有机微波化学研究进展

本文综述了近几年来微波技术在有机合成方面的研究和应用进展。初步探讨了微波催化有机反应的作用机理, 并展望了有机微波化学的发展前景。     

Microwaves as “Co-Catalysts” or as Substitute for Catalysts in Organophosphorus Chemistry

The purpose of this review is to summarize the importance of microwave (MW) irradiation as a kind of catalyst in organophosphorus chemistry. Slow or reluctant reactions, such as the Diels-Alder cycloaddition or an inverse-Wittig type reaction, may be performed efficiently under MW irradiation. The direct esterification of phosphinic and phosphonic acids, which is practically impossible on conventional heating, may be realized under MW conditions. Ionic liquid additives may promote further esterifications. The opposite reaction, the hydrolysis of P-esters, has also relevance among the MW-assisted transformations. A typical case is when the catalysts are substituted by MWs, which is exemplified by the reduction of phosphine oxides, and by the Kabachnik–Fields condensation affording α-aminophosphonic derivatives. Finally, the Hirao P–C coupling reaction may serve as an example, when the catalyst may be simplified under MW conditions. All of the examples discussed fulfill the expectations of green chemistry.     

Effective Application of Microwave Resonant Cavity System to Flow Chemistry

By applying advanced telecommunication solid state devices to microwave (MW) resonant cavity system for flow chemistry, it becomes possible to heat up low polarity solvents higher than 250 C, that are considered impossible to heat up by MW until now. The resonant cavity system is opening new process windows to production processes of specialty chemicals which require low cost, high yield and high productivity.     

Changing Perspectives on the Strategic Use of Microwave Heating in Organic Synthesis

This Personal Account describes collaborative investigations into apocryphal microwave effects in organic chemistry. Focused research on microwave‐assisted organic synthesis has been fraught with confusion, controversy, and misinformation. Microwave heating is an undoubtedly useful tactic for organic synthesis, but whether or not it can offer strategic advantages remains an open question in the minds of many people. (Ironically, those who do not consider it an open question are split as to whether it has been resolved affirmatively or negatively.) Our research in this area is guided by the hypothesis that microwave heating can alter reaction kinetics in ways distinct from what is observable under conventional heating. Here we provide a succinct record of the origins of our interests, our initial queries and associated controversies, and recent efforts to identify, quantify, and begin to leverage selective microwave heating for strategic advantage in organic synthesis.     

微波有机合成反应的新进展

综述了近来微波辐射技术在有机合成应用中的新进展。着重介绍了微波有机合 成反应技术及其在重要有机合成反应中的应用。     

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Silicon carbide (SiC) is a strongly microwave absorbing chemically inert ceramic material that can be utilized at extremely high temperatures due to its high melting point and very low thermal expansion coefficient. Microwave irradiation induces a flow of electrons in the semiconducting ceramic that heats the material very efficiently through resistance heating mechanisms. The use of SiC carbide reaction vessels in combination with a single‐mode microwave reactor provides an almost complete shielding of the contents inside from the electromagnetic field. Therefore, such experiments do not involve electromagnetic field effects on the chemistry, since the semiconducting ceramic vial effectively prevents microwave irradiation from penetrating the reaction mixture. The involvement of electromagnetic field effects (specific/nonthermal microwave effects) on 21 selected chemical transformations was evaluated by comparing the results obtained in microwave‐transparent Pyrex vials with experiments performed in SiC vials at the same reaction temperature. For most of the 21 reactions, the outcome in terms of conversion/purity/product yields using the two different vial types was virtually identical, indicating that the electromagnetic field had no direct influence on the reaction pathway. Due to the high chemical resistance of SiC, reactions involving corrosive reagents can be performed without degradation of the vessel material. Examples include high‐temperature fluorine–chlorine exchange reactions using triethylamine trihydrofluoride, and the hydrolysis of nitriles with aqueous potassium hydroxide. The unique combination of high microwave absorptivity, thermal conductivity, and effusivity on the one hand, and excellent temperature, pressure and corrosion resistance on the other hand, makes this material ideal for the fabrication of reaction vessels for use in microwave reactors.     

The microwave-to-flow paradigm: translating high-temperature batch microwave chemistry to scalable continuous-flow processes

The popularity of dedicated microwave reactors in many academic and industrial laboratories has produced a plethora of synthetic protocols that are based on this enabling technology. In the majority of examples, transformations that require several hours when performed using conventional heating under reflux conditions reach completion in a few minutes or even seconds in sealed-vessel, autoclave-type, microwave reactors. However, one severe drawback of microwave chemistry is the difficulty in scaling this technology to a production-scale level. This Concept article demonstrates that this limitation can be overcome by translating batch microwave chemistry to scalable continuous-flow processes. For this purpose, conventionally heated micro- or mesofluidic flow devices fitted with a back-pressure regulator are employed, in which the high temperatures and pressures attainable in a sealed-vessel microwave chemistry batch experiment can be mimicked.     

