Rotation about the Carbon-Carbon single bond is generally regarded to be unrestricted. About 50 years ago, rotational isomers were found to exhibit optical activity: substituted biphenyl derivatives. However, the author has investigated other classes of compounds that have stable rotational isomers at room temperature. These compounds can serve as models for studying very weak intermolecular interactions, that are difficult to observe. Even diastereotopic groups show different chemical reactivity. From the contents: The "Free Rotation Concept" - Rotamer Populations - Barriers to Rotation - Reactivity of Rotational Isomers.

Inhalt

1 Introduction.- 1.1 The "Free Rotation" Concept.- 1.2 Recognition of Rotational Isomers.- 1.3 Atropisomers.- 1.4 Isolation of Rotational Isomers.- 1.4.1 Factors Affecting Barrier Heights.- 1.4.2 Triptycene Derivatives.- 1.4.3 9-Arylfluorenes.- 1.5 Concepts That Need Modification.- 1.5.1 Are There Dextrorotatory and Levorotatory Groups?.- 1.5.2 Reactivity of Rotational Isomers.- 1.6 References.- 2 Rotamer Populations.- 2.1 Estimation of Rotamer Populations.- 2.1.1 Vibration Spectroscopy.- 2.1.2 Nuclear Magnetic Resonance Spectroscopy.- 2.2 Factors That Affect Rotamer Populations.- 2.3 Information on Molecular Interactions Obtained from Rotamer Populations.- 2.3.1 Triptycenes.- 2.3.1.1 Long-Range Coupling in NMR Spectra.- 2.3.1.2 Steric Repulsion.- 2.3.1.3 n-?* Interactions.- 2.3.1.4 n-?* Interactions.- 2.3.1.5 Hydrogen Bonding Involving a Methyl Group.- 2.3.2 9-Arylfluorenes.- 2.3.2.1 XH-? Interactions.- 2.3.2.2 ?-?* Interactions.- 2.4 References.- 3 Barriers to Rotation.- 3.1 Estimation of Barriers to Rotation.- 3.1.1 Coalescence Method.- 3.1.2 Line Shape Analysis.- 3.1.3 Saturation Transfer.- 3.1.4 The 2D-NMR Spectroscopy.- 3.1.5 Classical Kinetics.- 3.1.6 Reliability of the Data.- 3.2 High Barriers to Rotation in the 9-Arylfluorene Series.- 3.2.1 Isolation of Rotational Isomers.- 3.2.2 Factors Affecting the Rotational Barriers.- 3.2.2.1 Effects of the Steric Size of the Substituents.- 3.2.2.2 Ground State Destabilization.- 3.2.2.3 Ground State Stabilization.- 3.2.2.4 Solvent Effects.- 3.3 High Barriers to Rotation in 9-Substituted Triptycene Series.- 3.3.1 Triptycenes Carrying a 9-tert-Alkyl Substituent.- 3.3.2 Triptycenes Carrying a 9-sec-Alkyl Substituent.- 3.3.3 Triptycenes Carrying a 9-prim-Alkyl Substituent.- 3.3.4 Atropisomers About a Carbon-to-Heteroatom Bond.- 3.4 High Barriers to Rotation in Miscellaneous Compounds.- 3.4.1 Atropisomerism About an sp3-sp3 Bond.- 3.4.2 Atropisomerism About an sp3-sp2 Bond.- 3.5 References.- 4 Reactivity of Rotational Isomers.- 4.1 Reactivity of 9-Arylfluorenes.- 4.1.1 Additions and Eliminations.- 4.1.1.1 Additions to Olefins.- 4.1.1.2 Addition to Carbonyl Groups.- 4.1.1.3 Elimination Reactions.- 4.1.2 Substitution Reactions.- 4.1.3 Acid-Base Properties.- 4.1.3.1 Acids and Bases in Substituents of the Aryl Group.- 4.1.3.2 Deprotonation at the 9-Position of Fluorene.- 4.1.4 Other Types of Reaction.- 4.1.4.1 Radical Reactions of 9-Arylfluorene Rotamers.- 4.1.4.2 Nitrenes and Carbenes in Arylfluorene Rotamers.- 4.2 Reactivity of Substituted Triptycenes.- 4.2.1 Radical-Forming Reactions.- 4.2.1.1 Substitution of a tert-Butyl Group and Related Reactions.- 4.2.1.2 Thermolysis of Peroxyesters.- 4.2.2 Cation-Forming Reactions.- 4.2.2.1 Lewis Acid-Catalyzed Ionization of Halogen Compounds.- 4.2.2.2 Intramolecular Friedel-Crafts Acylation.- 4.2.2.3 Diazotization of Amines.- 4.2.3 Other Types of Reactions.- 4.2.3.1 Reactions of Chlorides with tert-Butyllithium.- 4.2.3.2 Chlorodecarboxylation.- 4.2.3.3 Bromine Addition Reactions.- 4.3 References.