8 research outputs found
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 C<sub>60</sub>
A wide
range of new dipoles and catalysts have been used in 1,3-dipolar
cycloadditions of N-metalated azomethine ylides onto C<sub>60</sub> 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 C<sub>60</sub> 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 H<sub>2</sub>O@C<sub>60</sub>
The stereochemical outcome of <i>cis</i>–<i>trans</i> isomerization of optically
pure [60], [70], and endohedral H<sub>2</sub>O@C<sub>60</sub> 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@C<sub>72</sub>, 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@C<sub>72</sub>(C<sub>6</sub>H<sub>3</sub>Cl<sub>2</sub>), catalyzed by a copper chiral complex. The chiral bis-adducts of La@C<sub>72</sub>, 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 C<sub>60</sub> Dimers?
Two
C<sub>60</sub> 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 C<sub>60</sub> 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