Abstract
Helicenes are unique helical chromophores possessing advanced and well-controlled spectral and chemical properties owing to their diverse functionalization and defined structures. Specific modification of these molecules by introducing aromatic rings of differing nature and different functional groups results in special chiroptical properties, making them effective chiral auxiliaries and supramolecular chirogenic hosts. This review aims to highlight these distinct structural features of helicenes; the different synthetic and supramolecular approaches responsible for their efficient chirality control; and their employment in the chirogenic systems, which are still not fully explored. It further covers the limitation, scope, and future prospects of helicene chromophores in chiral chemistry.
Highlights
It is of note that, whilst there is a great variety of aromatic structures used in different chiral fields [1], helicenes still lag behind, despite their attractive properties discussed below
It has been clearly demonstrated that helicene-based asymmetric organocatalysis is a highly promising and exciting field in terms of the enantioselectivity obtained with the lowest catalyst
It has been helicene-based asymmetric a highly promising and clearly excitingdemonstrated field in termsthat of the enantioselectivity obtainedorganocatalysis with the lowestiscatalyst promising and exciting field in terms of the enantioselectivity obtained with the lowest catalyst loading
Summary
Aromatic characteristics characteristicsin in organic compounds are interesting very interesting dueresonance to the resonance features, inter- and and interintramolecular π-π interactions along withalong the specific geometric electronicstability, features,and stability, and intramolecular π-π interactions with the specific properties and reactivity, making it an important topic for researchers. As the number of ortho-fused aromatic rings is increased beyond three units, such as in 4–6 (Figure 1), the space constraint in the molecule is enhanced, forcing it to units, such as in 4–6 (Figure 1), the space constraint in the molecule is enhanced, forcing it adopt a nonplanar helical arrangement. These types of aromatic structures are called helicenes. The diversity of helical aromatic structures makes helicenes into unique and advanced chromophoric systems with tunable electronic and steric properties for corresponding applications in all types of stereochemical processes, including chiral auxiliaries and supramolecular chirogenesis. It is of note that, whilst there is a great variety of aromatic structures used in different chiral fields [1], helicenes still lag behind, despite their attractive properties discussed below
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