Nicolas Vanthuyne

Metal−Bis(helicene) Assemblies Incorporating π-Conjugated Phosphole-Azahelicene Ligands: Impacting Chiroptical Properties by Metal Variation

The synthesis of phosphole-modified aza[6]helicenes and the complexation of these pi-conjugated ditopic ligands with metallic ions are described. The chiroptical properties of these metal-bis(helicene) assemblies were experimentally evaluated and studied by first-principles theoretical calculations. The results show that the metal impacts the chiroptical properties of these complexes through electronic interactions with the chiral pi-conjugated ligands.

Metallahelicenes: Easily Accessible Helicene Derivatives with Large and Tunable Chiroptical Properties†

Enantiopure metallahelicenes have been prepared by cyclometalation of 2-pyridyl-substituted benzophenanthrenes followed by resolution using chiral HPLC. They are red phosphors at room temperature and their chiroptical properties can be modulated by oxidation of the metal center to the oxidation state IV.

From Hetero- to Homochiral Bis(metallahelicene)s Based on a PtIII−PtIII Bonded Scaffold: Isomerization, Structure, and Chiroptical Properties

Hetero- and homochiral diastereomeric bis(metallahelicene)s have been synthesized. They possess a rare Pt(III)-Pt(III) scaffold bridged by benzoato ligands. It is shown that heterochiral (P,M)-bis(Pt(III)-[6]helicene) can isomerize into the homochiral (P,P)- and (M,M)-bis(Pt(III)-[6]helicene). A theoretical study shows a unique σ-π conjugation between the two π-helices and the σ-Pt(III)-Pt(III) scaffold that impacts the strong chiroptical properties.

Chirality in dynamic supramolecular nanotubes induced by a chiral solvent.

Amplification of chirality has been reported in polymeric systems. It has also been shown that related effects can occur in polymer-like dynamic supramolecular aggregates, if a subtle balance between noncovalent interactions allows the coupling between a chiral information and a cooperative aggregation process. In this context, we report a strong majority-rules effect in the formation of chiral dynamic nanotubes from chiral bisurea monomers. Furthermore, similar helical nanotubes (with the same circular dichroism signature) can be obtained from racemic monomers in a chiral solvent. Competition experiments reveal the relative strength of the helical bias induced by the chiral monomer or by the chiral solvent. The nanotube handedness is imposed by the monomer chirality, whatever the solvent chirality. However, the chirality of the solvent has a significant effect on the degree of chiral induction.

Atropisomerism and Axial Chirality in Heteroaromatic Compounds

Abstract The chapter “Atropisomerism and axial chirality in heteroaromatic compounds” provides a summary of the present knowledge of this important topic. The structure of the chapter is: first a detailed description of the methods used to study atropisomerism, from X-ray diffraction to theoretical calculations (14 items), then the application to different compounds according to the nature of the central bond. Rotation about a C–C bond (5–5, 5–6 and 6–6 rings), rotation about a C–N bond (azoles, oxo- and thioxo-five-membered rings, azines), rotation about an N–N bond (5–5, 5–6 and 6–6 rings), rotation about ortho-ortho’-linked compounds, rotation about N-metal bonds, multiple rotations, to end with axial chiral synthesis. In conclusion, a phenomenon that was discovered in 1922 for diphenic acids and that was extended to heterocycles in 1931 by Adams shows eighty years later to be very active, being expected to grow in importance because it is related to asymmetric synthesis, to materials and to biological properties.

Multifunctional and reactive enantiopure organometallic helicenes: tuning chiroptical properties by structural variations of mono- and bis(platinahelicene)s.

