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 The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction.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

Matrix studies on aromatic and heteroaromatic nitrenes and their rearrangements

Heterocyclic Rearrangements: New Cumulenes and Acetylenes

Reactions of excited molecules: VI reactions involved in pyrolysis of aromatic azides.

In the Dimroth rearrangement of heterocycles, often pyrimidines, an exocyclic and a ring substituent are interchanged. However, the term Dimroth rearrangement is frequently used even when there is no knowledge of the reaction mechanism and alternatives are likely. Here, we have employed density functional theory (DFT) calculations at the M06-2X/6-311+G(d,p) level to determine the most plausible rearrangement pathways of 3-aminothiocarbonylquinazoline 5, tetrahydrofuranylpyrimidine 21, and 5-allyltriazocine 30. For the rearrangement of quinazoline 5 to 9, the [1,3]-sigmatropic shift of the thioamido group with an activation barrier of 26.7 kcal/mol is much preferred over the Dimroth rearrangement (∼46 kcal/mol). An even lower barrier of 21.6 kcal/mol applies to a stepwise [1,3]-shift. The migration of the tetrahydrofuranyl unit in pyrimidines like 21 → 23 can take place by means of a [1,3]-sigmatropic shift with a low barrier (≤17.5 kcal/mol) rather than a Dimroth rearrangement under acidic conditions and most likely also under neutral conditions (∼30 kcal/mol). In the rearrangement of 5-allyl-6-iminotriazocine 30 to 32, the [3,3]-sigmatropic shift (aza-Cope rearrangement) is preferred over the Dimroth mechanism under neutral conditions, but in the presence of acid, the azonia-Cope rearrangement of an allyl group and the true Dimroth rearrangement have comparable activation energies.

When a "Dimroth Rearrangement" Is Not a Dimroth Rearrangement.

/pdf/the-structure-of-sodium-3-5-diacetylimino-1-2-dithiole-3lc1sh9zy2.pdf

Curt Wentrup

Papers

Matrix studies on aromatic and heteroaromatic nitrenes and their rearrangements

Heterocyclic Rearrangements: New Cumulenes and Acetylenes

Reactions of excited molecules: VI reactions involved in pyrolysis of aromatic azides.

When a "Dimroth Rearrangement" Is Not a Dimroth Rearrangement.

The Structure of Sodium 3,5-Diacetylimino-1,2-dithiole Trihydrate.