Contemporary Carbene Chemistry E-Book

Robert A. Moss

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley Series of Reactive Intermediates
• Sprache: Englisch

Seitenzahl: 907

Veröffentlichungsjahr: 2013

Beschreibung

Presents the most innovative results in carbene chemistry, setting the foundation for new discoveries and applications

The discovery of stable carbenes has reinvigorated carbene chemistry research, with investigators seeking to develop carbenes into new useful catalysts and ligands. Presenting the most innovative and promising areas of carbene research over the past decade, this book explores newly discovered structural, catalytic, and organometallic aspects of carbene chemistry, with an emphasis on new and emerging synthetic applications.

Contemporary Carbene Chemistry features contributions from an international team of pioneering carbene chemistry researchers. Collectively, these authors have highlighted the most interesting and promising areas of investigation in the field. The book is divided into two parts:

• Part 1, Properties and Reactions of Carbenes, explores new findings on carbene stability, acid-base behavior, and catalysis. Carbenic structure and reactivity are examined in chapters dedicated to stable carbenes, carbodicarbenes, carbenes as guests in supramolecular hosts, tunneling in carbene and oxacarbene reactions, and ultrafast kinetics of carbenes and their excited state precursors. Theoretical concerns are addressed in chapters on computational methods and dynamics applied to carbene reactions.
• Part 2, Metal Carbenes, is dedicated to the synthetic dimensions of carbenes, particularly the reactions and catalytic properties of metal carbenes. The authors discuss lithium, rhodium, ruthenium, chromium, molybdenum, tungsten, cobalt, and gold.

All the chapters conclude with a summary of the current situation, new challenges on the horizon, and promising new research directions. A list of key reviews and suggestions for further reading also accompanies every chapter.

Each volume of the Wiley Series on Reactive Intermediates in Chemistry and Biology focuses on a specific reactive intermediate, offering a broad range of perspectives from leading experts that sets the stage for new applications and further discoveries.

