Stereochemistry of Organic Compounds

Stereochemistry of Organic Compounds ... Resolving Agents 322 b. ... 14. CHIRALITY IN MOLECULES DEVOID OF CHIRAL CENTERS 1119...

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Stereochemistry of Organic Compounds ERNEST L ELIEL Department of Chemistry The University of North Carolina at Chapel Hill Chapel Hill, North Carolina

SAMUEL H. WILEN Department of Chemistry The City College of the City University of New York New York, New York With a Chapter on Stereoselective Synthesis by LEWIS N. MANDER Research School of Chemistry Australian National University Canberra, Australia

© A Wiley-Interscience Publication JOHN WILEY & SONS, INC. New York



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Contents

PREFACE

1. INTRODUCTION

1.1. Scope 1 1.2. History 2 1.3. Polarimetry and Optical Rotation 6 References 8 2. STRUCTURE

2.1. Meaning, Factorization, Internal Coordinates. Isomers 11 2.2. Constitution 15 2.3. Configuration 18 2.4. Conformation 20 2.5. Determination of Structure 24 2.6. A Priori Calculation of Structure 32 2.7. Molecular Models 40 References 42 3. STEREOISOMERS

3.1. Nature of Stereoisomers 49 a. General 49 b. Barriers between Stereoisomers. Residual Stereoisomers 54 3.2. Enantiomers 58 3.3. Diastereomers 62 a. General Cases 62 b. Degenerate Cases 65 References 69 4. SYMMETRY

4.1. Introduction 71 4.2. Symmetry Elements 71 4.3. Symmetry Operators. Symmetry Point Groups 74

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a. Point Groups Containing Chiral Molecules 76 b. Point Groups Containing Only Achiral Molecules 4.4. Desymmetrization 88 4.5. Averaged Symmetry 91 4.6. Symmetry and Molecular Properties 92 a. Rotation of Polarized Light 93 b. Dipole Moment 94 c. Symmetry Number 96 References 97

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5. CONFIGURATION

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5.1. Definitions: Relative and Absolute Configuration 101 5.2. Absolute Configuration and Notation 103 5.3. Determination of Absolute Configuration 113 a. Bijvoet Method 113 b. Theoretical Approaches 115 c. Modification of Crystal Morphology in the Presence of Additives 116 5.4. Relative Configuration and Notation 117 5.5. Determination of Relative Configuration of Saturated Aliphatic Compounds 124 a. X-Ray Structure Analysis 124 b. Chemical Interconversion Not Affecting Bonds to the Stereogenic Atom 126 c. Methods Based on Symmetry Considerations 128 d. Correlation via Compounds with Chiral Centers of Two Types 130 e. The Method of Quasi-racemates 132 f. Chemical Correlations Affecting Bonds to a Chiral Atom in a "Known" Way. 134 g. Correlation by Stereoselective Synthesis of "Known" Stereochemical Course 139 h. Chiroptical, Spectroscopic, and Other Physical Methods 144 5.6. Conclusion: Network Arguments 147 References 147

6. PROPERTIES OF STEREOISOMERS. STEREOISOMER DISCRIMINATION 6.1. 6.2. 6.3. 6.4.

Introduction 153 Stereoisomer Discrimination 153 The Nature of Racemates 159 Properties of Racemates and of Their Enantiomer Components 162 a. Introduction 162 b. Optical Activity 163 c. Crystal Shape 164

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d. Density and Racemate Type 165 e. Melting Point 167 f. Solubility 173 g. Vapor Pressure 179 h. Infrared Spectra 183 i. Electronic Spectra 184 j . Nuclear Magnetic Resonance Spectra 185 k. X-Ray Spectra 186 1. Other Physical Properties 187 m. Liquid State and Interfacial Properties 189 n. Chromatography 194 o. Mass Spectrometry 197 p. Interaction with Other Chiral Substances 197 q. Biological Properties 201 r. Origins of Enantiomeric Homogeneity in Nature 209 6.5. Determination of Enantiomar and Diastereomer Composition 214 a. Introduction 214 b. Chiroptical Methods 217 c. NMR Methods Based on Diastereotopicity 221 d. Chromatographie and Related Separation Methods Based on Diastereomeric Interactions 240 e. Kinetic Methods 265 f. Calorimetric Methods 268 g. Isotope Dilution 269 h. Miscellaneous Methods 272 References 275

7. SEPARATION OF STEREOISOMERS. RESOLUTION. RACEMIZATION 7.1. Introduction 297 7.2. Separation of Enantiomers by Crystallization 298 a. Crystal Picking. Triage 298 b. Conglomerates 299 c. Preferential Crystallization 304 d. Preferential Crystallization in the Presence of Additives 311 e. Asymmetrie Transformation of Racemates. Total Spontaneous Resolution 315 7.3. Chemical Separation of Enantiomers via Diastereomers 322 a. Formation and Separation of Diastereomers. Resolving Agents 322 b. Resolution Principles and Practice 344 c. Separation via Complexes and Inclusion Compounds 351 d. Chromatographie Resolution 359

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7.4. 7.5.

