AROMATICITY AND ANTIAROMATICITY

Electronic and Structural Aspects

VLADIMIR I. MINKIN MIKHAIL N. GLUKHOVTSEV BORIS YA. SIMKIN

® A Wiley-Interscience Publication

JOHN WILEY & SONS, INC. New York • Chichester • Brisbane • Toronto • Singapore

CONTENTS

Preface ix

List of Abbreviations xi

1 Introduction 1

References, 5

2 Criteria of Aromaticity and Antiaromaticity 6

2.1 Demands on these Criteria, 6 2.2 Energetic Criteria, 7

2.2.1 Various Types of Resonance Energies, 7 2.2.2 Hückel Resonance Energy, 10 2.2.3 Dewar's Resonance Energy, 11 2.2.4 Hess-Schaad Resonance Energies, 12 2.2.5 Topological Resonance Energy, 14 2.2.6 Conjugated Circuits Model, 19 2.2.7 Structure-Resonance Theory and Some Related Models, 24 2.2.8 PMO MethodoftheDewarResonance Energy Estimation, 28 2.2.9 Estimation of Energies of Aromatic Stabilization and

Antiaromatic Destabilization from the Energies of Isodesmic, Homodesmotic, and Hyperhomodesmotic Reactions, 29

2.2.10 Comparison Between Energetic Criteria, 35 2.3 Structural Criteria, 38

2.3.1 Distinguishing Characteristics in the Geometry of Aromatic and Antiaromatic Molecules, 38

V

CONTENTS

2.3.2 Indices of Aromaticity Based on Estimates of Bond Length Alternation in a Ring, 48

2.3.3 Aromaticity Gauged by Stability ofa High-Symmetry Structure Against Distortion into Kekule-Type Structures with Bond Alternation, 54

2.3.4 Effects of Nonplanarity, 60 2.4 Magnetic Criteria, 63

2.4.1 Concept of the Ring Current, 63 2.4.2 Magnetic Susceptibility Exaltation and Anisotropy, 67 2.4.3 Nuclear Magnetic Resonance Chemical Shifts, 71

2.5 Other Criteria of Aromaticity and Antiaromaticity, 74 2.5.1 Retention of the Structural Type, 75

2.5.1.1 Empirical Resonance Energies, 75 2.5.1.2 Reactivity Indices as a Measure of Aromaticity, 80

2.5.2 Specific Features of the Electron Distribution, 85 2.5.3 Anisotropie Optical Polarizability as an Index of Aromaticity, 88

2.6 Interrelation Between Various Types of Aromaticity Indices, 89 References, 92

3 Delocalization Modes and Electron-Count Rules 104

3.1 Aromaticity Types Stemming from the Ribbon Delocalization in a Cyclic System with Planar or Distorted Planar Geometry of a Ring, 104

3.2 Aromaticity Types Due to Surface Delocalization, 114 3.3 Volume Delocalization, 115 3.4 Electron-Count Rules: The Hückel Rule, 116

3.4.1 Formulation of the Hückel Rule, 117 3.4.2 Relation of the Hückel Rule with the Energetic and Magnetic

Criteria of Aromaticity and with the Reactivity of Cyclic Conjugated Molecules, 121

3.4.3 Limits of Applicability of the Hückel Rule, 123 3.4.4 The Generalized Hückel Rule, 127

References, 130

4 Annulenes, Monocyclic Conjugated Ions, and Annulenoannulenes 133

4.1 Annulenes, 133 4.1.1 Benzene and Cyclobutadiene, 133

4.1.1.1 Ground-State Structures and Their Thermal Automerizations, 133

4.1.1.2 The Excited States: Structures and Aromaticity, 142 4.1.1.3 Stability Relative to Valence Isomers, 149 4.1.1.4 Dications and Dianions of Benzene and

Cyclobutadiene, 157 4.1.2 Cyclooctatetraene and [10]Annulene, 165

CONTENTS vii

4.1.2.1 Structures oftheGround and Lowest Excited States, 165 4.1.2.2 COT's Dication and Dianion, 173

4.1.3 [18]Annulene, 175 4.2 Monocyclic Aromatic and Antiaromatic Ions, 177

4.2.1 Cyclopropenyl Cation and Cyclopropenide Anion, 177 4.2.2 Cyclopentadienyl Cation and Cyclopentadienide Anion, 183 4.2.3 Tropylium Cation and Cycloheptatrienide Anion, 186 4.2.4 Cyclononatetraenyl Cation and Cyclononatetraenide Anion, 188

4.3 Annulenoannulenes, 190 4.3.1 [3]Annuleno[3]annulene (Propalene), 192 4.3.2 [4]Annuleno[4]annulene (Butalene) and [3]Annuleno[5]-

annulene, 195 4.3.3 [5]Annuleno[5]annulene (Pentalene), [4]Annuleno[6]annulene,

and [3]Annuleno[7]annulene, 196 4.3.4 [6]Annuleno[6]annulene (Naphthalene), [5]Annuleno[7]-

annulene (Azulene), and [4]Annuleno[8]annulene, 201 References, 207

5 Heteroaromaticity 217

5.1 General Trends Observed in the Change of Aromatic Character due to Heterosubstitution, 217

5.2 Aromaticity and the Trend Toward Pyramidalization of the Heteroatom, 225

References, 227

6 Homoaromaticity 230

6.1 General Outline, 230 6.2 Qualitative Approaches, 232 6.3 Homoaromatic Cations, 235

6.3.1 Homotropylium Cation, 235 6.3.1.1 Structure of the Homotropylium Cation, 235 6.3.1.2 Energetic Criteria, 239

6.3.2 Other Homoaromatic Cations: Does Homoantiaromaticity Exist?, 241

6.4 Does Homoaromaticity Take Place for Anions?, 243 6.5 Can Neutral Molecules Manifest Homoaromaticity?, 246 References, 249

7 <7-Aromaticity 252

7.1 (T-Delocalization and cr-Conjugation, 252 7.2 cr-Aromaticity, 254

7.2.1 Cyclopropane: The Energy of Its cr-Aromatic Stabilization, 255 7.2.2 Cyclopropane: The Surface er-Delocalization, 259

V l l l CONTENTS

7.2.3 Cyclopropane and Benzene as Aromatic Analogs, 260 7.2.4 (T-Aromaticity of Five-Membered Ring Systems, 262

References, 264

8 In-Plane and Radial Aromaticity 267

References, 277

9 Three-Dimensional Aromaticity 279

9.1 Resonance Energies of Polyhedral Organometallics, 279 9.2 Pyramidal Organic Molecules and Ions, 281 9.3 Three-Dimensional Aromaticity and Electron Counting

in Clusters, 291 References, 294

10 Spherical Aromaticity 298

References, 303

11 Is the Physical Nature of Aromaticity Known? 305

References, 309

Index 311