Leseprobe

CONTENTS

PREFACE TO SERIES

PREFACE

CONTRIBUTORS

PART 1: PROPERTIES AND REACTIONS OF CARBENES

1 CARBENE STABILITY

1.1 INTRODUCTION

1.2 BACKGROUND

1.3 CARBENE STABILITY

1.4 CORRELATIONS INVOLVING CARBENE STABILITY

1.5 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

2 STABLE CARBENES

2.1 INTRODUCTION

2.2 TYPES OF STABLE CARBENES

2.3 SPECTROSCOPIC CHARACTERISTICS

2.4 CHEMICAL REACTIVITY

2.5 CONCLUSIONS AND OUTLOOK

SUGGESTED READING

REFERENCES

3 ACID–BASE CHEMISTRY OF CARBENES

3.1 INTRODUCTION

3.2 SOLUTION pKasOF THE CONJUGATE ACIDS OF CARBENES

3.3 GAS-PHASE BASICITIES AND PROTON AFFINITIES OF CARBENES

3.4 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

4 COMPUTATIONAL METHODS FOR THE STUDY OF CARBENES AND THEIR EXCITED STATES

4.1 INTRODUCTION

4.2 CARBENES

4.3 REARRANGEMENT IN EXCITED STATES (RIES)

4.4 ADVANCES IN COMPUTATIONAL INVESTIGATIONS OF CARBENES

4.5 THEORETICAL STUDIES OF THE PHOTOCHEMISTRY OF CARBENE PRECURSORS

4.6 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

5 DYNAMICS IN CARBENE REACTIONS

5.1 INTRODUCTION

5.2 DYNAMICS OF CARBENE CYCLOADDITIONS TO ALKENES AND ALKYNES

5.3 DYNAMICS OF OTHER CARBENE-MEDIATED REACTIONS

5.4 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

6 ULTRAFAST KINETICS OF CARBENE REACTIONS

6.1 INTRODUCTION

6.2 ULTRAFAST UV-VIS STUDIES OF THE INTERMOLECULAR REACTIVITY OF p-BIPHENYLYLCARBENE (BpCH)

6.3 REARRANGEMENTS IN THE EXCITED STATE OF THE CARBENE PRECURSOR

6.4 DYNAMICS OF CARBENE VIBRATIONAL COOLING AND SOLVATION

6.5 INFLUENCE OF SOLVENT ON CARBENE INTERSYSTEM CROSSING RATES

6.6 ELECTRONICALLY EXCITED (OPEN SHELL) SINGLET CARBENES

6.7 PARENT PHENYLDIAZIRINE—MECHANISTIC ASPECTS OF SINGLET CARBENE FORMATION

6.8 INFLUENCE OF HALO-SUBSTITUENT ELECTRON-DONATING CAPACITY ON DIAZIRINE DECAY IN THE FIRST EXCITED SINGLET STATE

6.9 THE INFLUENCE OF EXCITATION WAVELENGTH ON THE PHOTOCHEMISTRY OF DIAZIRINES

6.10 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

7 TUNNELING IN THE REACTIONS OF CARBENES AND OXACARBENES

7.1 INTRODUCTION: LIGHT- AND HEAVY-ATOM TUNNELING

7.2 ALKYL- AND HALOCARBENES

7.3 THE FORMOSE REACTION AND HYDROXYCARBENES

7.4 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

8 CARBODICARBENES

8.1 INTRODUCTION

8.2 CARBODICARBENES WITH N-HETEROCYCLIC LIGANDS C(NHC)2

8.3 TETRAAMINOALLENES AND “HIDDEN” CARBODICARBENES

8.4 BENT ALLENES

8.5 RELATED COMPOUNDS

8.6 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

9 CATALYTIC REACTIONS WITH N-MESITYL-SUBSTITUTED N-HETEROCYCLIC CARBENES

9.1 INTRODUCTION

9.2 THE N-MESITYL GROUP: A MECHANISTIC ASPECT

9.3 NHC CATALYSIS BY CLASS OF REACTIVE INTERMEDIATES

9.4 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

10 SUPRAMOLECULAR CARBENE CHEMISTRY

10.1 INTRODUCTION

10.2 TYPES OF HOSTS USED IN SUPRAMOLECULAR CARBENE CHEMISTRY

10.3 CHOOSING THE RIGHT CARBENE GUEST

10.4 DIAZIRINES AS SUITABLE SUPRAMOLECULAR CARBENE PRECURSORS

10.5 ARCHITECTURE OF THE GUEST@HOST COMPLEX

10.6 CASE STUDIES

10.7 CONCLUSION AND OUTLOOK

ACKNOWLEDGMENTS

SUGGESTED READING

REFERENCES

PART 2: METAL CARBENES

11 MODERN LITHIUM CARBENOID CHEMISTRY

11.1 INTRODUCTION

11.2 STRUCTURAL FEATURES OF LITHIUM CARBENOIDS

11.3 LITHIUM HALIDE CARBENOIDS

11.4 STRUCTURE–REACTIVITY RELATIONSHIPS

11.5 LITHIUM–OXYGEN CARBENOIDS

11.6 LITHIUM–NITROGEN CARBENOIDS

11.7 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

12 RHODIUM CARBENES

12.1 INTRODUCTION

12.2 OVERVIEW OF RHODIUM–CARBENOID INTERMEDIATES AND CHIRAL CATALYSTS

12.3 ENANTIOSELECTIVE CYCLOPROPANATION

12.4 CASCADE SEQUENCES INITIATED BY RHODIUM-CATALYZED CYCLOPROPANATION

12.5 ENANTIOSELECTIVE CYCLOPROPENATION

12.6 C–H FUNCTIONALIZATION BY CARBENOID-INDUCED C–H INSERTION

12.7 COMBINED C–H ACTIVATION/COPE REARRANGEMENT(CHCR)

12.8 FORMATION AND REACTIONS OF RHODIUM-BOUND YLIDES

12.9 VINYLOGOUS REACTIONS OF RHODIUM–VINYLCARBENOIDS

12.10 CONCLUSIONS AND FUTURE OUTLOOK

SUGGESTED READING

REFERENCES

13 RUTHENIUM CARBENES

13.1 INTRODUCTION

13.2 IMPROVED MECHANISTIC UNDERSTANDING

13.3 CATALYST DEVELOPMENT

13.4 ACHIEVING SELECTIVITY IN ALKENE METATHESIS

13.5 APPLICATIONS

13.6 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

14 NUCLEOPHILIC CARBENES OF THE CHROMIUM TRIAD

14.1 INTRODUCTION

14.2 CHROMIUM CARBENES

14.3 MOLYBDENUM CARBENES

14.4 TUNGSTEN CARBENES

14.5 CONCLUSIONS AND OUTLOOK

SUGGESTED READING

REFERENCES

15 COBALT-MEDIATED CARBENE TRANSFER REACTIONS

15.1 INTRODUCTION

