In the course of nearly every program of research in organic chemistry, the investigator finds it necessary to use several of the better‐known synthetic reactions. To discover the optimum conditions for the application of even the most familiar one to a compound not previously subjected to the reaction often requires an extensive search of the literature; even then a series of experiments may be necessary. When the results of the investigation are published, the synthesis, which may have required months of work, is usually described without comment. The background of knowledge and experience gained in the literature search and experimentation is thus lost to those who subsequently have occasion to apply the general method. The student of preparative organic chemistry faces similar difficulties. The textbooks and laboratory manuals furnish numerous examples of the application of various syntheses, but only rarely do they convey an accurate conception of the scope and usefulness of the processes.

For many years American organic chemists have discussed these problems. The plan of compiling critical discussions of the more important reactions thus was evolved. The volumes of Organic Reactions are collections of chapters each devoted to a single reaction, or a definite phase of a reaction, of wide applicability. The authors have had experience with the processes surveyed. The subjects are presented from the preparative viewpoint, and particular attention is given to limitations, interfering influences, effects of structure, and the selection of experimental techniques. Each chapter includes several detailed procedures illustrating the significant modifications of the method. Most of these procedures have been found satisfactory by the author or one of the editors, but unlike those in Organic Syntheses, they have not been subjected to careful testing in two or more laboratories. Each chapter contains tables that include all the examples of the reaction under consideration that the author has been able to find. It is inevitable, however, that in the search of the literature some examples will be missed, especially when the reaction is used as one step in an extended synthesis. Nevertheless, the investigator will be able to use the tables and their accompanying bibliographies in place of most or all of the literature search so often required. Because of the systematic arrangement of the material in the chapters and the entries in the tables, users of the books will be able to find information desired by reference to the table of contents of the appropriate chapter. In the interest of economy, the entries in the indices have been kept to a minimum, and, in particular, the compounds listed in the tables are not repeated in the indices.

The success of this publication, which will appear periodically, depends upon the cooperation of organic chemists and their willingness to devote time and effort to the preparation of the chapters. They have manifested their interest already by the almost unanimous acceptance of invitations to contribute to the work. The editors will welcome their continued interest and their suggestions for improvements in Organic Reactions.

In the intervening years since “The Chief” wrote this introduction to the second of his publishing creations, much in the world of chemistry has changed. In particular, the last decade has witnessed a revolution in the generation, dissemination, and availability of the chemical literature with the advent of electronic publication and abstracting services. Although the exponential growth in the chemical literature was one of the motivations for the creation of Organic Reactions, Adams could never have anticipated the impact of electronic access to the literature. Yet, as often happens with visionary advances, the value of this critical resource is now even greater than at its inception.

From 1942 to the 1980's the challenge that Organic Reactions successfully addressed was the difficulty in compiling an authoritative summary of a preparatively useful organic reaction from the primary literature. Practitioners interested in executing such a reaction (or simply learning about the features, advantages, and limitations of this process) would have a valuable resource to guide their experimentation. As abstracting services, in particular Chemical Abstracts and later Beilstein, entered the electronic age, the challenge for the practitioner was no longer to locate all of the literature on the subject. However, Organic Reactions chapters are much more than a surfeit of primary references; they constitute a distillation of this avalanche of information into the knowledge needed to correctly implement a reaction. It is in this capacity, namely to provide focused, scholarly, and comprehensive overviews of a given transformation, that Organic Reactions takes on even greater significance for the practice of chemical experimentation in the 21^st century.

Adams' description of the content of the intended chapters is still remarkably relevant today. The development of new chemical reactions over the past decades has greatly accelerated and has embraced more sophisticated reagents derived from elements representing all reaches of the Periodic Table. Accordingly, the successful implementation of these transformations requires more stringent adherence to important experimental details and conditions. The suitability of a given reaction for an unknown application is best judged from the informed vantage point provided by precedent and guidelines offered by a knowledgeable author.

As Adams clearly understood, the ultimate success of the enterprise depends on the willingness of organic chemists to devote their time and efforts to the preparation of chapters. The fact that, at the dawn of the 21^st century, the series continues to thrive is fitting testimony to those chemists whose contributions serve as the foundation of this edifice. Chemists who are considering the preparation of a manuscript for submission to Organic Reactions are urged to contact the Editor‐in‐Chief.

