Diastereoisomerically Pure Fulleropyrrolidines as Chiral Platforms for the Design of Optically Active Liquid Crystals

Abstract

Incorporation of [60]fullerene (C(60)) within self-organizing systems is conceptually challenging but allows us to obtain materials which combine the characteristics (anisotropy, organization) of condensed mesophases with the properties of C(60) (photo- and electrochemical activity). Here, we report on the synthesis, characterization, and liquid-crystalline properties of four optically active fullerodendrimers, which are chiral at the point of conjunction between the fullerene scaffold and the mesogenic moieties. Thus, the novelty of this study is to take advantage of the asymmetric carbon atom created during the 1,3-dipolar cycloaddition reaction on C(60) in order to induce mesoscopic chirality in the materials. Four diastereoisomeric fulleropyrrolidines ((R,S)-1, (R,R)-1, (S,R)-1, and (S,S)-1) were synthesized and associated with a second-generation nematic (N) dendron to give fullerodendrimers ((R,S)-2, (R,R)-2, (S,R)-2, and (S,S)-2) which display chiral nematic (N*) phases. The absolute configurations of the stereogenic centers were determined by X-ray crystallography, 1D and 2D NMR experiments, and circular dichroism (CD) spectroscopy. The liquid-crystalline properties of the fullerodendrimers were studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The fulleropyrrolidine derivatives 2 exhibit supramolecular helicoidal organizations with a right-handed helix for the (R,S)-2 and (R,R)-2 diastereoisomers and a left-handed helix for the (S,R)-2 and (S,S)-2 diastereoisomers. This result suggests that the self-organization of such supermolecular materials can be controlled at the molecular level by the introduction of only one chiral center.

Diastereoselective Synthesis of C60/Steroid Conjugates

“…The fullerene steroids conjugates (6)(7)(8)(9) were prepared by Prato reaction by treatment of The convenient transformation of the hydroxyl group on C3 of the epiandrosterone (1) by oxidation gave the 5β-androstan-3,17-dione (2) and by acetylation gave the 3α-acetoxy-5β-androstan-17-one (3) both with similar yields to those previously reported. 27,28 The Vilsmeier-Haack reaction of 2 and 3 with phosphorus oxychloride and dimethylformamide in refluxing dichlorometane, gave the corresponding 3-chloro-2-formyl-17-oxo-5β-androstan-2-ene (4) and 3-α-acetoxy-17-chloro-16-formyl-5β-androstan-16-ene (5), respectively.…”

“…Furthermore, the absence of significant potential shifts with respect to other fulleropyrrolidine monoadducts indicate that there is essentially no effect of the steroidic unit on the electronic properties of the C 60 core. 32 Inspection of the CD spectra shows that diastereoisomeric pairs 6, 7 and 8, 9 give nicely opposite signed curves in the 430 nm region of the electronic spectra, thus confirming opposite configurations in the new stereogenic center (see Figure 4). This relevant UV-visible band is considered the fingerprint for all fullerene monoadducts at 6,6 junctions (between two fused hexagons) regardless of the nature of the organic addend saturating the double bond.…”

“…7 The diastereoselective synthesis of fulleropyrrolidines endowed with enantiomerically pure functionalized cyclobutanes has also been previously reported. 8 Recently, the synthesis of diastereoisomerically pure fulleropyrrolidines as chiral platforms for the design of optically active liquid crystals, 9 and the diasteroselective preparation of fulleroprolines by lithium salt-assisted cycloaddition of azomethine ylides have been reported. 10 On the other hand, it is well-known that the principal obstacle for the potential use of C 60 in biomedical applications is its complete lack of solubility in water and quite low solubility in almost all organic solvents.…”

Accordion‐optimized DEPT experiments

“…Equations (1-3) have been used for plotting the curves. [28] All curves have been obtained using the range 120-260 Hz. In (A), (C) and (E), the theoretical curves were obtained using the DCT method.…”

Fullerene‐Containing Liquid Crystals

“…In 2010, Deschenaux and coworkers [13,14] developed an original synthetic methodology for the design of optically active liquid-crystalline fullerodendrimers ( Figure 6.2) from enantiomerically or diastereomerically pure fulleropyrrolidine. Such ingenious approach allowed exploring the relationship between molecular structure, intermolecular interactions, and macroscopic organization in relation to chirality transfer.…”