15.2 COBALT-CATALYZED CYCLOPROPANATION REACTIONS

15.3 OTHER COBALT-CATALYZED CARBENE TRANSFER REACTIONS

15.4 CONCLUSION AND OUTLOOK

ACKNOWLEDGMENTS

SUGGESTED READING

REFERENCES

16 GOLD CARBENES

16.1 INTRODUCTION

16.2 NATURE OF THE Au–CARBON DOUBLE BOND

16.3 GENERATION AND REACTIONS OF GOLD CARBENES

16.4 CONCLUSION AND OUTLOOK

SUGGESTED READING

REFERENCES

SUPPLEMENTAL IMAGES

INDEX

Wiley Series of Reactive Intermediates in Chemistry and Biology

Steven E. Rokita, Series Editor

Quinone MethidesEdited by Steven E. Rokita

Radical and Radical Ion Reactivity in Nucleic Acid ChemistryEdited by Marc Greenberg

Carbon-Centered Free Radicals and Radical CationsEdited by Malcolm D.E. Forbes

Copper-Oxygen ChemistryEdited by Kenneth D. Karlin and Shinobu Ihoh

Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and MechanismBy Virender K. Sharma

Nitrenes and Nitrenium IonsEdited by Daniel E. Falvey and Anna D. Gudmundsdottir

Contempopary Carbene ChemistryEdited by Robert A. Moss and Michael P. Doyle

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Moss, Robert A., author.    Contemporary carbene chemistry / Robert A. Moss, Michael P. Doyle.       pages cm. – (Wiley series of reactive intermediates in chemistry and biology)    Includes bibliographical references and index.    ISBN 978-1-118-23795-3 (hardback)    1. Carbenes (Methylene compounds) 2. Carbon compounds. I. Doyle, Michael P., author. II. Title.    QD305.H5M67 2014    547′.01–dc23

2013023529

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

PREFACE TO SERIES

Most stable compounds and functional groups have benefited from numerous monographs and series devoted to their unique chemistry, and most biological materials and processes have received similar attention. Chemical and biological mechanisms have also been the subject of individual reviews and compilations. When reactive intermediates are given center stage, presentations often focus on the details and approaches of one discipline despite their common prominence in the primary literature of physical, theoretical, organic, inorganic, and biological disciplines. The Wiley Series on Reactive Intermediates in Chemistryand Biology is designed to supply a complementary perspective from current publications by focusing each volume on a specific reactive intermediate or target and endowing it with the broadest possible context and outlook. Individual volumes may serve to supplement an advanced course, sustain a special topics course, and provide a ready resource for the research community. Readers should feel equally reassured by reviews in their specialty, inspired by helpful updates in allied areas, and intrigued by topics not yet familiar.

This series revels in the diversity of its perspectives and expertise. Where some books draw strength from their focused details, this series draws strength from the breadth of its presentations. The goal is to illustrate the widest possible range of literature that covers the subject of each volume. When appropriate, topics may span theoretical approaches for predicting reactivity, physical methods of analysis, strategies for generating intermediates, utility for chemical synthesis, applications in biochemistry and medicine, impact on the environment, occurrence in biology, and more. Experimental systems used to explore these topics may be equally broad and range from simple models to complex arrays and mixtures such as those found in the final frontiers of cells, organisms, earth, and space.

Advances in chemistry and biology gain from a mutual synergy. As new methods are developed for one field, they are often rapidly adapted for application in the other. Biological transformations and pathways often inspire analogous development of new procedures in chemical synthesis, and likewise, chemical characterization and identification of transient intermediates often provide the foundation for understanding the biosynthesis and reactivity of many new biological materials. While individual chapters may draw from a single expertise, the range of contributions contained within each volume should collectively offer readers with a multidisciplinary analysis and exposure to the full range of activities in the field. As this series grows, individualized compilations may also be created through electronic access to highlight a particular approach or application across many volumes that, together, cover a variety of different reactive intermediates.

