Switching the Stereoselectivity: (Fullero)Pyrrolidines “a la Carte”

Stereodivergent syntheses of <i>cis</i>/<i>trans</i> pyrrolidino­[3,4:1,2]­fullerenes and <i>endo</i>/<i>exo</i> pyrrolidines are reported with high enantioselectivity levels. Fullerenes are revealed as a useful benchmark to develop suitable catalysts to control the stereochemical outcome and to shed light on the mechanism involved in the related 1,3-dipolar cycloaddition

Enantioselective Cycloaddition of Münchnones onto [60]Fullerene: Organocatalysis versus Metal Catalysis

Novel chiral catalytic systems based on both organic compounds and metal salts have been developed for the enantioselective [3 + 2] cycloaddition of münchnones onto fullerenes and olefins. These two different approaches proved to be efficient and complementary in the synthesis of optically active pyrrolino­[3,4:1,2][60]­fullerenes with high levels of enantiomeric excess and moderate to good conversions. Further functionalization of the pyrrolinofullerene carboxylic acid derivatives has been carried out by esterification and amidation reactions

Stereodivergent Synthesis of Chiral Fullerenes by [3 + 2] Cycloadditions to C₆₀

A wide range of new dipoles and catalysts have been used in 1,3-dipolar cycloadditions of N-metalated azomethine ylides onto C₆₀ yielding a full stereodivergent synthesis of pyrrolidino[60]­fullerenes with complete diastereoselectivities and very high enantioselectivities. The use of less-explored chiral α-iminoamides as starting 1,3-dipoles leads to an interesting double asymmetric induction resulting in a matching/mismatching effect depending upon the absolute configuration of the stereocenter in the starting α-iminoamide. An enantioselective process was also found in the retrocycloaddition reaction as revealed by mass spectrometry analysis on quasi-enantiomeric pyrrolidino[60]­fullerenes. Theoretical DFT calculations are in very good agreement with the experimental data. On the basis of this agreement, a plausible reaction mechanism is proposed

Catalytic Stereodivergent Synthesis of Steroid–Fulleropyrrolidine Hybrids

The diastereoselective synthesis of <i>cis</i> and <i>trans</i> steroid–fulleropyrrolidines hybrids by reaction of <i>N</i>-metalated azomethine ylides [Cu­(II) or Ag­(I)] with the appropriate chiral ligand and C₆₀ is described. The experimental findings reveal that the azomethine ylide stabilized by an allylic group cycloadds to [60]­fullerene in an efficient manner and with a good diastereomeric excess. Furthermore, the new generated stereocenters are fully controlled by the catalytic systems used without being influenced by the chirality of the steroid. Interestingly, by this synthetic methodology the each one of the four possible stereoisomers have efficiently been obtained and characterized by CD spectra

Enantiospecific <i>cis</i>–<i>trans</i> Isomerization in Chiral Fulleropyrrolidines: Hydrogen-Bonding Assistance in the Carbanion Stabilization in H₂O@C₆₀

The stereochemical outcome of <i>cis</i>–<i>trans</i> isomerization of optically pure [60], [70], and endohedral H₂O@C₆₀ fulleropyrrolidines reveals that the electronic nature of substituents, fullerene size, and surprisingly the incarcerated water molecule plays a crucial role in this rearrangement process. Theoretical DFT calculations are in very good agreement with the experimental findings. On the basis of the experimental results and computational calculations, a plausible reaction mechanism involving the hydrogen-bonding assistance of the inner water molecule in the carbanion stabilization of endofullerene is proposed

Enantioselective Synthesis of Endohedral Metallofullerenes

Endohedral metallofullerenes are promising materials in biomedical and material sciences. In particular, they are of interest as agents for magnetic resonance imaging (MRI), photovoltaic devices, and semimetallic components. The synthesis of chiral endofullerenes represents one step further in the potential use of these carbon allotropes; however, this step has not been addressed so far. In this regard, enantiopure endofullerenes are expected to open new avenues in fields in which chirality is a key issue. Here, the synthesis and characterization of the first chiral endohedral metallofullerenes, namely, chiral bis-adducts of La@C₇₂, are reported. Eight optically active isomers were obtained by enantioselective 1,3-dipolar cycloaddition of a N-metalated azomethine ylide onto a non-isolated-pentagon rule metallofullerene derivative, La@C₇₂(C₆H₃Cl₂), catalyzed by a copper chiral complex. The chiral bis-adducts of La@C₇₂, isolated by nonchiral HPLC, showed optical purities as high as 98% as revealed by the remarkable positive or negative Cotton effects observed in the circular dichroic spectra

Does a Cyclopropane Ring Enhance the Electronic Communication in Dumbbell-Type C₆₀ Dimers?

Two C₆₀ dumbbell molecules have been synthesized containing either cyclopropane or pyrrolidine rings connecting two fullerenes to a central fluorene core. A combination of spectroscopic techniques reveals that the cyclopropane dumbbell possesses better electronic communication between the fullerenes and the fluorene. This observation is underpinned by DFT transport calculations, which show that the cyclopropane dumbbell gives a higher calculated single-molecule conductance, a result of an energetically lower-lying LUMO level that extends deeper into the backbone. This strengthens the idea that cyclopropane behaves as a quasi-double bond

Dumbbell-Type Fullerene-Steroid Hybrids: A Join Experimental and Theoretical Investigation for Conformational, Configurational, and Circular Dichroism Assignments

New [60]­fullerene-steroid conjugates (<b>4</b>–<b>6</b>) have been synthesized by 1,3-dipolar cycloaddition and Bingel–Hirsch cyclopropanation reactions from suitably functionalized epiandrosterone and [60]­fullerene. Since a new stereocenter is created in the formation of the Prato monoaduct, two different diastereomers were isolated by HPLC (<b>4</b>, <b>5</b>) whose absolute configurations were assigned according to the highly reliable “sector rule” on fullerenes. A further reaction of the malonate-containing diastereomer <b>5</b> with a second C₆₀ molecule has afforded dumbbell fullerene <b>6</b> in which the two fullerene units are covalently connected through an epiandrosterone moiety. The new compounds have been spectroscopically characterized and their redox potentials, determined by cyclic voltametry, reveal three reversible reduction waves for hybrids <b>4</b> and <b>5</b>, whereas these signals are split in dumbbell <b>6</b>. Theoretical calculations at semiempirical (AM1) and single point B3LYP/6-31G­(d) levels have predicted the most stable conformations for the hybrid compounds (<b>4</b>–<b>6</b>), showing the importance of the chlorine atom on the D ring of the steroid. Furthermore, TDDFT calculations have allowed assignments of the experimentally determined circular dichroism (CD) of the [60]­fullerene-steroid hybrids based on the sign and position of the Cotton effects, despite the exceptionally large systems under study