Organic Reactions, Volume 108 E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Organic Reactions
- Sprache: Englisch
The 108th volume in this series for organic chemists in academia and industry presents critical discussions of the following widely used organic reactions: CYCLIZATION REACTIONS OF NITROGEN-CENTERED RADICALS Stuart W. McCombie, Beatrice Quiclet-Sire, and Samir Z. Zard TRANSITION-METAL-CATALYZED AMINOOXYGENATION OF ALKENES Sherry R. Chemler, Dake Chen, Shuklendu D. Karyakarte, Jonathan M. Shikora, and Tomasz Wdowik
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Veröffentlichungsjahr: 2021
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Table of Contents
Cover
Title Page
Copyright
INTRODUCTION TO THE SERIES BY ROGER ADAMS, 1942
INTRODUCTION TO THE SERIES BY SCOTT E. DENMARK, 2008
PREFACE TO VOLUME 108
CHAPTER 1: CYCLIZATION REACTIONS OF NITROGEN‐CENTERED RADICALS
ACKNOWLEDGEMENTS
INTRODUCTION
MECHANISM AND STEREOCHEMISTRY
SCOPE AND LIMITATIONS
APPLICATIONS TO SYNTHESIS
COMPARISON WITH OTHER METHODS
EXPERIMENTAL CONDITIONS
EXPERIMENTAL PROCEDURES
Tabular Survey
References
Supplemental References
CHAPTER 2: TRANSITION‐METAL‐CATALYZED AMINOOXYGENATION OF ALKENES
DEDICATION
ACKNOWLEDGMENTS
INTRODUCTION
MECHANISM, REGIOCHEMISTRY, AND STEREOCHEMISTRY
SCOPE AND LIMITATIONS
APPLICATIONS TO SYNTHESIS
COMPARISON WITH OTHER METHODS
EXPERIMENTAL CONDITIONS
EXPERIMENTAL PROCEDURES
TABULAR SURVEY
REFERENCES
CUMULATIVE CHAPTER TITLES BY VOLUME
AUTHOR INDEX, VOLUMES 1–108
CHAPTER AND TOPIC INDEX, VOLUMES 1–108
End User License Agreement
List of Illustrations
Chapter 1
Scheme 1
Figure 1 Types of nitrogen-centered radicals.
Scheme 2
Scheme 3
Figure 2 Hydrogen‐atom abstraction rate constants (
k
T
) and cyclization rate ...
Scheme 4
Scheme 5
Scheme 6
Figure 3 Calculated activation energies for 5‐
exo
and 6‐
endo
cyclizations of...
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Scheme 54
Scheme 55
Figure 4. Derivatives of 1‐amino‐2,5‐cyclohexadiene and
N
‐amino‐1,4‐dihydrop...
Scheme 56
Scheme 57
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Chapter 2
Figure 1 Bioactive compounds that contain vicinal amino alcohols.
Scheme 1
Figure 2 Chiral ligands for osmium‐catalyzed aminohydroxylation.
Scheme 2
Scheme 3
Scheme 4
Scheme 5
Scheme 6
Scheme 8
Scheme 9
Scheme 10
Scheme 11
Scheme 12
Scheme 13
Scheme 14
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Scheme 16
Scheme 17
Figure 3 Proposed origin of selectivity for the PHAL‐ and AQN‐linked chiral ...
Figure 4 The Sharpless model for absolute configuration.
Scheme 18
Scheme 19
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Scheme 192
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Scheme 194
Scheme 195
Scheme 196
Scheme 197
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Scheme 199
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Scheme 204
Scheme 205
Scheme 206
Scheme 207
Scheme 208
Scheme 209
Scheme 210
Scheme 211
Scheme 212
Scheme 213
Scheme 214
Scheme 215
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Scheme 217
Scheme 218
Scheme 219
Scheme 220
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Scheme 222
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Scheme 224
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Guide
Cover Page
Table of Contents
Title Page
Copyright
Begin Reading
AUTHOR INDEX, VOLUMES 1–108
CHAPTER AND TOPIC INDEX, VOLUMES 1–108
WILEY END USER LICENSE AGREEMENT
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FORMER MEMBERS OF THE BOARDOF EDITORS AND DIRECTORS
JEFFREY
AUBÉ
ROBERT
C.
