Stable Radicals E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1206

Veröffentlichungsjahr: 2011

Beschreibung

Stable radicals - molecules with odd electrons which are sufficiently long lived to be studied or isolated using conventional techniques - have enjoyed a long history and are of current interest for a broad array of fundamental and applied reasons, for example to study and drive novel chemical reactions, in the development of rechargeable batteries or the study of free radical reactions in the body. In Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds a team of international experts provide a broad-based overview of stable radicals, from the fundamental aspects of specific classes of stable neutral radicals to their wide range of applications including synthesis, materials science and chemical biology. Topics covered include: * triphenylmethyl and related radicals * polychlorinated triphenylmethyl radicals: towards multifunctional molecular materials * phenalenyls, cyclopentadienyls, and other carbon-centered radicals * the nitrogen oxides: persistent radicals and van der Waals complex dimers * nitroxide radicals: properties, synthesis and applications * the only stable organic sigma radicals: di-tert-alkyliminoxyls. * delocalized radicals containing the hydrazyl [R2N-NR] unit * metal-coordinated phenoxyl radicals * stable radicals containing the thiazyl unit: synthesis, chemical, and materials properties * stable radicals of the heavy p-block elements * application of stable radicals as mediators in living-radical polymerization * nitroxide-catalyzed alcohol oxidations in organic synthesis * metal-nitroxide complexes: synthesis and magneto-structural correlations * rechargeable batteries using robust but redox-active organic radicals * spin labeling: a modern perspective * functional in vivo EPR spectroscopy and imaging using nitroxides and trityl radicals * biologically relevant chemistry of nitroxides Stable Free Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds is an essential guide to this fascinating area of chemistry for researchers and students working in organic and physical chemistry and materials science.

Leseprobe

Contents

Preface

List of Contributors

1. Triarylmethyl and Related RadicalsThomas T. Tidwell

1.1 Introduction

1.2 Free radical rearrangements

1.3 Other routes to triphenylmethyl radicals

1.4 The persistent radical effect

1.5 Properties of triphenylmethyl radicals

1.6 Steric effects and persistent radicals

1.7 Substituted triphenylmethyl radicals and dimers

1.8 Tris(heteroaryl)methyl and related triarylmethyl radicals

1.9 Delocalized persistent radicals: analogues of triarylmethyl radicals

1.10 Tetrathiatriarylmethyl (TAM) and related triarylmethyl radicals

1.11 Perchlorinated triarylmethyl radicals

1.12 Other triarylmethyl radicals

1.13 Diradicals and polyradicals related to triphenylmethyl

1.14 Outlook

2. Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular MaterialsJaume Veciana and Imrna Ratera

2.1 Introduction

2.2 Functional molecular materials based on PTM radicals

2.3 Multifunctional switchable molecular materials based on PTM radicals

2.4 Conclusions

3. Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered RadicalsYasushi Morita and Shinsuke Nishida

3.1 Introduction

3.2 Open shell graphene

3.3 Phenalenyl

3.4 2,5,8-Tri-ieri-butylphenalenyl radical

3.5 Perchlorophenalenyl radical

3.6 Dithiophenalenyl radicals

3.7 Nitrogen-containing phenalenyl systems

3.8 Oxophenalenoxyl systems

3.9 Phenalenyl-based zwitterionic radicals

3.10 7t-Extended phenalenyl systems

3.11 Curve-structured phenalenyl system

3.12 Non-alternant stable radicals

3.13 Stable triplet carbenes

3.14 Conclusions

4. The Nitrogen Oxides: Persistent Radicals and van der Waals Complex DimersD. Scott Bo hie

4.1 Introduction

4.2 Synthetic access

4.3 Physical properties

4.4 Structural chemistry of the monomers and dimers

4.5 Electronic structure of nitrogen oxides

4.6 Reactivity of nitric oxide and nitrogen dioxide and their van der Waals complexes

4.7 The kinetics of nitric oxide’s termolecular reactions

4.8 Biochemical and organic reactions of nitric oxide

4.9 General reactivity patterns

4.10 The colored species problem in nitric oxide chemistry

4.11 Conclusions

5. Nitroxide Radicals: Properties, Synthesis and ApplicationsHakim Karoui, François Le Moigne, Olivier Ouari and Paul Tordo

