Helicene-Based Chiral Auxiliaries and Chirogenesis

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.

Synthesis, crystal structures and properties of carbazole-based [6]helicenes fused with an azine ring

“…The steric strain releases by adopting either the P-or M-helix configuration. The helically extended π-conjugated system, axial chirality and associated with these structural peculiarities unique optical and electronic properties of helicenes have attracted scientific interest for decades [1][2][3][4][5][6][7][8][9][10][11]. Compared to other planar π-conjugated systems, helicenes are more thermally stable and soluble in common organic solvents [9].…”

“…Besides typical carbohelicenes, heterohelicenes, incorporating one or more heteroaromatic units in the skeleton, have also gained increasing attention [1][2][3][4][5][6][7][8][9][10][11]. The presence of heteroatoms (S, N, O, P) in the fused polycyclic π-systems additionally contributes to altering electronic structure and helps to fine tune optoelectronic properties [1][2][3][4][5][6][7][8][9][10][11]20,[35][36][37]. The last decades highlighted heterohelicenes, incorporating one or two carbazole fragments, as a very attractive class of molecules [38][39][40][41][42][43][44][45][46][47][48][49][50][51].…”

Synthesis and X‐ray diffraction study of a chiral bis‐phosphahelicene palladium (II) complex

“…After 2 hours stirring at rt, the solvent was removed, and the residue was purified by flash chromatography over silica gel (Petroleum ether/EtOAc, 80:20) to afford the palladium complex (S P ,S P ,P,P)-4 as a yellow solid (Rf = 0.5, 16 mg, 0.012 mmol, 35% yield over two steps). 1 ; Mp = 240°C. X-ray structure of (S P ,S P ,P,P)-4.…”

“…Yellow crystals were grown from CHCl 3 by slow evaporation. (S P ,S P ,P,P)-4: crystal size 0.40 × 0.17 × 0.05 mm, orthorhombic, P2 1 CCDC 1908942-1908943 (for (R P ,P)-2a and (S P ,S P ,P,P)-4, respectively) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.…”

“…Helicene and heterohelicene derivatives have found applications in a number of fields, related to their intrinsic chirality and unique chiroptical properties. [1][2][3] Among others, heterohelicenes with nitrogen and phosphorus functions have been used successfully as both enantioselective organocatalysts and chiral ligands in organometallic catalysis. [4][5][6] In this last field, the role of transition metal complexes of heterohelicenes is crucial, irrespective of whether they are generated in situ or handled as isolated pre-catalysts.…”