7.6.

7.7. 7.8.

e. Asymmetrie Transformations of Diastereomers 364 f. General Methods for the Separation of Diastereomers 374 Enantiomeric Enrichment. Resolution Strategy 381 Large Scale Resolution 388 a. Diastereomer-Mediated Resolution 389 b. Resolution by Preferential Crystallization 392 c. Kinetic Resolution 394 Kinetic Resolution 395 a. Theory. Stoiehiometric and Abiotic Catalytic Kinetic Resolution 395 b. Enzymatic Resolution 409 Miscellaneous Separation Methods 416 a. Partition in Heterogeneous Solvent Mixtures 416 b. Transport across Membranes 421 Racemization 424 a. Racemization Processes 426 b. Racemization of Amino Acids 436 References 440

8. HETEROTOPIC LIGANDS AND FACES (PROSTEREOISOMERISM, PROCHIRALITY) 8.1. Introduction. Terminology 465 8.2. Significance. History 467 8.3. Homotopic and Heterotopic Ligands and Faces 470 a. Homotopic Ligands and Faces 470 b. Enantiotopic Ligands and Faces 473 c. Diastereotopic Ligands and Faces 477 d. Concepts and Nomenclature 482 8.4. Heterotopicity and Nuclear Magnetic Resonance 488 a. General Principles. Anisochrony 488 b. NMR in Assignment of Configuration and of Descriptors of Prostereoisomerism 492 c. Origin of Anisochrony 499 d. Conformationally Mobile Systems 502 e. Spin Coupling Nonequivalence (Anisogamy) 507 8.5. Heterotopic Ligands and Faces in Enzyme-Catalyzed Reactions 508 a. Heterotopicity and Stereoelective Synthesis 508 b. Heterotopicity and Enzyme-Catalyzed Reactions 509 8.6. Pro2-Chiral Centers: Chiral Methyl, Phosphate, and Sulfate Groups 518 a. Chiral Methyl Groups 518 b. Chiral Phosphate Groups 526 c. Chiral Sulphate Groups 529 d. Pro3-Chiral Centers: The Chiral Thiophosphate Group 530 References 532

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9. STEREOCHEMISTRY OF ALKENES 9.1. Structure of Alkenes. Nature of cis-trans Isomerism 539 a. General 539 b. Nomenclature 541 c. Cumulenes 543 d. Alkenes with Low Rotational Barriers; Nonplanar Alkenes 544 e. The C=N and N=N Double Bonds 550 9.2. Determination of Configuration of cis-trans Isomers 555 a. Chemical Methods 555 b. Physical Methods 562 9.3. Interconversion of cis-trans Isomers: Position of Equilibrium and Methods of Isomerization 574 a. Position of cis-trans Equilibria 574 b. Methods of Equilibration 578 c. Directed cis-trans Interconversion 584 References 590 10. CONFORMATION OF ACYCLIC MOLECULES 10.1. Conformation of Ethane, Butane, and Other Simple Saturated Acyclic Molecules 597 a. Alkanes 597 b. Saturated Acyclic Molecules with Polar Substituents or Chains. The Anomeric Effect 606 10.2. Conformation of Unsaturated Acyclic and Miscellaneous Compounds 615 a. Unsaturated Acyclic Compounds 615 b. Alkylbenzenes 624 c. Miscellaneous Compounds 627 10.3. Diastereomer Equilibria in Acyclic Systems 629 10.4. Physical and Spectral Properties of Diastereomers and Conformers 634 a. General 634 b. Dipole Moments 635 c. Boiling Point, Refractive Index, and Density 638 d. Infrared Spectra 639 e. NMR Spectroscopy 641 10.5. Conformation and Reactivity: The Winstein-Holness Equation and the Curtin-Hammett Principle 647 References 656

11. CONFIGURATION AND CONFORMATION OF CYCLIC MOLECULES 11.1. Stereoisomerism and Configurational Nomenclature of Ring Compounds 665