5.1 Introduction

5.2 Nitroxide structure

5.3 Nitroxide multiradicals

5.4 Nitronyl nitroxides (NNOs)

5.5 Synthesis of nitroxides

5.6 Chemical properties of nitroxides

5.7 Nitroxides in supramolecular entities

5.8 Nitroxides for dynamic nuclear polarization (DNP) enhanced NMR

5.9 Nitroxides as pH-sensitive spin probes

5.10 Nitroxides as prefluorescent probes

5.11 EPR-spin trapping technique

5.12 Conclusions

6. The Only Stable Organic Sigma Radicals:\Y\-tert-\lkyliminoxylsKeith U. Ingold

6.1 Introduction

6.2 The discovery of stable iminoxvls

6.3 Hydrogen atom abstraction by di-ieri-butyliminoxyl

6.4 Other reactions and non-reactions of di-ieri-butyliminoxyl

6.5 Di-tot-alkyliminoxyls more sterically crowded than di-ieri-butyliminoxyl

6.6 Di-(l-Adamantyl)iminoxyl: a truly stable σ radical

7. Verdazyls and Related Radicals Containing the Hydrazyl [R2N—NR] GroupRobin G. Hicks

7.1 Introduction

7.2 Verdazyl radicals

7.3 Tetraazapentenyl radicals

7.4 Tetrazolinyl radicals

7.5 1,2,4-Triazolinyl radicals

7.6 1,2,4,5-Tetrazinyl radicals

7.7 Benzo-l,2,4-triazinyl radicals

7.8 Summary

8. Metal Coordinated Phenoxyl RadicalsFabrice Thomas

8.1 Introduction

8.2 General properties of phenoxyl radicals

8.3 Occurrence of tyrosyl radicals in proteins

8.4 Complexes with coordinated phenoxyl radicals

8.5 Conclusions

8.6 Abbreviations

9. The Synthesis and Characterization of Stable Radicals Containing the Thiazyl (SN) Fragment and Their Use as Building Blocks for Advanced Functional MaterialsRobin G. Hicks

9.1 Introduction

9.2 Radicals based exclusively on sulfur and nitrogen

9.3 “Organothiazyl” radicals

9.4 Thiazyl radicals as “advanced materials”

9.5 Conclusions

10. Stable Radicals of the Heavy p-Block ElementsJari Konu and Tristram Chivers

10.1 Introduction

10.2 Group 13 element radicals

10.3 Group 14 element radicals

10.4 Group 15 element radicals

10.5 Group 16 element radicals

10.6 Group 17 element radicals

10.7 Summary and future prospects

11. Application of Stable Radicals as Mediators in Living-Radical PolymerizationAndrea R. Szkurhan, Julie Lukkarila and Michael K. Georges