Something old, something new, Something borrowed, something blue, and a sixpence in her shoe.

Anon (1871)

St James' Magazine, London

The two chapters in this volume of Organic Reactions epitomize the sentiment of the traditional old English rhyme, in that the combination of something old with something new often provides a different perspective, and thus, important new possibilities. The first two lines of the verse were originally published in a magazine article on Marriage Superstitions in 1871. The something old is meant to ward off evil spirits, whereas something new offers optimism for the future. The something borrowed is thought to bring good luck, and the something blue represents purity and fidelity. The final line, and a sixpence in her shoe, was introduced during the Victorian era to represent prosperity. Although there were originally minor regional variations, this version has now been endorsed internationally, which exemplifies the impact of a simple piece of prose. The development of a synthetic transformation has many of the attributes delineated within this simple rhyme, wherein the combination of something old with something new can significantly extend the reaction scope and synthetic utility. The something borrowed and something blue could represent the serendipitous discoveries that often result in a very fertile area of investigation that advances a process in the context of efficiency and selectivity. In some cases, these findings can lead to commercial applications that can be lucrative to the inventor, which ties into the rhyme's last line. The two chapters in this volume focus on transformations connected by the preparation and reactions of alkenes. Notably, the cross‐metathesis reaction involves the catalytic construction of acyclic alkenes, whereas the Stetter reaction entails the catalytic enantioselective conjugate addition of aldehydes to alkenes.

The first chapter by Karol Grela, Anna Kajetanowicz, Anna Szadkowska, and Justyna Czaban‐Jóźwiak provides an extensive review on alkene cross‐metathesis, which completes the trilogy of chapters dealing with metathesis reactions. The two earlier chapters were on olefin ring‐closing metathesis by Larry Yet (Volume 89) and alkyne metathesis by Daesung Lee, Ivan Volchkov, and Sang Young Yun (Volume 102). The term “olefin metathesis” was coined by Calderon in 1967, wherein the word metathesis means “transposition” and is the combination of two Greek words–change (meta) and position (thesis). The reaction was serendipitously discovered in the 1950s and is often referred to as the “child of industry” because of the strong industrial connection. Chemists at Du Pont, Standard Oil, and Phillips Petroleum independently reported the metathesis of propene and other variants. In the late 1980s and early 1990s, Grubbs and Schrock, who received the 2005 Nobel Prize in Chemistry with Yves Chauvin for their work on olefin metathesis, developed well‐behaved ruthenium, molybdenum and tungsten catalysts that were compatible with functionalized alkenes required for fine chemical synthesis and the field exploded exponentially. This chapter delineates the historical development of the cross‐metathesis reaction from an academic/industrial curiosity to a sophisticated modern transformation for the catalytic construction of acyclic alkenes in a highly selective manner. The Mechanism and Stereochemistry section provides an insightful account of reaction initiation, catalyst regeneration, and stereocontrol. For example, the difference between the three initiation processes, the so‐called “boomerang” mechanism, and the various aspects of diastereo‐ and enantioselective cross‐metathesis reactions are discussed. Importantly, the latter part outlines the catalyst requirements for controlling (E)‐ and (Z)‐geometry, the desymmetrization of prochiral meso‐compounds through asymmetric ring‐opening cross‐metathesis (AROCM), and asymmetric cross‐metathesis (ACM). The Scope and Limitations component delineates the classification of alkenes (Types I‐IV according to Grubbs), the consequences of their ability to undergo cross‐metathesis, and the impact of the classification on the mechanism in terms of homodimerization. This section is organized by various alkene substituents, including boron, nitrogen, oxygen, halogens, silicon, tin, phosphorus, sulfur, etc. There is also a substantial section on (Z)‐selective processes that have proven particularly challenging, and, as such, makes the addition of this section very timely. The Applications to Synthesis section describes several impressive natural product syntheses that use this reaction, and the Comparison with Other Methods section provides a comprehensive assessment of more classical methods that are commonly deployed to construct alkenes. The Tabular Survey incorporates reactions reported up to March 2020. The tables are organized by the type of olefin metathesis (according to Grubbs) and then further subdivided by functional groups, which makes identifying a particular combination effortless. Overall, this is a superb chapter on a fundamentally important process that will be an invaluable resource to anyone wishing to construct an olefin using a cross‐metathesis reaction.