KELLY
JOHN
E.
BALDWIN
LAURA
KIESSLING
PETER
BEAK
MARISA
C.
KOZLOWSKI
DALE
L.
BOGER
STEVEN
V.
LEY
JIN
K.
CHA
JAMES
A.
MARSHALL
ANDRÉ
B.
CHARETTE
MICHAEL
J.
MARTINELLI
ENGELBERT
CIGANEK
STUART
W.
MC
COMBIE
DENNIS
CURRAN
SCOTT
J.
MILLER
SAMUEL
DANISHEFSKY
JOHN
MONTGOMERY
HUW
M. L.
DAVIES
LARRY
E.
OVERMAN
SCOTT
E.
DENMARK
T. V.
RAJANBABU
VICTOR
FARINA
JAMES
H.
RIGBY
PAUL
FELDMAN
WILLIAM
R.
ROUSH
JOHN
FRIED
TOMISLAV
ROVIS
JACQUELYN
GERVAY
‐
HAGUE
SCOTT
D.
RYCHNOVSKY
STEPHEN
HANESSIAN
MARTIN
SEMMELHACK
LOUIS
HEGEDUS
CHARLES
SIH
PAUL
J.
HERGENROTHER
AMOS
B.
SMITH
, III
DONNA
M.
HURYN
BARRY
M.
TROST
JEFFREY
S.
JOHNSON
PETER
WIPF
FORMER MEMBERS OF THE BOARDNOW DECEASED
ROGER
ADAMS
HERBERT
O.
HOUSE
HOMER
ADKINS
JOHN
R.
JOHNSON
WERNER
E.
BACHMANN
ROBERT
M.
JOYCE
ROBERT
BITTMAN
ANDREW
S.
KENDE
A. H.
BLATT
WILLY
LEIMGRUBER
VIRGIL
BOEKELHEIDE
FRANK
C.
MC
GREW
GEORGE
A.
BOSWELL
,
JR
.
BLAINE
C.
MC
KUSICK
THEODORE
L.
CAIRNS
JERROLD
MEINWALD
ARTHUR
C.
COPE
CARL
NIEMANN
DONALD
J.
CRAM
LEO
A.
PAQUETTE
DAVID
Y.
CURTIN
GARY
H.
POSNER
WILLIAM
G.
DAUBEN
HANS
J.
REICH
LOUIS
F.
FIESER
HAROLD
R.
SNYDER
HEINZ
W.
GSCHWEND
MILÁN
USKOKOVIC
RICHARD
F.
HECK
BORIS
WEINSTEIN
RALPH
F.
HIRSCHMANN
JAMES
D.
WHITE
Organic Reactions
Volume 108
EDITORIAL BOARD P.
ANDREWEVANS, Editor‐in‐Chief
STEVEN M. WEINREB, Executive Editor
DAVID
B.
BERKOWITZ
STEFAN
LUTZ
PAUL
R.
BLAKEMORE
ALBERT
PADWA
REBECCA
L.
GRANGE
JENNIFER
M.
SCHOMAKER
DENNIS
G.
HALL
KEVIN
H.
SHAUGHNESSY
JEFFREY
B.
JOHNSON
CHRISTOPHER
D.
VANDERWAL
JEFFREY
N.
JOHNSTON
MARY
P.
WATSON
MARISA
C.
KOZLOWSKI
BARRY B. SNIDER, Secretary
JEFFERY B. PRESS, Treasurer
DANIELLESOENEN, Editorial Coordinator
DENALINDSAY, Secretary and Processing Editor
LANDY K. BLASDEL, Processing Editor
TINAGRANT, Processing Editor
ENGELBERTCIGANEK, Editorial Advisor
ASSOCIATE EDITORS
SHERRY
R.
CHEMLER
DAKE
CHEN
SHUKLENDU
D.
KARYAKARTE
STUART
W.
MCCOMBIE
BÉATRICE
QUICLET-SIRE
JONATHAN
M.
SHIKORA
TOMASZ
WDOWIK
SAMIR
Z.
ZARD
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Library of Congress Cataloging‐in‐Publication Data: ISBN: 978‐1‐119‐83207‐2
INTRODUCTION TO THE SERIES BY ROGER ADAMS, 1942
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.
