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 The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction.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

Flash pyrolysis - synthetic and mechanistic aspects

Nitrenes derived from polymethylenetetrazoles

Alkoxyvinylketenes 4 are gener- ated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones 3. The thermal interconversion of 4 and allene carboxylic acid esters 5 under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxy- vinylketene 4 b undergoes thermal elimi- nation of ethene with formation of s-cis- and s-trans-acetylketene (8). An analog- ous aminovinylketene-to-allenecarbox- amide conversion is observed on FVT of 3-dimethylaminocyclobutenone 3 c. A facile 1,3-chlorine migration in 2J-buta- dienoyl chloride (5d) is also reported. Consistent with the experimental observa- tions, 1,3-methoxy, 1 ,3-chloro, and 1.3- dimethylamino migrations in vinylketene are calculated (G 2(MP2,SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol-l, respectively, significantly less than the corresponding 1,3-H shift barrier (273 kJ mol I). The stabilization of the four-center transition structures is rationalized in terms of the donor-accep- tor interaction between the lone pair elec- trons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migra- tory aptitude in the series of substituted vinylketenes, R-C(=CH,)-CH=C=O, is in the order N(CH,),> SCH, > SH > CI> NH, >OCH, >OH > F>H >CH,, and correlates well with the electron- donating ability of the R group.

The Vinylketene‐Acylallene Rearrangement: Theory and Experiment

N‐Ethynylamines of the Type RNHCCH

Dissociative ionization of 4,5-bis(methylthio)-1,2-dithiole-3-thione (1) has allowed the detection of a series of carbon disulfide radical cations, SCnS.+ (n = 1-3), and their S-methylated forms, CH3SCnS+, in the gas phase of a mass spectrometer. The connectivities and stabilities of these ions and the corresponding neutrals have been probed by collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Moreover, the sites of protonation and cationic methylation of the polycarbon disulfides have been investigated by a combination of flash vacuum pyrolysis, chemical ionization, and CAMS (FVP/CI/MS/MS). The experimental results are supported by ab initio MO calculations at the QCISD(T)/6-311+G(2d,p) + ZPVE level. Both experiments and calculations indicate that the most favorable site of protonation and methylation is the carbon atom for C2S2 and C3S2 but a sulfur atom for CS2. The proton affinities (298 K) of C2S2 and C3S2 are predicted to be 842 and 818 kJ mol(-1), respectively.

Curt Wentrup

Papers

Flash pyrolysis - synthetic and mechanistic aspects

Nitrenes derived from polymethylenetetrazoles

The Vinylketene‐Acylallene Rearrangement: Theory and Experiment

N‐Ethynylamines of the Type RNHCCH

Polycarbon Disulfides SCnS (n = 1-3) and Their Protonated and Methylated Forms (HCnS2+ and CH3CnS2+): Tandem Mass Spectrometry and ab Initio MO Study