Current Organic Chemistry - Volume 15, Issue 2, 2011

In order to perform a chemical reaction, energy transfer is needed in almost all cases. For many years the main source of heat for “chemical” reactions was fire and from 1855 the Bunsen burner, which provides flame by mixing air and gas, was predominantly used. Later these burners were largely replaced by electrical hotplates. Ultrasound and microwaves are considered as non-conventional energy sources when used to perform chemical reactions and they are regarded as having very interesting results compared to the conventional heating sources. R. Wood (1868-1955) and A. Loomis (1887-1975) in 1927 studied the motion of sonic waves through liquids and therefore inspired the scientists to utilize ultrasound in chemical reactions. Sonochemistry is mainly based on cavitation, i.e., the formation, growth and implosive collapse of bubbles in a liquid. Temperature rises of up to 5000 K and pressures can reach 1000 atm. Under these extreme conditions chemical reactions can be initiated. Since 1981, when a patent by B. Naresh (BASF) was filed dealing with the use of microwaves for the production of plasticizer esters, there has been a continuously increasing interest in the use of this source of energy. The principle of performing chemical reactions under microwave irradiation is based on the fact that only polar molecules or conducting ions can be heated. In this way, higher yields are obtained, reaction times are decreased and better selectivities are observed when compared to classical heating. In addition, microwaves allow reactions to occur that would not otherwise be possible. Today, the use of microwaves in chemical and pharmaceutical laboratories all over the world has increased dramatically. In this special issue of Current Organic Chemistry entitled “Organic Chemistry with Microwaves and Ultrasound” we present a number of reviews authored by eminent scientists which relate to the use of microwave and ultrasound in organic reactions. Thus: a) Christine Schmoger, Achim Stolle, Werner Bonrath and Bernd Ondruschka cover the topic of microwave-assisted reduction of organic compounds including catalytic and stoichiometric reactions. Special attention has been given with respect to the different functionalities: C-C multiple bonds, carbonyls, N-containing functional groups, aromatics and heteroaromatics, reductive deprotection strategies, and miscellaneous reactions such as dehalogenation or hydrogen-deuterium exchange....

The present review covers the topic of microwave-assisted reduction of organic compounds including catalytic (metalcatalyzed hydrogenation, catalytic transfer hydrogenation) and stochiometric reactions (ionic and transfer hydrogenation). The article is subcategorized with respect to the different functionalities which so far have been subjected to reduction: C-C multiple bonds, carbonyls, N-containing functional groups, aromatics and heteroaromatics, reductive deprotection strategies, and miscellaneous reactions like dehalogenation or hydrogen-deuterium exchange.

The destruction of toxic organic molecules using advanced oxidation processes (AOPs) is a potent tool for pollution control and environmental protection. Ultrasound is a convenient and effective method of generating hydroxyl radicals which is the key oxidant in AOPs. This review describes the use of ultrasound and associated chemical reactions, with and without additives, as a powerful means of remediating water contaminated with organic pollutants. After a brief introduction to ultrasound and sonochemistry, their application for the oxidation of polycyclic aromatic hydrocarbons, phenol and substituted phenols is considered. Next is the decomposition of chlorinated phenols, and other chlorinated organics, then removal of recalcitrant smaller organic molecules. A discussion follows of recent work that has investigated the effects of initial concentration of substrates; the use of different ultrasonic frequencies; the inclusion of oxidising species, inorganic particles, or salts and their contribution to enhanced degradation. Finally, brief comments are made on the status of ultrasound as an AOP treatment.

This review focuses on the synthesis of polymer-inorganic hybrid nanocoposites under microwave irradiation, a research area that has made rapid progress in the recent years. Nowadays, one of the cornerstones of the push towards the future improvements in electronics and optics technology together with the research and development is the decrease in size of the various components used for the device manufacture. The paper discusses the preparation and characterization of polymer nanocomposites composed of layer materials like clays and layered double hydroxides, metal nanoparticles and nanowires as well as carbon based materials (i.e., fullerenes and nanotubes) under microwave conditions. The polymer nanocomposites are grouped accordingly to a such classification.

