl sulfides

Coupling of microwave reactions with fluorous separations can dramatically increase the efficiency of high-speed synthesis. Described in this paper is a fluorous synthesis of aryl sulfides by palladium-catalyzed cross-coupling of aryl perfluoroalkylsulfonates (C8F17O2SOAr) with thiols (RSH) under microwave irradiation. Fluorous solid-phase extractions (F-SPE) are employed for the purification of reaction mixtures. No fluorous solvents are involved in reaction and separation processes. The fluorous synthesis is further extended to the multi-step synthesis of substituted hydantoin and amide scaffolds.     

Ultrasound-promoted synthesis of 3-trichloromethyl-5-alkyl(aryl)-1,2,4-oxadiazoles

The alternative synthesis of 12 1,2,4-oxadiazoles using ultrasound irradiation from trichloroacetoamidoxime and acyl chlorides is reported. Seven of them are novel compounds. The 3-trichloromethyl-5alkyl(aryl)-1,2,4-oxadiazoles have been synthesised in better yields and shorter reaction times compared to the conventional method. This protocol can be applicable for preparation of 1,2,4-oxadiazoles containing aryl or alkyl groups attached at their C-5 side-chain.     

Mechanistic insights into C‐C cross coupling activities of Pd/Ni‐doped heterofullerenes

Mechanistic insights into Heck and Suzuki‐Miyaura cross coupling reactions with C₅₉M (M = Pd/Ni) catalysts were developed. Density functional theory was used for the analysis of all the intermediates and transition states possible during C‐C cross coupling reactions over the catalysts under study. Oxidative addition, a step common to both Heck and Suzuki‐Miyaura cross coupling reactions, was observed to proceed with smaller activation barriers over C₅₉Pd. Heck coupling of iodobenzene with styrene was observed to proceed via oxidative addition, migratory insertion, and reductive elimination steps. The free energy barriers for oxidative addition, migratory insertion, and reductive elimination steps were 14.8, 11.6, and 4.8 kcal/mol, respectively, over C₅₉Pd, and 17.4, 79.3, and 17.4 kcal/mol, respectively, over C₅₉Ni, indicating oxidative addition and migratory insertion to be the rate‐determining steps over C₅₉Pd and C₅₉Ni, respectively. Similarly for Suzuki‐Miyaura coupling reaction, activation barriers for oxidative addition, transmetalation, and reductive elimination steps were 14.8, 52.4, and 7.9 kcal/mol, respectively, over C₅₉Pd, and 17.4, 64.7, and 60.2 kcal/mol, respectively, over C₅₉Ni, indicating transmetalation step to be the rate‐determining step over both the heterofullerenes.     

Microwave-assisted acylation of 7-amino-5-aryl-6-cyanopyrido[2,3-d]pyrimidines

A microwave-assisted method is described for monoacylating 7-amino-5-aryl-6-cyanopyrido[2,3-d]pyrimidines using excess acid chlorides in pyridine. A diacylated intermediate is effectively deacylated to the product amide by a macroporous-Tris resin. A small library of 17 amides was prepared to validate the method. The integration of commercial microwave technology into the ArQule chemistry platform is also discussed.     

Microwave-enhanced transition metal-catalyzed decoration of 2(1H)-pyrazinone scaffolds

The 2(1H)-pyrazinones have been demonstrated to be versatile building blocks for the synthesis of biologically active compounds. Here, an efficient method is described for the decoration of these interesting scaffolds. Microwave-assisted palladium catalyzed reactions allow the easy introduction of different substituents at the C3- and even at the rather unreactive C5-position of the pyrazinones. Stille, Suzuki, Heck, Sonogashira reactions, in addition to reductive dechlorinations, and cyanation reactions are investigated.     

Coupling-isomerization-coupling sequences switched on by propargyl alcohol-enone-isomerization

The coupling-isomerization-reaction (CIR) of an electron-deficient halide 1 with 1-(p-bromo phenyl) propyne-1-ol (2) leads to an in-situ activation of the carbon-bromine bond towards oxidative addition and has been elaborated to a consecutive CIR-coupling sequence where subsequent palladium catalyzed coupling reactions such as Sonogashira, CIR, Heck, or Suzuki reactions allow a rapid construction of more complex frameworks in a one-pot reaction and in moderate to good yields.     

Soft chemical routes to semiconductor nanostructures

Soft chemistry has emerged as an important means of generating nanocrystals, nanowires and other nanostructures of semiconducting materials. We describe the synthesis of CdS and other metal chalcogenide nanocrystals by a solvothermal route. We also describe the synthesis of nanocrystals of AlN, GaN and InN by the reaction of hexamethyldisilazane with the corresponding metal chloride or metal cupferronate under solvothermal conditions. Nanowires of Se and Te have been obtained by a self-seeding solution-based method. A single source precursor based on urea complexes of metal chlorides gives rise to metal nitride nanocrystals, nanowires and nanotubes. The liquidliquid interface provides an excellent medium for preparing single-crystalline films of metal chalcogenides.     

The effect of heterogeneous solvent systems on sonochemical reactions: accelerated degradation of alkyl thiols in emulsion

Sonochemical reactions of alkyl and aryl thiols in water–organic mixed solvent systems were kinetically investigated. The reaction in the liquid–liquid interface apparently depends on the polarities, surface activities, vapor pressures and hydrophobicities of organic solvents and thiols. Interestingly, the rate jump in sonochemical disappearance of alkyl thiols was observed under emulsified conditions.     

