Chirality in Supramolecular Assemblies

Causes and Consequences

F. Richard Keene

• Publisher: Wiley
• Year: 2017
• Print ISBN: 9781118867341
• eBook ISBN: 9781118867310
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

Supramolecular chemistry deals with the organisation of molecules into defined assemblies using non-covalent interactions, including weaker and reversible interactions such as hydrogen bonds, and metal-ligand interactions.  The aspect of stereochemistry within such chemical architectures, and in particular chirality, is of special interest as it impacts on considerations of molecular recognition, the development of functional materials, the vexed question of homochirality, nanoscale effects of interactions at interfaces, biocatalysis and enzymatic catalysis, and applications in organic synthesis.

Chirality in Supramolecular Assemblies addresses many of these aspects, presenting a broad overview of this important and rapidly developing interdisciplinary field. Topics covered include:

• Origins of molecular and topological chirality
• Homochirogenesis
• Chirality in crystallinity
• Host-guest behavior
• Chiral influences in functional materials
• Chirality in network solids and coordination solids
• Aspects of chirality at interfaces
• Chirality in organic assemblies
• Chirality related to biocatalysis and enzymes in organic synthesis.

This book is a valuable reference for researchers in the molecular sciences, materials science and biological science working with chiral supramolecular systems. It provides summaries and special insights by acknowledged international experts in the various fields.

1
Principles of Molecular Chirality

Jean‐Claude Chambron and F. Richard Keene

1.1 General Introduction

Chirality is probably one of the most significant topics in chemistry. The strong connection between chirality and symmetry has made it appealing from the mathematical and aesthetic viewpoints, and the recent interest in topologically chiral interlocked and knotted molecules has increased its intellectual attraction, raising the concept of a hierarchy in chirality [1]. The most fascinating aspect of chirality stems from the dynamic properties of molecules and supramolecular assemblies, rather than their static properties, because they are the cause of many intriguing and sometimes paradoxical issues. At the same time, dynamic chirality is also the most useful topic because of the numerous applications it underpins, from chiral recognition to molecular motors.

Historically, chirality is rooted into crystallography (the concept of hemiedry), and the first breakthrough into the field of molecular chirality was Louis Pasteur’s hypothesis that the dissymmetry of a crystal was a consequence of dissymmetry at the molecular level [2]. The second milestone was the Le Bel and van’t Hoff model of the tetrahedral carbon atom, which accounted for the chirality of the organic compounds known at that time, and several years later Werner was the first to study and provide evidence for the chirality of metal complexes. The discovery of organic molecules that did not owe their chirality to tetrahedral carbon atoms carrying four different substituents (e.g., allenes, biphenyls, cyclophanes), and of helical structures in nucleic acids and proteins, finally led Cahn, Ingold, and Prelog to establish a general system for the description of chiral structures. Since then, many novel chiral molecules have been reported, and most of them could be described in the frame of the CIP rules. The most notable developments in chirality in recent decades concern aspects of the generation and control of chirality: transfer by supramolecular interactions; chirality of molecular assemblies (chirality at the supramolecular level or “supramolecular chirality”); and finally, the concept of “topological chirality” brought forward by the development of interlocked and knotted molecules.

This chapter constitutes an introduction to molecular chirality from the rigid geometrical model to the topological model, but also from the isolated molecule to assemblies of molecules. As the first chapter in this book on the causes and consequences of chirality in supramolecular assemblies, it will, nevertheless, not cover all the aspects of chirality transfer – in particular those resulting from a covalent bond formation.

1.2 Geometrical Chirality

A chiral object is the one that does not coincide with its mirror image. The source object and its mirror image are called enantiomorphs. From the point of view of symmetry, enantiomorphic objects can have only rotation axes C_n, n ≥ 1, as symmetry elements: they are either asymmetric (C₁) or dissymmetric (C_n, n ≠ 1). There are many natural examples of enantiomorphic objects, the prototypical one being the human hand, the Greek word for which (χειρ) has been used to create the English word “chiral.” Molecules are objects at the nanometer scale that are made of atoms connected by chemical bonds. If molecules are considered as rigid nanoscale objects, the definition given above can be very easily transposed to the molecular level, with the term “enantiomorph” being replaced by “enantiomer.” However, molecules differ from macroscopic objects according to two criteria: (i) they are not rigid and can encompass a great variety of shapes called conformations, the distribution of which depends on time, temperature, and solvent; (ii) they are not usually handled as a single object, but as populations of very large number of individuals (~ Avogadro number). These two unique characteristics make the definition of molecular chirality not as simple as that of a rigid object (such as a quartz crystal), and therefore it needs further developments in order to be refined [2].

The object molecule can be described at different levels of complexity, which are represented by models [3, 4]. The chemical formula, which uses atomic symbols for the atoms and lines for the bonds (traditionally, dashed lines for the weakest bonds), is no more than what has been termed a molecular graph, a concept derived from mathematics that has been introduced and used fruitfully in various areas of chemistry, in particular in molecular topology (see section 1.3). The structural formula is more informative because it shows the spatial relationships between the atoms and the bonds, which can be, for example, probed by nOe effects in NMR spectroscopy. The most accomplished description of the molecule as a rigid object is the 3D representation resulting from an X‐ray crystal structure analysis, as it gives the distances between the ato...

Table of contents

• Cover
• Title Page
• Table of Contents
• List of Contributors
• Preface
• 1 Principles of Molecular Chirality
• 2 Homochirogenesis and the Emergence of Lifelike Structures
• 3 Aspects of Crystallization and Chirality
• 4 Complexity of Supramolecular Assemblies
• 5 Chirality in the Host‐Guest Behaviour of Supramolecular Systems
• 6 Chiral Influences in Functional Molecular Materials
• 7 Chirality in Network Solids
• 8 Chiral Metallosupramolecular Polyhedra
• 9 Chirality at the Solution/Solid‐State Interface
• 10 Nanoscale Aspects of Chiral Nucleation and Propagation
• 11 Chirality in Organic Hosts
• 12 Chirality Related to Biocatalysis and Enzymes in Organic Synthesis
• Index
• End User License Agreement