INTRODUCTION TO THE SERIES BY SCOTT E. DENMARK, 2008
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 21st 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 21st 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.
PREFACE TO VOLUME 108
Don't be trapped by dogma — which is living with the results of other people's thinking.
Steve Jobs, 2005
Stanford University Commencement Speech
Dogma is omnipresent in nearly all aspects of everyday life and can either be an obstacle or an inspiration to free thinking. Apple's Steve Jobs, arguably one of the most innovative minds of our time, laid down the gauntlet with the notion of questioning other people's thinking to avoid being trapped in a specific mindset. Indeed, the freedom to challenge mainstream thinking is a significant justification for the academic tenure system. Despite the ability of chemistry faculty to express different points of view and thus challenge the status quo, it is human nature that new ideas are often stifled and met with skepticism and controversy. The development of a synthetic transformation is similar in that a specific mechanistic construct evolves, promoting the associated dogma of a reaction's scope and limitations. Notably, the discovery of a new mechanism, reagent, or catalyst can address some of the underlying issues and expand the scope and utility in a way that was inconceivable before their introduction. This is one of the strengths of an Organic Reactions chapter since the in‐depth mechanistic analysis, in conjunction with the associated applications, provides a critical analysis that highlights gaps in our knowledge and thereby represents an invaluable resource for garnering insight into new research areas.
The chapters in this volume of Organic Reactions describe the cyclization of nitrogen‐centered radicals and the transition‐metal‐catalyzed aminooxygenation of alkenes. Both chapters feature nitrogen‐centered radicals, albeit the first chapter focuses entirely on this topic, whereas they provide an alternative reactive intermediate in the second chapter. Ironically, free‐radical intermediates epitomize the notion of dogma in a chemical reaction stemming from the yin and yang of controlling both reactivity and selectivity. Hence, to varying extents, both chapters address the dogma associated with free‐radical intermediates, which were once deemed much too reactive to be used in efficient and selective chemical transformations. Moreover, each chapter includes some of the more recent work with photoredox catalysis that provides exciting new possibilities for accessing free radical intermediates. Hence, the volume represents another stellar contribution to the Organic Reactions series, which delineates the seminal developments and defines the critical elements that change the narrative, albeit with the unavoidable introduction of new dogma!
The first chapter by Stuart W. McCombie, Béatrice Quiclet‐Sire, and Samir Z. Zard provides an excellent review of the cyclization of nitrogen‐centered radicals, which like carbon‐centered radicals, have a chameleon‐like character in that they can either be electrophilic or nucleophilic. Nitrogen‐centered radicals can be traced back to the early work of A. W. Hoffmann in 1878, who, in the course of conducting an unsaturation test on a piperidine intermediate, discovered the eponymous Hofmann‐Löffler‐Freytag reaction in which 1,5‐H atom abstraction and subsequent cyclization forms a new pyrrolidine ring. Wawzonek and Thelan later provided evidence for a chain process involving a nitrogen‐centered radical. The current chapter starts with a brief background of the various classes of nitrogen‐centered radicals to orient the reader and then focuses on the intramolecular addition of nitrogen‐centered free radicals to unsaturated groups and the homolytic substitution at a heteroatom. The authors also discuss the advantage of free radical‐based domino reactions, which permit further functionalization of the radical intermediate formed upon initial ring‐closure through atom/group transfer, fragmentation, and other cyclization reactions. The Mechanism and Stereochemistry section summarizes the specific rate constants for the hydrogen abstraction and 5‐hexenyl radical cyclizations to be able to determine the optimal substrate for a proposed cyclization. The section also describes aspects of regio‐ and stereocontrol, wherein the former is described in the context of ring‐size (i.e., exo‐ vs. endo‐selectivity) and the latter in the context of 5‐hexenyl radical cyclization reactions that afford similar stereochemical outcomes to carbon‐centered radicals that are rationalized using the venerable Beckwith model. The Scope and Limitations section is structured according to the nature of the bond that is broken to produce the initial nitrogen‐centered radical, namely, the scission of nitrogen‐halogen, nitrogen‐nitrogen, nitrogen‐oxygen, nitrogen‐sulfur, nitrogen‐carbon, and nitrogen‐hydrogen bonds. As noted by Zard, “Progress in this area has been slow, hampered hitherto by a dearth of convenient routes for generating these reactive species and a lack of awareness concerning their reactivity.” The section nicely categorizes the various types of nitrogen‐centered radical precursors and the associated cyclization reactions that are now available. The Applications to Synthesis section describes the relatively limited applications to natural product syntheses, which unsurprisingly focus on alkaloids. The Comparison with Other Methods section provides a critical assessment of alternative methods for constructing C‐N bonds using ionic and organometallic methods, which are rarely incorporated into domino reactions to increase molecular complexity (vide supra). The Tabular Survey organization follows that of the Scope and Limitations, wherein the specific type of nitrogen‐centered radical can be readily identified for a particular cyclization reaction. Overall, this is an outstanding chapter on a venerable reaction that continues to generate interest and thus provides an important resource for the synthetic organic and medicinal chemistry communities.