NMR and AM1 Study of the Tautomeric Equilibrium of Isochroman-1,2-diones

Reaction of homophtalic anhydrid with aliphatic or aryl anhydrids (or acid chlorides) and an appropriate base gives 4-acyl and 4-aroylisochroman-1,3-diones. The structure of these isochroman-1,3-diones was determined by ¹H, ¹³C and ¹⁷O NMR spectrometry. In solution, among the three possible tautomers, only the enolic one with exocyclic double bond was observed. These results are in good agreements with AMI calculations ones.     

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH₃OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH₃O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.     

甲烷微尺度催化燃烧的数值模拟

本文联合使用计算流体力学软件FLUENT和可以计算表面反应的化学反应动力学软件DETCHEM对有逆流换热的微尺度燃烧器进行了数值计算。计算中忽略空间反应。燃料-空气混合物的当量比为0.4,反应器壁面采用等温边界条件。计算结果表明,采用催化燃烧可以实现微尺度下通常情况下无法实现的甲烷稳定燃烧。通过适当设置催化表面,可以实现燃料低温、高效转变。甲烷的总转变率受流动状态、反应温度和催化表面的大小等因素的影响。     

Experimental and kinetic modeling studies of phenyl acetate pyrolysis at atmospheric pressure

Phenyl acetate (CH₃COOC₆H₅, PA) shares a similar aryl acetate group with vitamin E acetate, which is thought to be responsible for producing pulmonary toxic ketene in e-cigarettes. Hence, PA is reported to be a model compound of vitamin E acetate in producing ketene. To better understand the pyrolysis chemistry of vitamin E acetate, pyrolysis of PA in a jet-stirred reactor was investigated by using synchrotron vacuum ultraviolet photoionization mass spectrometry at atmospheric pressure and at temperature range of 700 – 1025 K. Several key products such as acetylene, ethylene, carbon monoxide, formaldehyde, carbon dioxide, vinyl acetylene, 1,3-butadiene, 1,3-cyclopentadiene, benzene, phenol, etc., and especially, ketene, were identified and measured. By extending the phenyl formate pyrolysis model, a detailed PA pyrolysis model containing 735 species and 3365 reactions was constructed and validated against the current experimental results of PA pyrolysis. Rate of production analysis and sensitivity analysis show that the main reaction pathways of PA pyrolysis are the unimolecular decomposition forming phenol and ketene, followed by the C–CH₃ bond cleavage forming phenoxycarbonyl and methyl. The corresponding products of these two reactions and of the subsequent reactions, including phenol, ketene and carbon monoxide, etc., are demonstrated to be the key products in PA pyrolysis. Toxic aromatic compounds, such as benzene, toluene and ethylbenzene, etc., also have relatively high mole fractions in PA pyrolysis.     

Sonochemical synthesis of novel pyrano[3,4-e][1,3]oxazines: A green protocol

The atom-efficient and green protocol for formation of pyrano[3,4-e][1,3]oxazines utilizing dimethyl carbonate under ultrasound irradiation in a presence of KF/basic alumina was reported. We provide a novel series of pyrano[3,4-e][1,3]oxazine derivatives interesting for biological screening tests. In general, it was found that ultrasound irradiations enable the reactions to occur which could not be carried out under silent conditions. These remarkable effects appeared in sonicated reactions can be reasonably interpreted in terms of acoustic cavitation phenomenon. Structures of the products were established on analytical and spectral data. This protocol offers several advantages attain many principles of green chemistry including, save energy, atom economy, clean reactions, inexpensive green reagent and use catalysts rather than stoichiometric reagents.     

Educational aspects of sonochemistry: The role of sonochemistry at high school level

The applications of sonochemistry have attracted a great deal of attention. However, conventional chemistry textbooks used in universities and high schools cover traditional chemistry plus thermo,- photo- and electrochemistry. The effect of sound on chemical systems has completely been ignored. Although the mechanisms of many sonochemical reactions remain unresolved, the rapid expansion of knowledge in sonochemistry has already justified its inclusion in present chemistry curricula. Moreover, the interesting and unusual phenomena encountered under sonication conditions can be used for motivation purposes. Furthermore, exploring and equipping students with a new tool for investigating chemistry is also important based on educational principles. This paper examines some chemical systems (in a high school chemistry syllabus) which are affected by sound energy. It is hoped that these sonochemical applications will facilitate the learning of chemistry in terms of cognitive, psychomotor and affective domains.     

Recent trends in the chemistry of Sandmeyer reaction: a review

Metal-catalyzed reactions play a vital part to construct a variety of pharmaceutically important scaffolds from past few decades. To carry out these reactions under mild conditions with low-cost easily available precursors, various new methodologies have been reported day by day. Sandmeyer reaction is one of these, first discovered by Sandmeyer in 1884. It is a well-known reaction mainly used for the conversion of an aryl amine to an aryl halide in the presence of Cu(I) halide via formation of diazonium salt intermediate. This reaction can be processed with or without copper catalysts for the formation of C–X (X?=?Cl, Br, I, etc.), C-CF₃/CF₂, C–CN, C–S, etc., linkages. As a result, corresponding aryl halides, trifluoromethylated compounds, aryl nitriles and aryl thioethers can be obtained which are effectively used for the construction of biologically active compounds. This review article discloses various literature reports about Sandmeyer-related transformations developed during 2000–2021 which give different ideas to synthetic chemists about further development of new and efficient protocols for Sandmeyer reaction.

Graphical abstract

An updated compilation of new approaches for Sandmeyer reaction is described in this review to construct a variety of carbon-halogen, carbon-phosphorous, carbon-sulfur, carbon-boron etc. linkages.