Kryptoracemates

Racemic crystals normally crystallise in centrosymmetric spacegroups containing equal numbers of enantiomers. More rarely, racemates may crystallise in non-centrosymmetric space-groups having glide symmetry or, even more rarely, in space-groups devoid of a centre of inversion, having no rotary-inversion axes nor glide plane. The latter class of crystals form the subject of the present bibliographic review – a survey of kryptoracemic behaviour. The term kryptoracemic alludes to the presence of a hidden or non-crystallographic centre of inversion between two molecules that might otherwise be expected to crystallise in an achiral space-group, often about a centre of inversion. Herein, examples of molecules with stereogenic centres crystallising in one of the 65 Sohncke space-groups are described. Genuine kryptoracemates, i.e. crystals comprising only enantiomorphous pairs are described followed by an overview of non-genuine kryptoracemates whereby the crystal also contains other species such as solvent and/or counterions. A full range, i.e. one to six, stereogenic centres are noted in genuine kryptoracemates. Examples will also be described whereby there are more that one enantiomeric pair of molecules in the crystallographic asymmetric unit. A more diverse range of examples are available for non-genuine kryptoracemates. There are unbalanced species where in addition to the enantiomeric pair of molecules, there is another enantiomeric molecule present. There are examples of genuine co-crystals, solvated species and of salts. Finally, special examples will be highlighted where the counterions are chiral and where they are disparate, both circumstances promoting kryptoracemic behaviour

Increased Speed Of Rotation For The Smallest Light-driven Molecular Motor: Supporting information CIF-file for C28H24

In this paper we present the smallest artificial light-driven molecular motor consisting of only 28 carbon and 24 hydrogen atoms. The concept of controlling directionality of rotary movement at the molecular level by introduction of a stereogenic center next to the central olefinic bond of a sterically overcrowded alkene does not only hold for molecular motors with six-membered rings, but is also applicable to achieve the unidirectional movement for molecular motors having five-membered rings. Although X-ray analyses show that the five-membered rings in the cis-and trans-isomer of the new molecular motor are nearly flat, the energy differences between the (pseudo-)diaxial and (pseudo-)diequatorial conformations of the methyl substituents in both isomers are still large enough to direct the rotation of one-half of the molecule with respect to the other half in a clockwise fashion. The full rotary cycle comprises four consecutive steps: two photochemical isomerizations each followed by a thermal helix inversion. Both photochemical cis- transisomerizations proceed with a preference for the unstable diequatorial isomers over the stable diaxial isomers. The thermal barriers for helix inversion of this motor molecule have decreased dramatically compared to its six-membered ring analogue, the half-life of the fastest step being only 18 s at room temperature.

Unidirectional Light-Driven Molecular Motors Based on Overcrowded Alkenes

Over the last two decades, interest in nanotechnology has led to the design and synthesis of a toolbox of nanoscale versions of macroscopic devices and components. In molecular nanotechnology, linear motors based on rotaxanes and rotary motors based on overcrowded alkenes are particularly promising for performing work at the nanoscale. In this chapter, progress on light-driven molecular motors based on overcrowded alkenes is reviewed. Both the so-called first and second generation molecular motors are discussed, as well as their potential applications.</p

Stereochemistry of Bistricyclic Aromatic Enes and Related Polycyclic Systems

Bistricyclic aromatic enes (BAEs) and related polycyclic systems are a class of molecular materials that display a rich variety of conformations, dynamic stereochemistry and switchable chirality, color, and spectroscopic properties. This is due to the a subtle interplay of the inherent preference for planarity of aromatic systems and the competing necessity of non-planarity due to intramolecular overcrowding in the fjord regions built into the general molecular structure of BAEs. The conformational, dynamic, and spectroscopic properties may be designed and fine-tuned, e.g., by variation of the bridging groups X and Y, the overcrowding in the fjord regions, extensions of the aromatic system, or other modifications of the general BAE structure, based on the fundamental understanding of the structure-property relationships (SPR). The present review provides an analysis of the conformational spaces and the dynamic stereochemistry of overcrowded bistricyclic aromatic enes applying fundamental symmetry considerations. The symmetry analysis presented here allows deeper insight into the conformations, chirality, and the mechanisms of the dynamic stereochemistry, and will be instrumental in future computational studies