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) is generally recognized as the most striking example of “click reaction”. CuAAC fit so well into Sharpless' definition that it became almost synonymous with “click chemistry” itself. The most common catalyst systems employ Cu(II) salt in the presence of a reducing agent (i.e. sodium ascorbate) to generate the required Cu(I) catalyst in situ or as an alternative the comproportionation of Cu(II)/Cu(0) species. Although, Cu(I) catalyzes the reaction with a rate enhancement of ∼107 even in the absence of ligands and provides a clean and selective conversion to the 1,4-substituted triazoles, some bulky and scantily reactive substrates still require long reaction times and often few side products are formed. Outstanding results have been achieved by performing CuAAC under microwave (MW) irradiation. Several authors described excellent yields, high purity and short reaction times. In few cases also power ultrasound (US) accelerated the reaction, especially when heterogeneous catalysts or metallic copper are employed. The aim of this review is to summarize and highlight the huge advantages offered by MW- and US-promoted CuAAC. In the growing scenario of innovative synthetic strategies, we intend to emphasize the complementary role played by these non-conventional energy sources and click chemistry to achieve process intensification in organic synthesis.

Multicomponent reactions are valuable tools for the generation of diverse heterocycles. As in many fields or organic chemistry, microwave irradiation is rapidly replacing conventional heating methods in multicomponent chemistry. In this review, we present an overview of recent applications of the use of microwaves in multicomponent heterocycle synthesis. Where possible, the yield and chemo-, regio- and stereoselectivity of microwave-assisted reactions are compared with those using conventional heating.

Over the centuries mankind has benefited from the natural materials that occur in plants. In earlier times the whole plant or an extract was used in cooking or as a medicine but nowadays the active constituents of plant extracts provide targets for the synthetic chemist. In this review we will explore the advantages that accrue from the incorporation of either ultrasound or microwaves in the extraction process. The two techniques offer different approaches in that ultrasound is generally used to improve conventional solvent extraction whereas microwaves are known for their ability to remove constituents via heating without solvents.

The coupled activation of photochemical and photocatalytic reactions by using of two different types of radiation, microwave and UV/Vis, is covered by the new discipline called microwave photochemistry and photocatalysis. Such a connection might have a synergic effect on reaction efficiencies or, at least, enhance them by summing up the individual effects. The objective of this discipline is frequently, but not necessarily, connected to the electrodeless discharge lamp (EDL) as a novel light source which generates efficiently UV/Vis radiation when placed into a microwave field. This review article is focused on the general principles of microwave photochemistry and photocatalysis, i.e. generation of UV/Vis discharge in EDL (theory of the microwave discharges, construction of EDL, preparation of the thin titania films on EDL, spectral characteristics of EDL, and performance of EDL). Likewise, the various microwave photochemical and photocatalytic reactor types (batch with external or internal light source, flow-through with external light source, annular flow-through with internal EDL, and cylindrical flow-through surrounded with EDL) with different arrangement of the lamps are described. The concept of microwave photochemistry and photocatalysis as an important issue in synthetic chemistry and material science is presented in several tables.

The application of microwave irradiation for the synthesis and decoration of the 2(1H)-pyrazinone scaffold as well as for the synthesis of various heterocycles starting from this core structure has been reviewed.

An innovative approach in the field of nanomaterials is to develop modern and mild synthetic protocols that enable controlled and integrated organization of specific functional organic and biological building blocks. In this context, research on the preparation of clay-based organic-inorganic hybrid materials, i.e. organoclays, has received considerable attention because these lamellar materials not only possess highly ordered structure in 2D, but also provide interesting chemical intercalation and surface properties. As a result entrapment and orientation of various functional guest molecules into layered inorganic solids, has been well exploited. At present there is a great deal of interest in the rational design of hybrid organic-/bio-inorganic composites on the nanometer to micrometer length scales. These hybrids encompass highly selective recognition properties associated with organic and biological species, combined with catalytic, optical, and electronic properties of the inorganic lamellar framework. This paper presents an overview of microwave application in different preparation steps of organophilic clays, their modification and their addition to polymeric matrices. Particular attention has been given to the microwave-mediated hydrothermal technique, as innovative and eco-friendly protocol for the intercalation of different kinds of organic hosts into the interlayer of clay minerals. The few pioneering complete microwave-mediated preparation procedures, exploiting advantages of dielectric heating in both the synthesis of the organic host molecule and the intercalation in the clay structure, have been also highlighted.