Acetylacetonato-platina[6]- and -platina[7]helicenes have been prepared from 2-pyridyl-substituted benzophenanthrene ligands by following a two-step cycloplatination reaction. The photophysical properties (UV-visible absorption and emission behavior) and chiroptical properties (circular dichroism and molar rotation) of the resolved enantiomers have been measured. These metallahelicenes constitute a novel family of easily accessible helicene derivatives that exhibit large and tuneable chiroptical properties that can be rationalized theoretically and compared to the parent [6]- and [7]carbohelicenes. Furthermore, they are red phosphors at room temperature and their large chiroptical properties can be modulated by oxidation of the metal center to Pt(IV). Hetero- and homochiral diastereomeric bis(metallahelicene)s that possess a rare Pt(III)-Pt(III) scaffold bridged by benzoato ligands have also been prepared. It is shown that the heterochiral (P,M)-bis(Pt(III)-[6]helicene) 9a(1) can isomerize into the homochiral (P,P)- and (M,M)-bis(Pt(III)-[6]helicene) 9a(2). Spectral assignments and an analysis of the optical rotation of these systems were made with the help of time-dependent density functional theory. The calculations highlight the contributions of the metal centers to the chiroptical properties. For 9a(1) and 9a(2), σ-π conjugation between the helicenes and the Pt-Pt moiety may contribute strongly to the optical rotation and electronic circular dichroism.

Memory of chirality in cascade rearrangements of enediynes.

The cascade rearrangement of chiral enediynes 1c-e, involving successively 1,3-proton shift, Saito-Myers cyclization, 1,5-hydrogen atom transfer, and intramolecular coupling of the resulting biradical, proceeded at 80 °C to form tri- and tetracyclic heterocycles possessing a quaternary stereogenic center with a very high level of memory of chirality.

En Route to (S)-Selective Chemoenzymatic Dynamic Kinetic Resolution of Aliphatic Amines. One-Pot KR/Racemization/KR Sequence Leading to (S)-Amides

A one-pot sequential process, involving a radical racemization and an enzymatic resolution, provides access to (S)-amides, from racemic amines, with ee and yields ranging from 78 to 94% and 58 to 80%, respectively.

Chemoenzymatic Dynamic Kinetic Resolution of Primary Amines Catalyzed by CAL-B at 38-40 °C

The (R)-selective chemoenzymatic dynamic kinetic resolution of primary amines was performed at 38-40 °C in MTBE, in good to high yields and with high enantiomeric excesses. These reactions associating CAL-B to octanethiol as radical racemizing agent were carried out in the presence of methyl β-methoxy propanoate as acyl donor, under photochemical irradiation at 350 nm in glassware.

Mimics of the self-assembling chlorosomal bacteriochlorophylls: regio- and stereoselective synthesis and stereoanalysis of acyl(1-hydroxyalkyl)porphyrins.

Diacylation of copper 10,20-bis(3,5-di-tert-butylphenylporphyrin) using Friedel-Crafts conditions at short reaction times, high concentrations of catalyst, and 0-4 degrees C affords only the 3,17-diacyl-substituted porphyrins, out of the 12 possible regioisomers. At longer reaction times and higher temperatures, the 3,13-diacyl compounds are also formed, and the two isomers can be conveniently separated by normal chromatographic techniques. Monoreduction of these diketones affords in good yields the corresponding acyl(1-hydroxyalkyl)porphyrins, which after zinc metalation are mimics of the natural chlorosomal bacteriochlorophyll (BChl) d. Racemate resolution by HPLC on a variety of chiral columns was achieved and further optimized, thus permitting easy access to enantiopure porphyrins. Enantioselective reductions proved to be less effective in this respect, giving moderate yields and only 79% ee in the best case. The absolute configuration of the 3(1)-stereocenter was assigned by independent chemical and spectroscopic methods. Self-assembly of a variety of these zinc BChl d mimics proves that a collinear arrangement of the hydroxyalkyl substituent with the zinc atom and the carbonyl substituent is not a stringent requirement, since both the 3,13 and the 3,17 regioisomers self-assemble readily as the racemates. Interestingly, the separated enantiomers self-assemble less readily, as judged by absorption, fluorescence, and transmission electron microscopy studies. Circular dichroism spectra of the self-assemblies show intense Cotton effects, which are mirror-images for the two 3(1)-enantiomers, proving that the supramolecular chirality is dependent on the configuration at the 3(1)-stereocenter. Upon disruption of these self-assemblies with methanol, which competes with zinc ligation, only very weak monomeric Cotton effects are present. The favored heterochiral self-assembly process may also be encountered for the natural BChls. This touches upon the long-standing problem of why both 3(1)-epimers are encountered in BChls in ratios that vary with the illumination and culturing conditions.