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11.2. Determination of Configuration of Substituted Ring Compounds 669 a. Introduction 669 b. Symmetry-Based Methods 669 c. Methods Based on Physical and Chemical Properties 671 d. Correlation Methods 675 11.3. Stability of Cyclic Molecules 675 a. Strain 675 b. Ease of Cyclization as a Function of Ring Size 678 c. Ease of Ring Closure as a Function of the Ring Atoms and Substituents. The Thorpe-Ingold Effect 682 d. Baldwin's Rules 684 11.4. Conformational Aspects of the Chemistry of SixMembered Ring Compounds 686 a. Cyclohexane 686 b. Monosubstituted Cyclohexanes 690 c. Disubstituted and Polysubstituted Cyclohexanes 700 d. Conformation and Physical Properties in Cyclohexane Derivatives 709 e. Conformation and Reactivity in Cyclohexanes 720 f. sp2 Hybridized Cyclohexyl Systems 726 g. Six-Membered Saturated Heterocycles 740 11.5. Chemistry of Ring Compounds Other Than SixMembered Ones 754 a. Three-Membered Rings 754 b. Four-Membered Rings 755 c. Five-Membered Rings 758 d. Rings Larger Than Six-Membered 762 e. The Concept of I Strain 769 11.6. Stereochemistry of Fused, Bridged, and Caged Ring Systems 771 a. Fused Rings 771 b. Bridged Rings 787 c. Paddlanes and Propellanes 794 d. Catenanes, Rotaxanes, Knots, and Möbius Strips 800 e. Cubane, Tetrahedrane, Dodecahedrane, Adamantane, and Buckminsterfullerene 806 References 811 12. STEREOSELECTIVE SYNTHESIS

12.1. Introduction 835 a. Terminology 837 b. Stereoselective Synthesis 838 c. Categories of Stereoselective Synthesis 839 d. Convergent Syntheses 843

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12.2. Diastereoselective Synthesis of Achiral Compounds 845 a. Cyclanes 845 b. Diastereoselective Syntheses of Alkenes 846 12.3. Diastereoselective Synthesis 858 a. Introduction 858 b. Strategies for Stereocontrol in Diastereoselective Synthesis 859 c. Diastereoselective Syntheses Based on Chiral Substrates of Natural Origin 872 d. Nucleophilic Additions 875 e. Electrophilic Reactions of Alkenes 894 f. The Aldol Reaction 913 g. Pericyclic Reactions 920 h. Catalytic Hydrogenations 932 i. Free Radical Reactions 935 12.4. Enantioselective Syntheses 939 a. Introduction 939 b. Enantioselective Syntheses with Chiral Nonracemic Reagents 941 c. Enantioselective Reactions with Chiral Nonracemic Catalysts 947 d. Nonlinear Effects in Catalysis 959 e. Enzyme Based Processes 960 f. Enantioselective Synthesis Involving Discrimination between Enantiotopic Groups 962 g. Enantioconvergent Syntheses 963 12.5. Double Stereodifferentiation 965 a. Introduction 965 b. Interactions between Principal Chiral Reactants 967 c. Reagent Control 969 d. Kinetic Amplification 970 12.6. Conclusion 971 References 971

13. CHIROPTICAL PROPERTIES 13.1. Introduction 991 13.2. Optical Activity. Anisotropie Refraction 992 a. Origin. Theory 992 b. Optical Rotatory Dispersion 999 13.3. Circular Dichroism. Anisotropie Absorption 1003 13.4. Applications of Optical Rotary Dispersion and Circular Dichroism 1007 a. Determination of Configuration and Conformation. Theory 1007 b. Classification of Chromophores 1013 c. Sector and Helicity Rules 1019

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d. Exciton Chirality 1043 e. Other Applications. Induced ORD and CD 1050 f. Fluorescence Detected Circular Dichroism 1059 g. Circular Dichroism of Chiral Polymers 1060 13.5. Applications of Optical Activity 1071 a. Polarimetry 1071 b. Empirical Rules and Correlations. Calculation of Optical Rotation 1080 13.6. Vibrational Optical Activity 1093 13.7. Circular Polarization of Emission. Anisotropie Emission 1100 References 1105 14. CHIRALITY IN MOLECULES DEVOID OF CHIRAL CENTERS 14.1. Introduction. Nomenclature 1119 14.2. Allenes 1122 a. Historical. Natural Occurrence 1122 b. Synthesis of Optically Active Allenes 1124 c. Determination of Configuration and Enantiomeric Purity of Allenes 1125 d. Cyclic Allenes, Cumulenes, Ketene Imines 1132 14.3. Alkylidenecycloalkanes 1133 14.4. Spiranes 1138 14.5. Biphenyls. Atropisomerism 1142 * a. Introduction 1142 b. Biphenyls and Other Atropisomers of the sp2-sp2 Single-Bond Type 1143 c. Atropisomerism about sp2-sp3 Single Bonds 1150 d. Atropisomerism about sp3-sp3 Bonds 1153 14.6. Molecular Propellers and Gears 1156 a. Molecular Propellers 1156 b. Gears 1160 14.7. Helicenes 1163 14.8. Molecules with Planar Chirality 1166 a. Introduction 1166 b. Cyclophanes 1166 c. Annulenes 1170 d. trans-Cycloalkenes 1172 e. Metallocenes and Related Compounds 1175 14.9. Cyclostereoisomerism 1176 References 1181

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GLOSSARY

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INDEX

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