11.1 Introduction

11.2 Living polymerizations

11.3 Stable free radical polymerization

11.4 Non-nitroxide-based radicals as mediating agents

11.5 Aqueous stable free radical polymerization processes

11.6 The application of stable free radical polymerization to new materials

11.7 Conclusions

12. Nitroxide-Catalyzed Alcohol Oxidations in Organic SynthesisChristkin Brückner

12.1 Introduction

12.2 Mechanism of TEMPO-catalyzed alcohol oxidations

12.3 Nitroxides used as catalysts

12.4 Chemoselectivity: oxidation of primary vs secondary alcohols

12.5 Chemoselectivity: oxidation of primary vs benzylic alcohols

12.6 Oxidation of secondary alcohols to ketones

12.7 Oxidations of alcohols to carboxylic acids

12.8 Stereoselective nitroxide-catalyzed oxidations

12.9 Secondary oxidants used in nitroxide-catalyzed reactions

12.10 Use of nitroxide-catalyzed oxidations in tandem reactions

12.11 Predictable side reactions

12.12 Comparison with other oxidation methods

12.13 Nitroxide-catalyzed oxidations and green chemistry

13. Metal-Nitroxide Complexes: Synthesis and Magnetostructural CorrelationsVictor Ovcharenko

13.1 Introduction

13.2 Two types of nitroxide for direct coordination of the metal to the nitroxyl group

13.3 Ferro- and ferrimagnets based on metal-nitroxide complexes

13.4 Heterospin systems based on polynuclear compounds of metals with nitroxides

13.5 Breathing crystals

13.6 Other studies of metal-nitroxides

13.7 Conclusions

14. Rechargeable Batteries Using Robust but Redox Active Organic RadicalsTakeo Suga and Hiroyuki Nishide

14.1 Introduction

14.2 Redox reaction of organic radicals

14.3 Mechanism and performance of an organic radical battery

14.4 Molecular design and synthesis of redox active radical polymers

14.5 A totally organic-based radical battery

14.6 Conclusions

15. Spin Labeling: A Modern PerspectiveLawrence J. Berliner

15.1 Introduction

15.2 The early years

15.3 Advantages of nitroxides

15.4 Applications of spin labeling to biochemical and biological systems

15.5 Distance measurements

15.6 Site directed spin labeling (SDSL): how is it done?

15.7 Other spin labeling applications

15.8 Conclusions

16. Functionalin vivoEPR Spectroscopy and Imaging Using Nitroxide and Trityl RadicalsValéry V. Khramtsov and Jay L. Zweier

16.1 Introduction

16.2 Nitroxyl radicals

16.3 Triarylmethyl (trityl) radicals

16.4 In vivo EPR oximetry using nitroxyl and trityl probes

16.5 EPR spectroscopy and imaging of pH using nitroxyl and trityl probes

16.6 Redox- and thiol-sensitive nitroxide probes

16.7 Conclusions Acknowledgements References

17. Biologically Relevant Chemistry of NitroxidesSara Goldstein and Amram Samuni

17.1 Introduction

17.2 Mechanisms of nitroxide reactions with biologically relevant small radicals

17.3 Nitroxides as SOD mimics

17.4 Nitroxides as catalytic antioxidants in biological systems

17.5 Conclusions Acknowledgements References

Index

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Library of Congress Cataloging-in-Publication Data

Stable radicals : fundamentals and applied aspects of odd-electron compounds /editor, Robin G. Hicks.p. cm.

Includes bibliographical references and index.ISBN 978-0-470-77083-2 (hardback)

1. Radicals (Chemistry) – Stability. 2. Electron mobility. I. Hicks, Robin G. QD471.S73 2010541’.224 – dc22

2010010754

A catalogue record for this book is available from the British Library.ISBN 978-0-470-77083-2

Preface

Valency is a core tenet of chemical structure and bonding. In essence, valency can be thought of as a “bond-forming capability” parameter that permits understanding of the chemical formula and properties of known compounds, the prediction of the existence of new ones, and the rationalization of the non-existence (or instability) of species with incompletely satisfied valencies. This last point provides a rationale for why radicals – with an unpaired electron and, as such, quintessential examples of subvalent compounds – are typically described as short-lived, highly reactive species. To be sure, the vast majority of radicals based on s- and p-block elements are indeed thermodynamically and kinetically unstable. Thus, it should not be surprising that, since Gomberg’s paradigm-shifting discovery of the triphenylmethyl radical in 1900, the very existence of radicals stable enough to be observed or isolated has always been met with wonder (and at times skepticism) by the general chemistry community. The ongoing spate of publications highlighting the discovery of a new molecule that can be isolated but has an unpaired electron serves as evidence that stable radicals are generally regarded as exotic and esoteric compounds.

