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 The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction.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.

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

Asymmetric 1,3-Dipolar Cycloaddition Reactions

The Diels-Alder reaction has both enabled and shaped the art and science of total synthesis over the last few decades to an extent which, arguably, has yet to be eclipsed by any other transformation in the current synthetic repertoire. With myriad applications of this magnificent pericyclic reaction, often as a crucial element in elegant and programmed cascade sequences facilitating complex molecule construction, the Diels-Alder cycloaddition has afforded numerous and unparalleled solutions to a diverse range of synthetic puzzles provided by nature in the form of natural products. In celebration of the 100th anniversary of Alder's birth, selected examples of the awesome power of the reaction he helped to discover are discussed in this review in the context of total synthesis to illustrate its overall versatility and underscore its vast potential which has yet to be fully realized.

https://www.ch.ic.ac.uk/local/organic/pericyclic/DA.pdf

The Diels--Alder reaction in total synthesis.

Introduction Cationic domino reactions Anionic domino reactions Radical domino reactions Pericyclic domino reactions Photochemically induced domino processes Transition metal catalysis Domino reactions initiated by oxidation or reduction Enzymes in domino reactions Multicomponent reactions Special techniques in domino reactions

Domino Reactions in Organic Synthesis

Methyleneketenes 2 and imidoylketenes 4, generated from Meldrum's acid derivatives 1 and 2,3-dihydropyrrole-2,3-diones 3, respectively, both rearrange thermally to oxoketenimines 5; the imidoylketene–oxoketenimine rearrangement is reversible and much accelerated by electropositive migrating groups (SMe).

Methyleneketene–imidoylketene–oxoketenimine rearrangements

4-Alkylideneisoxazol-5-Ones - Synthesis, Tautomerism, and Rearrangement to Pyrroles

Pyridylnitrenes. Mechanism of ring contraction to cyanopyrroles

The history of pyrolysis equipment, methods, and reactions is narrated in the Introduction. Detailed descriptions of flash vacuum pyrolysis (FVP) (or thermolysis, FVT) apparatus for preparative and spectroscopic (UV, IR, electron spin resonance) purposes with product isolation at 77 K or in Ar matrices at ~10 K are presented. Very low pressure pyrolysis (VLPP), laser pyrolysis, and pulsed pyrolysis (jet flash pyrolysis) are also described together with illustrations of apparatus. The solvent spray flash vacuum pyrolysis (SS-FVP) of liquids or solutions of compounds of low volatility is described together with methods for the addition of solids to a pyrolysis tube, in particular details of pipto-pyrolysis (‘falling solid pyrolysis’). Methods used for catalytic vacuum gas–solid reactions (VGSR) are also summarised.

Flash (Vacuum) Pyrolysis Apparatus and Methods

Several aromatic and heteroaromatic nitrenes and carbenes undergo photochemical and sometimes also thermal ring opening. Depending on benz-annelation, the ring-opened species may have the character of either nitrenes (for α-annelation) or 1,5-(1,7-)-biradicals (for β-annelation). Both types have been observed, and they are clearly distinguished by their characteristic electron spin resonance spectra. In addition, ring opening of hetarylnitrenes to nitrile ylides can be observed whenever there is a meta-relationship between a ring nitrogen atom and the nitrene (or carbene) centre. The factors governing the two types of ring opening have been investigated. The nitrenes and carbenes are generated by either low temperature Ar matrix photolysis or flash vacuum thermolysis of azides, tetrazoles, triazoles, or diazo compounds with matrix isolation of the products.

Curt Wentrup

Papers

Methyleneketene–imidoylketene–oxoketenimine rearrangements

4-Alkylideneisoxazol-5-Ones - Synthesis, Tautomerism, and Rearrangement to Pyrroles

Pyridylnitrenes. Mechanism of ring contraction to cyanopyrroles

Flash (Vacuum) Pyrolysis Apparatus and Methods

1,5-(1,7)-Biradicals and Nitrenes Formed by Ring Opening of Hetarylnitrenes