The Cationminus signpi Interaction.

Cations bind to the π face of an aromatic structure through a surprisingly strong, noncovalent force termed the cation-π interaction. The magnitude and generality of the effect have been established by gas-phase measurements and by studies of model receptors in aqueous media. To first order, the interaction can be considered an electrostatic attraction between a positive charge and the quadrupole moment of the aromatic. A great deal of direct and circumstantial evidence indicates that cation-π interactions are important in a variety of proteins that bind cationic ligands or substrates. In this context, the amino acids phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) can be viewed as polar, yet hydrophobic, residues.

https://science.sciencemag.org/content/sci/271/5246/163.full.pdf

Cation-π Interactions in Chemistry and Biology: A New View of Benzene, Phe, Tyr, and Trp

The nature and characteristics of the CH/π interaction are discussed by comparison with other weak molecular forces such as the CH/O and OH/π interaction. The CH/π interaction is a kind of hydrogen bond operating between a soft acid CH and a soft base π-system (double and triple bonds, C6 and C5 aromatic rings, heteroaromatics, convex surfaces of fullerenes and nanotubes). The consequences of CH/π hydrogen bonds in supramolecular chemistry are reviewed on grounds of recent crystallographic findings and database analyses. The topics include intramolecular interactions, crystal packing (organic and organometallic compounds), host/guest complexes (cavity-type inclusion compounds of cyclodextrins and synthetic macrocyclic hosts such as calixarenes, catenanes, rotaxanes and pseudorotaxanes), lattice-inclusion type clathrates (including liquid crystals, porphyrin derivatives, cyclopentadienyl compounds and C60 fullerenes), enantioselective clathrate formation, catalytic enantioface discriminating reactions and solid-state photoreaction. The implications of the CH/π concept for crystal engineering and drug design are evident.

CH/π hydrogen bonds in crystals

Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

Hydrogen bonds are like human beings in the sense that they exhibit typical grouplike behavior. As an individual they are feeble, easy to break, and sometimes hard to detect. However, when acting together they become much stronger and lean on each other. This phenomenon, which in scientific terms is called cooperativity, is based on the fact that 1+1 is more than 2. By using this principle, chemists have developed a wide variety of chemically stable structures that are based on the reversible formation of multiple hydrogen bonds. More than 20 years of fundamental studies on these phenomena have gradually developed into a new discipline within the field of organic synthesis, and is nowadays called noncovalent synthesis. This review describes noncovalent synthesis based on the reversible formation of multiple hydrogen bonds. Starting with a thorough description of what the hydrogen bond really is, it guides the reader through a variety of bimolecular and higher order assemblies and exemplifies the general principles that determine their stability. Special focus is given to reversible capsules based on hydrogen-bonding interactions that exhibit interesting encapsulation phenomena. Furthermore, the role of hydrogen-bond formation in self-replicating processes is actively discussed, and finally the review briefly summarizes the development of novel materials (nanotubes, liquid crystals, polymers, etc.) and principles (dynamic libraries) that recently have emanated from this intriguing field of research.

Noncovalent synthesis using hydrogen bonding

RACEMATES AND THEIR ENANTIOMER CONSTITUENTS Types of Crystalline Racemates Binary Mixtures of Enantiomers Solution Properties of Enantiomers and Their Mixtures RESOLUTION OF ENANTIOMER MIXTURES Resolution by Direct Crystallization Formation and Separation of Diastereomers Crystallization-Induced Asymmetric Transformations Experimental Aspects and Art of Resolutions Index

Enantiomers, Racemates, and Resolutions

The potential interest of vanillin, isovanillin, thiovanillin, and isothiovanillin as starting units for the design of hyperpolarizable molecules such as polyenovanillin (Ar-(CH=CH) n -CH=O) and bisvanillin derivatives was investigated by a CNDO/S method using a specific program that allowed the calculation of all the components of the first-ordcr molecular hyperpolarizability tensor β in nonpolar solvents. The reliability of the calculations was assesscd by comparison with EFISH measurements on model systems. Among the vanillin derivatives, thiovanillin, where the sulfur group is in para to the aldehyde group, showed the largest hyperpolarizability, β

Theoretical and experimental investigations of the nonlinear optical properties of vanillin, polyenovanillin, and bisvanillin derivatives

Optical resolution by direct crystallization of enantiomer mixtures

A l'aide de la spectrometrie RMN, on etudie le dedoublement optique du bromochlorofluoromethane qui est inclus dans une cavite de compose chiral le cryptophane C

Analytical optical resolution of bromochlorofluoromethane by enantioselective inclusion into a tailor-made cryptophane and determination of its maximum rotation

André Collet

Papers

Enantiomers, Racemates, and Resolutions

Theoretical and experimental investigations of the nonlinear optical properties of vanillin, polyenovanillin, and bisvanillin derivatives

Optical resolution by direct crystallization of enantiomer mixtures

Analytical optical resolution of bromochlorofluoromethane by enantioselective inclusion into a tailor-made cryptophane and determination of its maximum rotation

Remarkable effect of the receptor size in the binding of acetylcholine and related ammonium ions to water-soluble cryptophanes