The second chapter by Sherry R. Chemler, Dake Chen, Shuklendu D. Karyakarte, Jonathan M. Shikora, and Tomasz Wdowik delineates the development of the transition‐metal‐catalyzed aminooxygenation of alkenes to prepare vicinal amino alcohol derivatives. Sharpless pioneered the development of this reaction with the first osmium‐mediated stereospecific vicinal oxyamination of olefins in 1975, which was followed over twenty years later by the first enantioselective osmium‐catalyzed aminooxygenation. Notably, the reaction has evolved beyond the early work, and this chapter describes the many adaptations reported in the context of other mechanistic constructs that can be accessed using different metal complexes. The Mechanism and Stereochemistry section is organized by the type of reactive intermediate, namely, metal nitrenes, nitrogen‐centered radicals and heteroatom‐metalated intermediates. Each reactive intermediate is then subdivided by the metal, which further describes aspects of kinetics, in addition to regio‐, diastereo‐, and enantioselectivity. For instance, the reactions of nitrenes using osmium, rhodium, iron, and copper outline the difference between the concerted approach to one using a nitrogen‐centered radical. The Scope and Limitations section is organized by inter‐ and intramolecular aminooxygenation reactions, subdivided by substrate and product. For example, the intermolecular reactions are organized by type of olefin (e.g., unactivated alkenes, dienes, enynes, styrenes, enamides, indoles, and enol ethers). In contrast, the intramolecular process is organized by the product formed using a specific metal complex, namely the preparation of β‐, γ‐, and δ‐lactams, pyrrolidines, piperidines, and indolines. Additional sections follow the same format based on the products derived from the reactions with O‐allyl carbamates, sulfamates, N‐allyl ureas, hydroxylamines, indoles and glycals, in addition to unsaturated alcohols, carboxylic acids, and oximes. The Applications to Synthesis section describes several impressive natural product syntheses that encompass this process. The Comparison with Other Methods section details a few related methods that utilize stoichiometric metal‐free reagents, highlighting the power of the metal‐catalyzed variant. The Tabular Survey incorporates reactions reported up to mid‐December 2020 and mirrors the Scope and Limitations in that the intra‐ and intermolecular reactions are organized by starting material and product, respectively, to allow the reader to traverse these two reaction manifolds. Overall, this is an excellent chapter on a fundamentally important transformation valuable to one wishing to construct vicinal amino alcohols.
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 editorial process's various stages. I want to thank Paul R. Blakemore (Chapter 1), Marisa Kozlowski and Kevin H. Shaughnessy (both Chapter 2), who served as the Responsible Editors to marshal the chapters through the various phases of development. I am also deeply indebted to Dr. Danielle Soenen for her continued and 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 authors', editors', and publishers' 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. Tina Grant (Processing Editor). I would also like to acknowledge Dr. Barry B. Snider (Secretary) for keeping everyone on task and Dr. Jeffery Press (Treasurer) for ensuring 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, makes this series of reviews both unique and exceptionally valuable to the practicing synthetic organic chemist.
P. Andrew Evans
Kingston
Ontario, Canada
CHAPTER 1CYCLIZATION REACTIONS OF NITROGEN‐CENTERED RADICALS
STUART W. McCOMBIE
Caldwell, NJ, 07006, U.S.A.