Interest in starting this series came easily, but the creation of each volume of this series required vision, hard work, enthusiasm, and persistence. I thank all of the contributors and editors who graciously accepted the challenge.

STEVEN E. ROKITAJohn Hopkins University

PREFACE

The origins of modern carbene chemistry can be found in Jack Hine’s incisive mechanistic studies of chloroform hydrolysis, which implicated dichlorocarbene (or “carbon dichloride,” as Hine termed it) as the key intermediate. Published in 1950, Hine’s work initiated a myriad of studies, which shows no sign of abating even after 60 years.

Along with the continuing stream of primary research reports, pertinent monographs have appeared regularly, summarizing progress and solidifying our increasing understanding of the physical organic chemistry and synthetic potential of divalent carbon compounds. It is instructive to examine the contents and foci of the earliest texts, not only to chart the developmental history of carbene chemistry, but also to trace the temporal succession and evolution of those problems that have been of central concern.

Hine’s Divalent Carbon (1964) focused on the formation and reactions of key carbene types, e.g., methylene, halocarbenes, and alkoxycarbenes. The formation of methylene and its derivatives from diazo compounds and ketenes was also reviewed. Significantly, Hine recognized isonitriles as “double-bonded divalent carbon,” relatives of today’s stable carbenes. Kirmse’s Carbene Chemistry (1964) adopted a functional group approach to carbene structure, considering methylene, alkyl- and arylcarbenes, as well as carboalkoxy-, alkoxy-, amino-, halo-, and ketocarbenes. Importantly, Gaspar and Hammond’s now-classic chapter on carbene spin states was included. A second edition of CarbeneChemistry (1971) added chapters on structure and reactivity, structural theory, spectroscopy, carbene analogs, and reaction types. The two volumes of Carbenes by Jones and Moss (1973, 1975) featured essays on carbenes from diazo compounds, carbene–alkene addition reactions, reactions of carbon atoms and unsaturated carbenes, organometallic carbene precursors, physical methods applied to carbenes in solution, and an updated version of Gaspar and Hammond’s spin state chapter. Carbene (Carbenoide), a landmark, multiauthored, two-volume set edited by Manfred Regitz (1989), summarized in exceptional depth, scope, and detail nearly 40 years of modern divalent carbon chemistry. Finally, Advances in Carbene Chemistry, a three volume set edited by Udo Brinker (1994, 1998, 2001), exemplified the breadth and depth of twentieth century carbene chemistry, while Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, edited by Guy Bertrand (2002), heralded the new country’s focus on selectivity.

Now, some 10 years later, carbene chemistry has experienced a remarkable renaissance, entering a new era, driven by advances in spectroscopy—especially fast and ultrafast laser flash photolysis, continuing progress in theory and computational methodology, and signal developments in synthetic chemistry. The latter feature an ever-expanding arsenal of stable carbenes and metal carbenes, useful both as catalysts and as reagents. Spectroscopy now allows the visualization of transient carbenes, together with their excited state precursors, on the nanosecond to femtosecond time scales, while theory enables increasingly accurate calculation of the carbenes’ energies and spectra, as well as many features of the energy surfaces upon which their reactions occur. Additionally, developments in reaction dynamics help to characterize the optimum reaction channels for transient carbenes generated on relatively flat energy surfaces.

At the same time, carbene chemistry now provides widely used, highly selective synthetic methodologies that were only nascent 20 years ago. Chemical transformations of carbenes, including metathesis and carbon–hydrogen insertion reactions, have claimed their place in the synthetic chemistry lexicon, controlling carbene reactivity via transition metal-bound intermediates. Moreover, the role of carbenes in these catalytic processes is well-understood in both mechanistic and synthetic terms. From “unselective” as a characteristic description in the 1960s, carbenes, tamed by association with transition metals, are now paragons of catalytic selectivity, while stable carbenes themselves are important catalysts and ligands in an ever-widening spectrum of reactions.