In fact, several kinds of stable molecules containing one (or more) unpaired electrons have been known for decades, and several more general classes of stable radicals have been developed in recent times. Moreover, applications which explicitly make use of stable radicals abound, spanning synthesis, materials science, and medicine. Many review articles and books have dealt with particular kinds or uses of stable radicals. However, the only attempt to provide unified coverage of the stable radical literature was a book by Forrester, Hay, and Thomson (Organic Chemistry of Stable Free Radicals, Academic Press) published in 1968. The present book is, in essence, an update on the huge developments in (and diversification of) the field of stable radicals over the four decades since the book by Forrester et al. My ambitions for this book were to bring together the diverse range of topics and interests whose common theme is stable neutral radicals (the omission of radical ions was a deliberate decision, made largely due to space limitations). Thus, Chapters 1–10 introduce readers to the many different kinds (organic, inorganic, old, new) of existing stable radicals, and Chapters 11–17 provide accounts of various applications of stable radicals, ranging from synthesis (oxidation catalysts, living radical polymerization) to materials science (magnetochemistry, battery components) to chemical biology/medicine (spin labeling, EPR imaging, redox biochemistry). I offer my sincere thanks to all of the contributors to each chapter who have contributed to what I hope will be viewed as an informative, interesting, and illuminating text.

Robin G. HicksVictoria, Canada

List of Contributors

D. Scott Bohle, Department of Chemistry, McGill University, 801 Sherbrooke St.W., Montreal, Canada H3A 2K6

Lawrence J. Berliner, Department of Chemistry and Biochemistry, University of Denver, 2190 E. Iliff Avenue, Denver, CO 80208, USA

Christian Brückner, Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA

Tristram Chivers, Department of Chemistry, University of Calgary, Calgary, AB, Canada T2N 1N4

Michael K. Georges, Department of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario, Canada L5L 1C6

Sara Goldstein, Institute of Chemistry and the Accelerator Laboratory, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Robin G. Hicks, Department of Chemistry, University of Victoria, Victoria, British Columbia, Canada V8W 3V6

Keith U. Ingold, National Research Council, Ottawa, Ontario, Canada K1A 0R6

Hakim Karoui, Laboratoire Chimie Provence, UMR 6264, Aix-Marseille Université and CNRS, Centre de St Jérôme, 13397, Marseille Cedex 20, France

Valery V. Khramtsov, Dorothy M. Davis Heart and Lung Research Institute, 201 HLRI, 473 W 12th Ave, The Ohio State University, Columbus, OH 43210, USA

Jari Konu, Department of Chemistry, University of Calgary, Calgary, AB, Canada T2N 1N4

François Le Moigne, Laboratoire Chimie Provence, UMR 6264, Aix-Marseille Université and CNRS, Centre de St Jérôme, 13397, Marseille Cédex 20, France

Julie Lukkarila, Department of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario, Canada L5L 1C6

Yasushi Morita, Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

Shinsuke Nishida, Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan

Hiroyuki Nishide, Department of Applied Chemistry, Waseda University, Tokyo 169–8555, Japan

Olivier Ouari, Laboratoire Chimie Provence, UMR 6264, Aix-Marseille Université and CNRS, Centre de St Jérôme, 13397, Marseille Cédex 20, France

Victor Ovcharenko, International Tomography Center, Novosibirsk, Russia

Imma Ratera, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra (Barcelona), Spain

Amram Samuni, Department of Molecular Biology, The Hebrew University of Jerusalem - Hadassah Medical School, Jerusalem 91120, Israel

Takeo Suga, Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan

Andrea R. Szkurhan, Department of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario, Canada L5L 1C6

Fabrice Thomas, Inorganic Redox Chemistry, Department of Molecular Chemistry (UMR-5250), University of Grenoble, BP 53, 38041, Grenoble Cedex 9, France

Thomas T. Tidwell, Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6

Paul Tordo, Laboratoire Chimie Provence, UMR 6264, Aix-Marseille Université and CNRS, Centre de St Jérôme, 13397, Marseille Cédex 20, France

Jaume Veciana, Institut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra (Barcelona), Spain

Jay L. Zweier, Dorothy M. Davis Heart and Lung Research Institute, 611 HLRI, 473W 12th Ave, The Ohio State University, Columbus, OH 43210, USA

1Triarylmethyl and Related Radicals

Thomas T. Tidwell

Department of Chemistry, University of Toronto, Ontario, Canada

1.1 Introduction

1.1.1 Discovery of the triphenylmethyl radical

During the Nineteenth Century the understanding of the structure of organic compounds was beginning to evolve. The theory of free radicals had risen to prominence, and then fallen into disrepute. This changed abruptly with the bold announcement in 1900 by Moses Gomberg of the formation of the stable and persistent free radical triphenylmethyl 1, with its radical character shown by its facile reaction with oxygen forming the peroxide 2 (Equation l.l).1 This had an immediate impact, and was a major landmark that set the stage for the rapid development of free radical chemistry in the Twentieth Century. Gomberg’s work attracted the attention of the world chemical community, and led to careful scrutiny and the ultimate acceptance of this controversial discovery.2

