Helicenes: synthesis and applications.

Carbohelicenes generally incorporate a helical, distorted, conjugated, polyaromatic system with ortho-fused benzenoid rings, which is a fundamental molecular characteristic of this class of compounds. They have been described as “molecules in distress” due to their distortion. The generation of a chiral helicity in helicenes was observed because of a severe intramolecular steric strain. Helicity is a molecular necessity in the higher series of carbohelicenes, when at some point, a helical pitch occurs when a second coil is formed. The most interesting properties resulting from such molecular distortion are the very high chiroptical and circular dichroism values. For instance, the resolution of some helicene racemates by “hand picking” of a few homochiral single enantiomeric crystals allowed for a measurement of their optical rotation. Due to that intrinsic chirality spanned over a large polyaromatic template, preliminary results clearly established the efficiency of carbohelicenes to induce asymmetry and chirality in organic synthesis and in supramolecular chemistry. Additionally, they have some potential uses in several fields: materials science, nanoscience, chemical biology and supramolecular chemistry. It has encouraged many attempts to develop new asymmetric syntheses of carbohelicenes, as well as some chiral separations of enantiomers and diastereoisomers. This review is thus dedicated to carbohelicene chirality. It gathered a substantial collection of data, and a comprehensive review on the preparations of enantioenriched helicenes, either from an asymmetric synthesis or from a chiral separation. Utilizations of non-racemic helicenes and their applications will be treated in the following review (Part 3), and will not be the subject of this manuscript.

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

In this review, the delicate interplay between stereochemical control, monitoring at the subnanometer level, and an understanding of crystal nucleation is probed Control of crystal nucleation may be achieved employing tailor-made auxiliaries, which are either nucleation inhibitors or promoters The process may be monitored at an interface via grazing incidence X-ray diffraction (GIXD) By these means, we can glean experimental knowledge of crystal nucleation in various molecular systems A hypothesis was invoked that supersaturated solutions containing molecular clusters adopt various arrangements and shapes, some of which resemble the crystals into which they develop This hypothesis was taken advantage of for the design of tailored inhibitors in achieving kinetic resolution of enantiomers and induced precipitation of particular crystal polymorphs The control and behavior of polymorphic crystallization may be understood at the molecular level through the interplay between inhibitor, solvent, solute, cr

Toward Stereochemical Control, Monitoring, and Understanding of Crystal Nucleation

In this review, we discuss the rich chemistry of helicenes and helicenoids containing main-group elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N, and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, and emission, together with other photochemical properties and applications.

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Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P).

Controlled racemization of optically active organic compounds: Prospects for asymmetric transformation

Thermal racemisation of hepta-, octa-, and nonahelicene : Kinetic results, reaction path and experimental proofs that the racemisation of hexa- and heptahelicene does not involve an intramolecular double diels-alder reaction

Resolution and optical properties ([α]max, ord and cd) of hepta-, octa- and nonahelicene

Thermal racemisation of [6], [7], [8] and [9] helicene.

A two-step synthesis of hexahelicene (scheme 1; 53% over-all yield), a three-step synthesis of hexaheliene (scheme 2; 33% over-all yield), and a two-step synthesis of heptahelicene (scheme 3; 14% over-all yield) are described.

Rapid Syntheses of Hexa and Heptahelicene

“Photochemical synthesis of polycyclic aromatic hydrocarbons and NMR-1H study of bay protons. Specific solvent effects and nuclear Overhauser effects”. Dinaphtho[1,2-a 2′,1′-j]anthracene (4a), 9-bromo-dinaphto [1,2-a; 2′,1′-j]anthracene (4b), 9-methyldinaphtho[1,2-a; 2′,1′-j]anthracene (4c), benzo [c]naphtho [2,1-1]chrysene (7), benzo [a]naphtho [2,1-j]anthracene (3), naphtho [2,1-c]chrysene (9) and dibenzo [c,1] chrysene (12) have been synthesized by the photocyclodehydrogenation of 1,2-diarylethylenes (5,8) and bis (arylvinyl)arenes (6, 10). The NMR study has been mainly focused on the bay protons involved in specific solvent effects (Table 4) and nuclear Overhauser effects (Table 5).

Synthèse Photochimique D'Hydrocarbures Polycycliques Aromatiques et Étude en RMN-1H Des Protons de Baie. Effets de Solvant Specifiques et Effets Nucleaires Overhauser

Marie-Jeanne Marchant

Papers

Thermal racemisation of hepta-, octa-, and nonahelicene : Kinetic results, reaction path and experimental proofs that the racemisation of hexa- and heptahelicene does not involve an intramolecular double diels-alder reaction

Resolution and optical properties ([α]max, ord and cd) of hepta-, octa- and nonahelicene

Thermal racemisation of [6], [7], [8] and [9] helicene.

Rapid Syntheses of Hexa and Heptahelicene

Synthèse Photochimique D'Hydrocarbures Polycycliques Aromatiques et Étude en RMN-1H Des Protons de Baie. Effets de Solvant Specifiques et Effets Nucleaires Overhauser