Research on analytical chemistry in Brazil: an overview

An overview is presented of the beginnings, evolution and current status of research on analytical chemistry in Brazil. Among the various fields of chemistry two decades ago, Analytical Chemistry was considered one of the least developed in Brazil. In the last 15 years, however, research and development in this field have expanded considerably and today it is one of the most highly developed fields of Chemistry in Brazil. This paper offers a general overview of this evolution and some suggestions for possible routes for the future of analytical chemistry in Brazil.     

Electromagnetic Relationship between Microwaves and Flow Reactor Systems

One of the important points in handling microwaves is that the order of magnitude is decimeter order. Laboratory equipment is not too big for wavelength, so intensity nonuniformity may occur. Another important point is that microwave propagation changes with material parameter changes. To heat material in the planned shape, the consideration with electromagnetism about the equipment and the system will be necessary. Herein, the phenomena which should be considered when a flow reactor is irradiated with a microwave is described.     

复杂化学体系的分子模拟研究——2013年诺贝尔化学奖浅析

随着化学科学研究水平的日益提高,人们对物质世界的分子水平的理解也在逐步加深。化学研究中对原理性理解的要求越来越高,也要求越来越准确的定量研究。化学传统上以实验为主,而现代化学对理论具有了强烈依赖性。理论化学的发展取得了巨大进步,成为化学学科的一个重要组成部分。理论与计算化学不但帮助解释实验结果,更提供对实验观察的预言和对材料药物等的合理设计。通过与统计力学和量子力学的紧密结合,在现代计算机技术的支持下分子模拟在化学、材料、生物等学科中得到广泛应用,处理多尺度实际体系的能力越来越强,具有非常广阔的发展前景。但分子模拟作为一个研究手段还需要大力发展,以提高其普适性,使之具有更坚实的理论基础,同时更加系统化和实用化。     

Microwave Flow: A Perspective on Reactor and Microwave Configurations and the Emergence of Tunable Single‐Mode Heating Toward Large‐Scale Applications

Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications.     

The Impact of Microwaves on Organophosphorus Chemistry

A personal account on the pioneering results on microwave (MW)‐assisted organophosphorus syntheses obtained in the last decade is presented. The broad spectrum of the reactions studied exemplifies the advantages of the MW technique as a green tool regarding efficiency and the simplification of catalyst systems. Theoretical calculations for a few typical models helped us to explore the scope and limitations of the application of MW irradiation.     

Microwaves and chemistry: The catalysis of an exciting marriage

The potential of microwave power as a tool to facilitate chemical reactions has not whetted the chemist’s appetite in the past and the phenomenon and uses of microwaves have remained in the comer of spectroscopists and engineers for a long time. The possibility of microwaves initiating chemical changes has nevertheless excited our imagination for the past ten years. We will present the original development of the concept of microwave catalysis/sensitization in chemistry and the coming of age of the techniques as an enabling technology in the industrial world. A number of demonstrated applications ranging from hydrocarbon oxidations to environmental technology will be illustrated, as well as the most recently developed technique and applications of the microwave-induced acoustic phenomenon.     

The Microwave‐Assisted Ionic‐Liquid Method: A Promising Methodology in Nanomaterials

In recent years, the microwave‐assisted ionic‐liquid method has been accepted as a promising methodology for the preparation of nanomaterials and cellulose‐based nanocomposites. Applications of this method in the preparation of cellulose‐based nanocomposites comply with the major principles of green chemistry, that is, they use an environmentally friendly method in environmentally preferable solvents to make use of renewable materials. This minireview focuses on the recent development of the synthesis of nanomaterials and cellulose‐based nanocomposites by means of the microwave‐assisted ionic‐liquid method. We first discuss the preparation of nanomaterials including noble metals, metal oxides, complex metal oxides, metal sulfides, and other nanomaterials by means of this method. Then we provide an overview of the synthesis of cellulose‐based nanocomposites by using this method. The emphasis is on the synthesis, microstructure, and properties of nanostructured materials obtained through this methodology. Our recent research on nanomaterials and cellulose‐based nanocomposites by this rapid method is summarized. In addition, the formation mechanisms involved in the microwave‐assisted ionic‐liquid synthesis of nanostructured materials are discussed briefly. Finally, the future perspectives of this methodology in the synthesis of nanostructured materials are proposed.     

