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 The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction.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

A Novel and Convenient Approach to Functionalized 2,6,9-Trioxabicyclo( 3.3.1)nona-3,7-dienes (Bridged Bis-Dioxines).

2-Pyridyliminopropadienone 6 is formed together with 2-aminopyridine on flash vacuum thermolysis (FVT) of 2-pyridylamino-pyridopyrimidinone 16a and observed by Ar matrix IR spectroscopy, but the two products recombine on warming to room temperature to regenerate the starting material. FVT of the picolinylamino-pyridopyrimidinones 16b and 16c generated mixtures of pyridyliminopropadienone 6 and picolinyliminopropadienones 19b and 19c, respectively. These reactions can be understood in terms of fragmentation of the open-chain bis(2-pyridylamino)methyleneketene intermediates 20 or the thermal interconversion of pyridopyrimidinones 16 and mesoionic pyridopyrimidinium olates 21. 2-Thiazoyliminopropadienone 28 was obtained in an analogous manner by FVT of the 2-(methylthio)thiazolopyrimidinone 24. However, the corresponding dihydro derivative 31 yielded cyanoketene 36 as the major product.

Pyrido[1,2-a]pyrimidinones and Thiazolo[3,2-a]pyrimidinones: Precursors for Pyridyl- and Thiazolyliminopropadienones, R–N=C=C=C=O

This essay details the author’s work with high-energy molecules based on sulfur or nitrogen, or both, which started with amateur rocket propellants like zinc dust and sulfur followed by experiments with the highly sensitive compounds nitrogen trichloride and fulminating gold. Research on the inorganic and organic fulminates and the isomeric cyanates led to detailed investigations of reactive intermediates generated by flash vacuum pyrolysis or photolysis, in particular nitrenes and carbenes derived from azides, diazo compounds, triazoles, and tetrazoles and characterized in low temperature matrices.

/pdf/nitrogen-and-sulfur-containing-energetic-compounds-64-years-4psfy04s9x.pdf

Nitrogen-and Sulfur-Containing Energetic Compounds. 64 Years of Fascinating Chemistry

Interconversions between alpha-oxoketenes, imidoylketenes and alpha-oxoketenimines, thioacylketenes and acylthioketenes, vinylketenes and acylallenes, isocyanates, and thioacylisocyanates and acylisothiocyanates take place by means of 1,3-shifts of substituents, which are facilitated by electron-rich migrating groups, especially those containing lone pairs on the migrating atoms. A bonding interaction between the lone pair orbital and the LUMO of the cumulene moiety stabilizes the transition state and can even make it become an intermediate. The 1,3-shifts can also be said to be pseudopericyclic reactions. The 1,3-migration of aryl groups is accelerated by electron-donating substituents in the phenyl ring. In general, for like substituents, migratory aptitudes decrease in the series alpha-oxoketene > imidoylketenes > acylallene > vinylketene.

Cumulene Rearrangements: Ketene—Ketene, Isocyanate—Isocyanate, Thioketene—Ketene, Imidoylketene—Ketenimine, and Ketene—Allene Rearrangements

Chlorination of the N,N'-di(2-pyridyl) malonamide 13a affords 2-chloro-8-methyl-4-(2-(4-picolinyl) imino-4-H-pyrido[1,2-a] pyrimidine 17a. Flash vacuum thermolysis of 17a causes efficient ring opening to the valence-tautomeric ketenimine 18a/19a, elimination of HCl, and formation of the bis(4-methyl-2-pyridyl) iminopropadiene, R-N=C=C=C=N-R 20a.

Curt Wentrup

Papers

A Novel and Convenient Approach to Functionalized 2,6,9-Trioxabicyclo( 3.3.1)nona-3,7-dienes (Bridged Bis-Dioxines).

Pyrido[1,2-a]pyrimidinones and Thiazolo[3,2-a]pyrimidinones: Precursors for Pyridyl- and Thiazolyliminopropadienones, R–N=C=C=C=O

Nitrogen-and Sulfur-Containing Energetic Compounds. 64 Years of Fascinating Chemistry

Cumulene Rearrangements: Ketene—Ketene, Isocyanate—Isocyanate, Thioketene—Ketene, Imidoylketene—Ketenimine, and Ketene—Allene Rearrangements

Bisiminopropadienes R-N=C=C=C=N-R from Pyridopyrimidines