Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200400655

Controlled microwave heating in modern organic synthesis.

Microwave assisted organic synthesis-a review

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

Starting from the early 1990’s, the chemistry of polyvalent iodine organic compounds has experienced an explosive development. This surging interest in iodine compounds is mainly due to the very useful oxidizing properties of polyvalent organic iodine reagents, combined with their benign environmental character and commercial availability. Iodine(III) and iodine(V) derivatives are now routinely used in organic synthesis as reagents for various selective oxidative transformations of complex organic molecules. Several areas of hypervalent organoiodine chemistry have recently attracted especially active interest and research activity. These areas, in particular, include the synthetic applications of 2-iodoxybenzoic acid (IBX) and similar oxidizing reagents based on the iodine(V) derivatives, the development and synthetic use of polymer-supported and recyclable polyvalent iodine reagents, the catalytic applications of organoiodine compounds, and structural studies of complexes and supramolecular assemblies of polyvalent iodine compounds.

The chemistry of polyvalent iodine has previously been covered in four books1–4 and several comprehensive review papers.5–17 Numerous reviews on specific classes of polyvalent iodine compounds and their synthetic applications have recently been published.18–61 Most notable are the specialized reviews on [hydroxy(tosyloxy)iodo]benzene,41 the chemistry and synthetic applications of iodonium salts,29,36,38,42,43,46,47,54,55 the chemistry of iodonium ylides,56–58 the chemistry of iminoiodanes,28 hypervalent iodine fluorides,27 electrophilic perfluoroalkylations,44 perfluoroorgano hypervalent iodine compounds,61 the chemistry of benziodoxoles,24,45 polymer-supported hypervalent iodine reagents,30 hypervalent iodine-mediated ring contraction reactions,21 application of hypervalent iodine in the synthesis of heterocycles,25,40 application of hypervalent iodine in the oxidation of phenolic compounds,32,34,50–53,60 oxidation of carbonyl compounds with organohypervalent iodine reagents,37 application of hypervalent iodine in (hetero)biaryl coupling reactions,31 phosphorolytic reactivity of o-iodosylcarboxylates,33 coordination of hypervalent iodine,19 transition metal catalyzed reactions of hypervalent iodine compounds,18 radical reactions of hypervalent iodine,35,39 stereoselective reactions of hypervalent iodine electrophiles,48 catalytic applications of organoiodine compounds,20,49 and synthetic applications of pentavalent iodine reagents.22,23,26,59

The main purpose of the present review is to summarize the data that appeared in the literature following publication of our previous reviews in 1996 and 2002. In addition, a brief introductory discussion of the most important earlier works is provided in each section. The review is organized according to the classes of organic polyvalent iodine compounds with emphasis on their synthetic application. Literature coverage is through July 2008.

Chemistry of Polyvalent Iodine

Microwave assisted organic reactions

The preparation and infrared and n.m.r. spectra of monodeuterated pyridine and 3-picoline

Thermolysis of 1-(N-acetyl-N-aryl)amino-2,4,6-triphenylpyridinium tetrafluoroborates provides a new and synthetically useful source of arylnitrenium ions under non-acidic conditions; a p-semidine-type byproduct is obtained from the phenylamino compound.

Thermolysis of 1-(N-acetyl-N-aryl)amino-2,4,6-triphenylpyridinium tetrafluoroborates: a new source of arylnitrenium ions

The reactions of pyridine and 3-picoline with cyclohexanone and 2-methylcyclohexanone, and of pyridine with 4-t-butylcyclohexanone, in the presence of magnesium amalgam have been studied quantitatively. In the latter case the conformations of the products have been established. The cis--trans-isomer ratio was 54·2:45·8. With 3-picoline, attack took place predominantly at C-6. The results are thought to fit a 1,3-dipolar cycloaddition or a co-ordination of R2[graphic omitted]·O[graphic omitted] at nitrogen followed by nucleophilic cyclisation mechanism.

Aromatic substitution. Part XXIV. The Emmert reaction: effects of substituents on orientation

The reaction of quinoline-1-oxides with -phenylbenzimidoyl chloride

A convenient synthesis of 1,2-benzisothiazole 1,1-dioxide from toluene-o-sulphonamide is reported, and this allows the preparation of a number of 1,2-benzothiazepine 1,1-dioxides bearing hydrogen atoms in the seven-membered ring; 1H n.m.r. spectroscopy suggests that the later is not aromatic.

Rudolph A. Abramovitch

Papers

The preparation and infrared and n.m.r. spectra of monodeuterated pyridine and 3-picoline

Thermolysis of 1-(N-acetyl-N-aryl)amino-2,4,6-triphenylpyridinium tetrafluoroborates: a new source of arylnitrenium ions

Aromatic substitution. Part XXIV. The Emmert reaction: effects of substituents on orientation

The reaction of quinoline-1-oxides with -phenylbenzimidoyl chloride

1,2-Benzisothiazole 1,1-dioxide: a convenient synthesis. The question of the possible aromaticity of 1,2-benzothiazepine 1,1-dioxides