Contemporary Carbene Chemistry focuses on the most innovative and promising aspects of carbene research over the past decade. We do not dwell on those classical mechanistic concerns, which, for the most part, are now resolved. Rather, we explore newer structural, catalytic, and organometallic aspects of carbene chemistry, with a particular emphasis on synthetic applications. The essays that follow are not intended to be exhaustive reviews. Instead, they highlight the most interesting, fruitful, and promising initiatives in their topical areas. In Part 1, Properties and Reactions of Carbenes, we consider new paradigms of carbene stability, acid–base behavior, and catalysis. Aspects of carbenic structure and reactivity are highlighted by chapters on stable carbenes, carbodicarbenes, carbenes as guests in supramolecular hosts, tunneling in the reactions of carbenes and oxacarbenes, and the ultrafast kinetics of carbenes and their excited state precursors. Theoretical concerns are explored in chapters on computational methods, and dynamics applied to the reactions of carbenes. In Part 2, Metal Carbenes, our attention turns to the synthetic dimension, particularly the reactions and catalytic properties of metal carbenes. The metals considered include, lithium, rhodium, ruthenium, chromium, molybdenum, tungsten, cobalt, and gold. In addition, each chapter features a concluding section summarizing the current situation, highlighting near-term problems, and pointing to likely research directions. A list of key reviews and suggestions for further reading also accompanies each chapter.

Any book that bears “contemporary” in its title challenges the future. We welcome this challenge. If Contemporary Carbene Chemistry aids the research that renders it redundant, then it will have served its purpose; its authors and editors will be more than content.

New BrunswickCollege Park

MICHAEL P. DOYLEROBERT A. MOSS

CONTRIBUTORS

Christopher W. Bielawski*, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX, USAJeffrey W. Bode*, Laboratorium fur Organische Chemie, Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zürich, Zürich, SwitzerlandUdo H. Brinker*, Department of Chemistry, Institute of Organic Chemistry, University of Vienna, Vienna, Austria; Department of Chemistry, The State University of New York at Binghamton, Binghamton, NY, USAGotard Burdzinski*, Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, Poznan, PolandVito Capriati*, Dipartimento di Farmacia, Università degli Studi di Bari “Aldo Moro,” Consorzio Interuniversitario Nazionale Metodologie e Processi Innovativi di Sintesi, Bari, ItalyXin Cui, Department of Chemistry, University of South Florida, Tampa, FL, USAHuw M. L. Davies*, Department of Chemistry, Emory University, Atlanta, GA, USASteven T. Diver*, Department of Chemistry, University at Buffalo, The State University of New York, Amherst, NY, USAJonathan M. French, Department of Chemistry, University at Buffalo, The State University of New York, Amherst, NY, USAGernot Frenking*, Fachbereich Chemie, Philipps-Universität Marburg, Marburg, GermanyDennis Gerbig, Institute of Organic Chemistry, Justus-Liebig University, Giessen, GermanyScott Gronert*, Department of Chemistry, Virginia Commonwealth University, Richmond, VA, USAChristopher M. Hadad*, Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH, USAK. N. Houk*, Department of Chemistry and Biochemistry, University of California, Los Angeles, CA, USAJames R. Keeffe, Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, CA, USAHoi Ling Luk, Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH, USAJessada Mahatthananchai, Laboratorium fur Organische Chemie, Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zürich, Zürich, SwitzerlandRichard S. Massey, Department of Chemistry, Durham University, Durham, UKDina C. Merrer*, Department of Chemistry, Barnard College, New York, NY, USAJean-Luc Mieusset, Department of Chemistry, Institute of Organic Chemistry, University of Vienna, Vienna, AustriaJonathan P. Moerdyk, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX, USARory A. More O’Ferrall*, School of Chemistry and Chemical Biology, University College Dublin, Dublin, IrelandAnnMarie C. O’Donoghue*, Department of Chemistry, Durham University, Durham, UKBrendan T. Parr, Department of Chemistry, Emory University, Atlanta, GA, USAMathew S. Platz*, Department of Chemistry, The Ohio State University, Columbus, OH, USAMurray G. Rosenberg, Department of Chemistry, The State University of New York at Binghamton, Binghamton, NY, USAPeter R. Schreiner*, Institute of Organic Chemistry, Justus-Liebig University, Giessen, GermanyRalf Tonner, Fachbereich Chemie, Philipps-Universität Marburg, Marburg, GermanyZachary J. Tonzetich*, Department of Chemistry, University of Texas at San Antonio, San Antonio, TX, USAShubham Vyas, Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH, USALai Xu, Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, USALiming Zhang*, Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USAX. Peter Zhang*, Department of Chemistry, University of South Florida, Tampa, FL, USA

*Denotes lead author.

