Helicity in a molecule arises when the molecule contains a stereogenic axis instead of a stereogenic centre. In a molecule that is not inherently helically chiral, helicity can be induced by designing the molecule such that an unfavourable steric interaction, or strain, is present in its planar conformation. The release of this strain forces the molecule to adopt a helical twist against the cost of the torsional strain induced in the backbone, an interplay of forces, which must be balanced in favour of the helical conformation over the planar one. In this tutorial review, design principles that govern this process are analysed and the selected examples are categorised into three main (I, II and III) and two related (IV and V) classes, simply by their relation to one of the three types of helically twisted ribbons or two types of helically twisted cyclic ribbons, respectively. The presented examples were selected such that they illustrate their category in the best possible way, as well as based on availability of their solid-state structures and racemisation energy barriers. Finally, the relationship between the structure and properties is discussed, highlighting the cases in which induced helicity gave rise to unprecedented phenomena.
Carbon allotropes constituted of sp-hybridised carbon atoms display a variety of properties that arise from their delocalised π-conjugated electronic structure. Apart from carbon's planar allotropic form graphene, bent or curved structures, such as carbon nanotubes or fullerenes, respectively, have been discovered. In this Tutorial Review, we analyse and conceptually categorise chiral synthetic molecular fragments of non-planar sp-carbon allotropes, including hypothetical forms of carbon that have been proposed to exist as stable entities. Two types of molecular systems composed of equally or differently sized rings are examined: bent with zero Gaussian curvature and curved with positive or negative Gaussian curvature. To affirm that a system is chiral, two conditions must be fulfilled: (1) both reflective symmetry elements, an inversion centre and a mirror plane, must be absent and (2) the system must be stereochemically rigid. It is therefore crucial to not only consider the symmetry of a given system as if it was a rigid object but also its structural dynamics. These principles serve as guidelines for the design of molecular fragments that encode and transcribe chirality into larger systems.
Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that molecular rings with [4 n +2] π-electrons are aromatic, with an induced magnetisation that opposes the external field inside the ring, whereas those with 4 n π-electrons are antiaromatic, with the opposite magnetisation. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons ( n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel’s rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons ( n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule. The largest ring currents occur when the porphyrins units have fractional oxidation states.
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