BÉATRICE QUICLET‐SIRE, SAMIR Z. ZARD AND PAUL R. BLAKEMORE
Laboratoire de SynthÈse Organique, Ecole Polytechnique, 91128, Palaiseau, France
Edited by Paul R. Blakemore
CONTENTS
ACKNOWLEDGEMENTS
INTRODUCTION
MECHANISM AND STEREOCHEMISTRY
Rate Constants for Ring Closures
Regiochemistry
Formation of Small Rings
Formation of Five‐Membered Rings
Formation of Six‐Membered and Larger Rings,
Stereoselectivity
SCOPE AND LIMITATIONS
Cleavage of Nitrogen‐Halogen Bonds
Nitrogen‐Chlorine Bonds
Nitrogen‐Bromine Bonds
Nitrogen‐Iodine Bonds
Cleavage of Nitrogen‐Nitrogen Bonds
N
‐Nitroso Compounds
N
‐Acyl Triazenes
N
‐(Benzotriazolyl)imines
Azides
Aminodihydropyridines and Aminocyclohexadienes
Thiosemicarbazides and Thiosemicarbazones
Cleavage of Nitrogen‐Oxygen Bonds
Ethers and Esters of Oximes, Hydroxamic Acids, and Related Derivatives
Peresters and Barton Esters
O
‐(1‐Carboxyalkyl)oximes
Dithiocarbonates of Oximes
Sulfinates and Phosphinates of Oximes and Hydroxamic Acids
Oxaziridines
Cleavage of Nitrogen‐Sulfur Bonds
N
‐(Arylthio)amines, ‐amides, and ‐imines
N
‐(Alkoxythiocarbonylthio) Derivatives
Extrusion of Sulfur Dioxide
Cleavage of Nitrogen‐Carbon Bonds
N
‐Allylanilines
Aziridines
Cleavage of Nitrogen‐Hydrogen Bonds
Indirect Methods
APPLICATIONS TO SYNTHESIS
COMPARISON WITH OTHER METHODS
EXPERIMENTAL CONDITIONS
Hazards
Reaction Conditions
EXPERIMENTAL PROCEDURES
Ethyl 8‐Oxo‐azocane‐4‐carboxylate [Addition of an Aminyl Radical to a Ketone with Ring Expansion].
cis
‐2‐Benzyl‐1,5‐dimethylpyrrolidine [5‐
exo
‐Cyclization of an Electrochemically Generated Aminyl Radical].
4‐Methyl‐2‐phenylselenanyl‐4‐azatricyclo[4.2.1.0
0,0
]nonan‐5‐one [5‐
exo
‐Cyclization of a Hydroxamate‐Derived Aminyl Radical with Selenide Trapping].
10‐Methoxy‐7‐oxo‐2,3,4,5,11b,11c‐hexahydro‐1
H
,7
H
‐pyrrolo[3,2,1‐
de
]-phenanthridine‐3a‐carboxylic Acid Ethyl Ester [Tandem Cyclization Initiated by an Amidyl Radical Generated by N–N Scission].
1‐Benzyl‐5‐methylpyrrolidin‐2‐one [Cyclization of a Photoredox‐Generated Amidyl Radical].
(
4R
,
S
,5
S
,
R
)‐4‐(Chloromethyl)‐1,3‐oxazolidin‐2‐one [Fe(II)‐Catalyzed Generation and 5‐
exo
‐Cyclization of a Carbamyl Radical with Cl Capture].
2‐Methyl‐2‐azaspiro[4.5]dec‐1‐ene [5‐
exo
‐Cyclization of an Iminyl Radical Generated by Bu
3
SnH Reduction of a Sulfenylimine].
3,4‐Dihydro‐2‐methyl‐5‐(3,3‐methylenedioxydecyl)‐2
H
‐pyrrole [Reductive 5‐
exo
Cyclization of an Iminyl Radical Generated by Phenolate Reduction of an Oxime
O
‐(2,4‐dinitrophenyl) Ether].
3‐(4‐Methylbenzenesulfonylmethyl)‐2,3‐dihydropyrrolo[2,1‐
b
]quinazolin‐9[1
H
]‐one [Arylsulfonyl Radical Addition to Generate an Amidinyl Radical, with Cyclization onto an Aromatic Ring].
3‐Bromo‐3‐iodo‐1‐(4‐methylbenzenesulfonyl)piperidine [6‐
endo
‐Iodocyclization of a Sulfonamidyl Radical].