(1.1)

Gomberg treated triphenylmethyl chloride with silver or zinc metal and obtained a colored solution, which upon reaction with oxygen yielded peroxide 2. The species in solution was confidently identified by Gomberg as the triphenylmethyl radical 1, and he published his discovery in both German and English.1 Over the next decade there was much dispute as to the identification of 1, but Wilhelm Schlenk and coworkers in 1910 obtained tris(4-biphenylyl)methyl 3 as a deeply colored solid that was almost completely dissociated in solution, which confirmed the existence of l.2i

Upon removal of the solvent for the isolation of 1, a solid dimer was obtained, for which the symmetrical head-to-head structure 4 as well as the unsymmetrical structure 5 (head-to-tail, Jacobson structure)3a and 6 (tail-to-tail, Heintschel structure)3b,c were given serious consideration. However, as recounted by McBride,3d the wrong structure for the dimer, namely the head-to-head structure 4, became accepted for more than half a century, before this was corrected to the unsymmetrical structure 5 based on spectroscopic data.3e In retrospect, not only was the original evidence for the misidentified structure rather flimsy, but techniques, such as NMR, IR, and UV, were also widely available that would have permitted correction of this structure well before 1968. This provides a cautionary tale that skepticism and a critical look at the evidence available for supposed chemical truths is warranted even in the face of conventional wisdom.

The preparation and identification of the stable triphenylmethyl radical was one of the great chemical discoveries of the Twentieth Century, but surprisingly this was not honored by the award of the Nobel Prize. As revealed by the investigation by Lennart Eberson in the Nobel archives, Gomberg was repeatedly nominated for the award but, due to a series of unfortunate circumstances, the nominations were not accepted.3f Despite Gomberg’s confident assertion in his first paper that he had proof of the existence of the radical there was later some equivocation, and some contrary opinions, which were enough for the Nobel Committee not to approve the award. With the later preparation of the tris(4-biphenylyl)methyl radical 3 as a stable solid the uncertainty vanished,2i but then subsequent nominations were turned down either because both Gomberg and Schlenk were not nominated in the same year, or because too much time had elapsed since the initial discovery. Even in 1940 Gomberg was still being nominated, but without success. Gomberg’s discovery was a clearly momentous discovery by a single individual, and although well recognized he did not receive the ultimate accolade he deserved. Paradoxically, Gerhard Herzberg was awarded the 1971 prize in chemistry “for his contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals”, studies which had occupied him for 30 years.

By 1968 the study of stable free radicals had advanced to the stage that the book Organic Chemistry of Stable Free Radicals appeared,4 but in Chapter 2 entitled “Triarylmethyls and Other Carbon Radicals” the introductory paragraph describing the discovery of triphenylmethyl in 1900 ended with the dispiriting words “The behaviour of such radicals was elucidated during the following twenty years, mainly by the work of Gomberg, Schlenk and Wieland, since then little new chemistry has come to light although numerous triarylmethyls have been prepared and more physical data are available.” However six pages later, under “Dimerisation” there was the alarming statement “Much of what has been said in this section may require revision in light of the recent communication by Lankamp, Nauta and MacLean”, which described the surprising but in retrospect completely predictable finding3d,e that the triphenylmethyl dimer had the head-to-tail structure 5. The rapid development in triarylmethyl radical chemistry since 1968 also belies the tacit assumption of the authors noted above that such studies had become an intellectual backwater. Much of the rather extensive chemistry of triarylmethyl radicals described in this earlier review is not repeated here. The period from 1968 has been a new golden age for free radical chemistry, and this was given great impetus by the widespread use of electron paramagnetic resonance (EPR) spectroscopy, which led to a rapid development of free radical chemistry, and includes many advances in the study of triarylmethyl radicals.