The second chapter by Darrin M. Flanigan, Kerem E. Ozboya, Alberto Muñoz, Tomislav Rovis, Subhash D. Tanpure, and Paul R. Blakemore chronicles the development of the catalytic enantioselective Stetter reaction, which represents an update to the original chapter by Stetter and Kuhlmann in Volume 40. This process is closely related to the venerable acyloin condensation, which is catalyzed by cyanide and vitamin B₁ (thiamine) in Nature. While plants tend to employ cyanide derived from cyanoglucosides, other living organisms use non‐toxic thiamine and the reaction proceeds via the so‐called “Breslow Intermediate” from the addition of the deprotonated quaternary thiazolium salt to pyruvate. The recognition that an aldehyde's natural electrophilicity can be reversed to afford the requisite acyl anion equivalent prompted Hermann Stetter to examine the cyanide‐catalyzed addition of aromatic aldehydes to α,β‐unsaturated esters, ketones, and nitriles in the early 1970s. He later demonstrated that thiazolylidenes also catalyze this process to permit the coupling of two formally electrophilic species for construction of 1,4‐dicarbonyls that constitute useful intermediates for target‐directed synthesis. Although the catalyst requirement mitigates any background reaction, the first enantioselective process was not described until the mid‐1990s by Dieter Enders using a chiral triazolium catalyst. Consequently, this important and seminal report set the wheels in motion for others to develop more selective and versatile catalysts that significantly broaden the substrate scope. This chapter catalogs the development of the enantioselective variant that has emerged as a powerful synthetic tool for target‐directed synthesis. The Mechanism and Stereochemistry section outlines the various methods for generating the NHC catalysts from the corresponding salts and the subsequent formation and reactivity of Breslow intermediates, including associated mechanistic experiments, namely, deuterium isotope and competition studies. The section on stereochemistry presents a series of models that rationalize the origin of stereocontrol, including computational studies on the preferred enaminol geometry and the transition state for the key C‐C bond‐forming event. The Scope and Limitations component documents the preparation of triazolium salts, and the remainder of the discussion is organized by the different intra‐ and intermolecular variants. The authors subdivide this section by the type of aldehyde donor (aryl, aliphatic, α,β‐unsaturated, etc.). A section on the recent aza‐Stetter reaction with imine donors may also be of interest to the reader. The Applications to Synthesis section provides some unique applications to the synthesis of natural products, and the Comparison with Other Methods section provides a detailed comparison with several alternative methods. The organization of the Tabular Survey mirrors the Scope and Limitations section, thereby making it easy for the reader to identify a specific transformation. Overall, this is an outstanding chapter on a particularly important and useful process that will be a valuable resource to the synthetic community.

I would be remiss if I did not acknowledge the entire Organic Reactions Editorial Board for their collective efforts in steering this volume through the various stages of the editorial process. I want to thank Steven M. Weinreb (Chapter 1 and 2), the Responsible Editor for the two chapters, albeit I had some early input into Chapter 2 through the early phases of development. I am also deeply indebted to Dr. Danielle Soenen for her heroic efforts as the Editorial Coordinator; her knowledge of Organic Reactions is critical to maintaining consistency in the series. Dr. Dena Lindsay (Secretary to the Editorial Board) is thanked for coordinating the author's, editor's, and publisher's contributions. In addition, the Organic Reactions enterprise could not maintain the quality of production without the efforts of Dr. Steven M. Weinreb (Executive Editor), Dr. Engelbert Ciganek (Editorial Advisor), Dr. Landy Blasdel (Processing Editor), and Dr. Debra Dolliver (Processing Editor). I would also like to acknowledge Dr. Barry R. Snider (Secretary) for keeping everyone on task and Dr. Jeffery Press (Treasurer) for making sure that we are fiscally solvent!

I am also indebted to past and present members of the Board of Editors and Directors for ensuring the enduring quality of Organic Reactions. The unique format of the chapters, in conjunction with the collated tables of examples, make this series of reviews both unique and exceptionally valuable to the practicing synthetic organic chemist.