TABULAR SURVEY
Table 1. Cyclizations of Aminyl Radicals
Table 2. Cyclizations of Amidyl Radicals
Table 3. Cyclizations of Carbamyl Radicals
Table 4. Cyclizations of Iminyl Radicals
Table 5. Cyclizations of Amidinyl Radicals
Table 6. Cyclizations of Other Nitrogen‐Centered Radicals
REFERENCES
SUPPLEMENTAL REFERENCES
ACKNOWLEDGEMENTS
The authors thank Professor P. Andrew Evans and other members of the editorial board of Organic Reactions for invaluable assistance with the editing and processing of this chapter.
INTRODUCTION
Nitrogen‐centered radicals can be generated and subsequently captured by a tethered unsaturated unit to furnish nitrogen‐containing rings. This transformation is generally useful in the total synthesis of natural products, especially alkaloids, and for the synthesis of nitrogen heterocycles in general, including many of interest to medicinal chemists. The creation of a nitrogen‐carbon bond through alternative methods, e.g., ionic or organometallic processes, can suffer from serious limitations owing to incompatibility with other functionality present in the precursor, the relative inaccessibility of the precursors, sensitivity to steric hindrance, and the formation of side‐products arising from ionic rearrangements and eliminations. In contrast, the nitrogen‐centered‐radical approach often exhibits remarkable functional‐group tolerance because of the general mildness of the experimental conditions, and the facts that the radical intermediate is less sterically encumbered. The ring‐closure step may be followed by further radical processes, including atom or group transfers, fragmentations, and additional cyclization reactions. These domino sequences allow a rapid increase in molecular complexity and level of functionalization in the products. The transformations outlined in Scheme 11 are illustrative and combine the cyclization of an iminyl radical with an intermolecular addition to methyl acrylate and subsequent ionic lactam formation.
Scheme 1
Whereas the use of cyclization reactions of carbon‐centered radicals in synthesis is now reasonably well established,2 cyclizations of nitrogen‐centered radicals are less exploited, despite their obvious potential. Their implementation into synthetic planning has lagged considerably, one reason being the comparative dearth of methods for generating such species. That situation has changed in recent years.
Many types of nitrogen‐centered radicals exist (Figure 1), including aminyl 1, aminium 2 and 3, amidyl 4, carbamyl 5, ureidyl 6, cyanamidyl 7, sulfonamidyl 8, phosphoramidyl 9, iminyl 10, alkoxyiminyl 11, amidinyl 12, and alkoxyaminyl 13, and their capture by a suitably located unsaturated bond leads to different nitrogen‐containing heterocycles. The reactivity of these species varies greatly depending on the substituent on the nitrogen atom and, in the case of the basic aminyl radicals, on the pH of the medium or the presence of Lewis acids, including transition‐metal salts or other metal complexes that permit the same outcome (cf. 1, 2, and 3; M = metal salt or complex). Radical stabilization energies (RSE) for some of these nitrogen‐centered radicals and their protonated counterparts have been calculated ab initio at the G3(MP2)‐RAD and G3B3 levels.3
Figure 1 Types of nitrogen-centered radicals.
Scheme 2
This chapter covers only those transformations wherein a ring is created by a nitrogen‐centered radical that either adds to an unsaturated group or carries out a homolytic substitution on a heteroatom; however, examples of the latter are rare. Insertions into C–H bonds and processes in which nitrogen radicals are produced but do not participate in the ring‐forming step are not included. For instance, although it is the earliest transformation involving a nitrogen‐centered (aminium) radical, the Hoffmann‐Löffler‐Freytag reaction3a that is shown in general form in Scheme 2, is not discussed because the ring closure to give the pyrrolidine ring involves an ionic substitution. Formation of pyrrolidine rings by sequences initiated by intermolecular additions of nitrogen‐centered radicals, illustrated in general form in Scheme 34 are also not within the scope of this chapter. It should be noted that in the transformation delineated in Scheme 3, and throughout the chapter, half‐headed “curly arrows” are used to indicate the sense of sequential radical formation, cyclization, and trapping processes and do not imply any synchronicity in those processes. Reciprocating half‐headed arrows for single‐electron motion are not illustrated to simplify the presentation; however, their presence is implied.
Scheme 3