PART 1

PROPERTIES AND REACTIONS OF CARBENES

1

CARBENE STABILITY

SCOTT GRONERT

Department of Chemistry, Virginia Commonwealth University, Richmond, VA, USA

JAMES R. KEEFFE

Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, CA, USA

RORY A. MORE O’FERRALL

School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin, Ireland

1.1 INTRODUCTION

Assessing the stabilities of carbenes has been a richly rewarding, but frustrating endeavor in organic chemistry over the past several decades.1–9 The investigations have required heroic efforts in the synthesis of precursors, validation of carbene formation, and extraction of thermodynamic parameters. The field is studded with examples of elegant and sophisticated approaches to avoid the complications presented by these often highly reactive intermediates. The pursuit of some carbenes has been so extensive, controversial, and fraught with difficulty, that it offers material possibly better suited for a novel than a scientific review. In this chapter, space limitations do not allow for a full historical review and accounting of all the major contributions that have been made in the study of carbene stability. The review will focus on the latest or most accurate values that are available for selected species of fundamental interest (multiple values will be given if a consensus value is not available). As a result, some classical and seminal work may not be directly referenced in this chapter, but instead be found in the cited references. We will begin with a brief discussion of strategies for assessing carbene stability, and the experimental and computational methods that have been used, and then move directly to the stabilities of various classes of carbenes.

1.2 BACKGROUND

1.2.1 Measures of Carbene Stability

Although stability is a familiar concept in all of organic chemistry, describing it precisely can be problematic in practical applications because appropriate, universal reference states cannot always be defined. In some cases, such as carbanions, a single reaction process, protonation in this case, provides a stability measure, proton affinity (PA), which has been broadly embraced by the organic chemistry community. For carbocations, hydride affinities have also been widely used to characterize stability. This has not been true for carbenes, in part because they have varied reaction patterns, but also because they have not been amenable to some physical measurements. As a result, carbene stability has been described in a number of ways, including singlet–triplet energy gaps, reaction energetics such as hydrogenation energies, and kinetic reactivity. Each of these approaches probes a somewhat different aspect of carbene stability and generally correlates with a different reference state. In this chapter, the goal will be to provide a broad overview of these measures of stability, along with a very detailed analysis of the stability of fundamental species on the basis of computed hydrogenation energies.

When choosing a reference state for judging carbene stability, there are generally two considerations that drive the decision process. In the first approach, the decision is driven by a chemical process of interest. This is usually straightforward and can be determined on the basis of relative reaction energies or kinetics. However, this approach is specific to the process and may or may not answer the question of stability in the most general sense. For example, the singlet–triplet gap is a well-defined measure of the relative stability of electronic states of carbenes and offers insight into their spectral properties, but it has only indirect relevance to the ease of formation or the bond-forming reactivity of the carbene.

The second approach is to define a universal reference state that offers no strain or special stabilization. Heats of formation are a simple example of this approach, but they offer limited utility for comparative work because they scale with the molecular formula and therefore direct comparisons are only valid with isomers. A more practical application of this strategy is to relate stability to a model that lacks any of the strain or stabilization that is being probed in the target. These approaches often refer back to alkyl groups and/or employ isodesmic/homodesmotic reactions to extract the stabilization or strain energies.

In this chapter, we will focus on two measures of carbene stability. The primary measure will be hydrogenation energies for the conversion of the carbene center to a tetravalent carbon (Eq. 1.1). The motivations for this choice are: (i) the conversion generally eliminates special orbital interactions that potentially stabilize or destabilize the carbene; (ii) stabilization of singlet and triplet carbenes can be determined separately; and (iii) the reference states are often stable species with well-established thermochemistry.