微波化学的应用研究进展

对近年来发表在化工和化学及相关领域内公开出版物中微波化学的相关论文进行分析讨论,介绍了微波及其加热机理和国内外微波有机化学、无机化学及其它方向上的研究现状和进展,阐述了数值模拟计算在微波化学中的应用。引用参考文献46篇。     

Encouragements for the Use of Microwaves in Industrial Chemistry

Microwave travels at the speed of light, and transfers energy solely to materials. This holds great promise for energy conservation in industrial processes. However, due to differences with common heating principles, and misunderstanding of the correct way to handle them, the effectiveness of microwaves has been underestimated, and development of technologies using microwaves often stops due to this. This paper has focused on the use of microwave heating for organic/polymer synthesis, specifically for a highly effective condensation reaction and for use with ionic reactants. In addition to covering the process of ascertaining which reactions are suitable for the application of microwave heating, and introducing studies on scaling these up, this paper covers points of caution, especially those relating to the all‐important measurement/control of temperature. Based on their accumulation of expertise in the area, the authors present the design for equipment/plants for industrial use and introduce their research into the practical application of such technology.     

Microwaves in organic synthesis. Thermal and non-thermal microwave effects

Microwave irradiation has been successfully applied in organic chemistry. Spectacular accelerations, higher yields under milder reaction conditions and higher product purities have all been reported. Indeed, a number of authors have described success in reactions that do not occur by conventional heating and even modifications of selectivity (chemo-, regio- and stereoselectivity). The effect of microwave irradiation in organic synthesis is a combination of thermal effects, arising from the heating rate, superheating or "hot spots" and the selective absorption of radiation by polar substances. Such phenomena are not usually accessible by classical heating and the existence of non-thermal effects of highly polarizing radiation--the "specific microwave effect"--is still a controversial topic. An overview of the thermal effects and the current state of non-thermal microwave effects is presented in this critical review along with a view on how these phenomena can be effectively used in organic synthesis.     

Real‐time quantum chemistry

Significant progress in the development of efficient and fast algorithms for quantum chemical calculations has been made in the past two decades. The main focus has always been the desire to be able to treat ever larger molecules or molecular assemblies—especially linear and sublinear scaling techniques are devoted to the accomplishment of this goal. However, as many chemical reactions are rather local, they usually involve only a limited number of atoms so that models of about 200 (or even less) atoms embedded in a suitable environment are sufficient to study their mechanisms. Thus, the system size does not need to be enlarged, but remains constant for reactions of this type that can be described by less than 200 atoms. The question then arises how fast one can obtain the quantum chemical results. This question is not directly answered by linear‐scaling techniques. In fact, ideas such as haptic quantum chemistry (HQC) or interactive quantum chemistry require an immediate provision of quantum chemical information which demands the calculation of data in “real time.” In this perspective, we aim at a definition of real‐time quantum chemistry, explore its realm and eventually discuss applications in the field of HQC. For the latter, we elaborate whether a direct approach is possible by virtue of real‐time quantum chemistry. © 2012 Wiley Periodicals, Inc.     

Direct Arylation of Adenine by Fluoro- and Chloronitrobenzenes: Effect of Microwaves

Direct arylation of adenine with fluoro- and chloronitrobenzenes leads to mixtures of N9 and N7 substituted adenines. After separation by column chromatography, the individual isomers can be efficiently hydrogenated on Pd to give the corresponding aminophenyladenines. A significant enhancement of the reaction rate by microwave irradiation was observed. This two-step procedure was found to be a feasible route to otherwise hardly available 7-aminophenyladenines. Correlation between calculated shielding constants and experimental values of chemical shifts in ¹³C and ¹⁵N NMR was used for assignment of the site of substitution.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]     

Improving the performances of direct-current triboelectric nanogenerators with surface chemistry

Over the past decade, triboelectric nanogenerators (TENGs) – small and portable devices designed to harvest electricity from mechanical vibrations and friction – have matured from a niche theme of electrical engineering research into multidisciplinary research encompassing materials science, physics, and chemistry. Recent advances in both the fundamental understanding and performances of TENGs have been made possible by surface chemistry, electrochemistry, and theoretical chemistry research entering this active and promising field. This short review focuses on the recent developments of direct-current (DC) TENGs, where sliding friction or repetitive contact–separation cycles between the surface of polymers, metals, chemically modified semiconductors, and more recently even by the simple contact of surfaces with water solutions, can output DC suitable to power electronic devices without the need of additional rectification. We critically analyze the role of surface chemistry toward maximizing DC TENG outputs and device longevity. The major current hypotheses about their working mechanism(s) are also discussed.