Ivana Cisarova

An organometallic route to long helicenes

Along with the recent progress in the development of advanced synthetic methods, the chemical community has witnessed an increasing interest in promising carbon-rich materials. Among them, helicenes are unique 3D aromatic systems that are inherently chiral and attractive for asymmetric catalysis, chiral recognition and material science. However, there have been only limited attempts at synthesizing long helicenes, which represent challenging targets. Here, we report on an organometallic approach to the derivatives of undecacyclic helicene, which is based on intramolecular [2 + 2 + 2] cycloisomerization of aromatic hexaynes under metal catalysis closing 6 new cycles of a helicene backbone in a single operation. The preparation of nonracemic compounds relied on racemate resolution or diastereoselective synthesis supported by quantum chemical (density functional theory) calculations. The fully aromatic [11]helicene was studied in detail including the measurement and theoretical calculation of its racemization barrier and its organization on the InSb (001) surface by STM. This research provides a strategy for the synthesis of long helical aromatics that inherently comprise 2 possible channels for charge transport: Along a π-conjugated pathway and across an intramolecularly π-π stacked aromatic scaffold.

Unprecedented meta-substitution of calixarenes: direct way to inherently chiral derivatives.

Electrophilic aromatic substitution in the calix[n]arene series is a well-established procedure leading exclusively to para-substituted derivatives. An unprecedented regioselectivity of the mercuration reaction leading to the meta-substituted calix[4]arenes is described. These compounds represent a new type of substitution pattern in classical calixarene chemistry and open the door for the straightforward synthesis of inherently chiral receptors based on calixarenes.

Lanthanide(III) complexes of pyridine-N-oxide analogues of DOTA in solution and in the solid state. A new kind of isomerism in complexes of DOTA-like ligands.

The replacement of one of the acetate pendant arms with a 2-methylpyridine-N-oxide group in the molecule of H4dota significantly alters the coordination properties of the ligand in Ln(III) complexes. The structural properties of the complexes are investigated both in solution and in the solid state. The variable-temperature 1H NMR spectra of Nd(III), Eu(III), and Yb(III) complexes show that the twisted-square-antiprismatic (TSA) isomer is strongly destabilized and suppressed in solution and the complexes exist mostly as the square-antiprismatic (SA) isomers (98% for Eu(III) at -35 degrees C). The exchange between the TSA and SA isomers is fast at room temperature compared to that of the NMR time scale. The flexibility of the six-membered chelate ring formed by coordination of the 2-methylpyridine-N-oxide group to the central ion allows two orientations of this pendant arm relative to the acetate arms: syn-SA (pyridine in the direction of the acetates) and anti-SA (pyridine opposite to the acetates). The syn-SA form was found in the X-ray structure of the Nd(III) complex; the anti-SA forms were found in the structures of Dy(III), Tm(III), and Yb(III) complexes. The UV-vis and 1H NMR spectra of the Eu(III) complex suggest that both forms are in dynamic equilibrium in solution. A derivatization of the pyridine-N-oxide group with a carboxylic group in the 4 position has no significant effect on the properties of the Ln(III) complexes.

A biscalix[4]arene-based ditopic hard/soft receptor for K+/Ag+ complexation

Abstract A biscalixarene bearing tert -butylated and de- tert -butylated calix[4]arene unit connected by four ethylenoxy bridges has been prepared. This compound adopts the pinched cone conformation ( C 2 v symmetry) and hence, is suitably preorganised for Ag + complexation. X-ray crystallography showed that the silver cation (soft) is sandwiched by the two distal coplanar phenyl rings of the de- tert -butylated part of the molecule possessing a η 1 binding mode. The complexation of the K + cation (hard) into the central cavity created by the bridging moieties leads to a change of conformation ( C 4 v symmetry) and hence, to the complete loss of complexation ability towards the Ag + cation.

Uncommon regioselectivity in thiacalix[4]arene formylation.

To reveal the alternative ways for upper-rim thiacalixarene derivatization, the formylation reactions (Gross and/or Duff conditions) of the corresponding tetrapropoxythiacalix[4]arene immobilized in the 1,3-alternate conformation were systematically studied. Surprisingly, albeit using an excess of the formylation agent, only two formyl groups were introduced exclusively into the meta positions of thiacalixarene skeleton. Unexpected regioselectivity of these reactions opens the door for a unique substitution pattern in thiacalixarene chemistry. The formation of meta-substituted aldehydes is another illustration showing remarkably different reactivity of the thiacalix[4]arene system compared with that of a classical calyx[4]arene analogue.

Unusual Intramolecular Bridging Reaction in Thiacalix[4]arene Series

Thiacalix[4]arene immobilized in the cone conformation undergoes a direct formylation reaction giving unusual products in high yields. The Duff reaction (urotropine/TFA) leads to unprecedented intramolecularly bridged compounds possessing two formyl groups on the opposite para- or para-/meta-positions. The comparison with the corresponding classical calix[4]arene analogues indicates an amazingly different reactivity of the thiacalix[4]arene system.

Upper rim substitution of thiacalix[4]arene

Abstract Upper rim bromination of thiacalix[4]arene has been achieved for the first time using a distally disubstituted thiacalix[4]arene as a starting material. Depending on the bromine:thiacalix[4]arene ratios, the corresponding dibromo- and tetrabromo derivatives were obtained which opens the door for subsequent derivatisation of the upper rim of thiacalix[4]arenes. Interestingly, direct bromination of tetraalkylated compounds did not yield brominated derivatives. The structure of the tetrabromo derivative in the 1,3-alternate conformation was proven by X-ray crystallography.

Inversion twinning in troilite

Abstract The crystal structure of troilite from chondrites Etter and Georgetown and a troilite analog synthesized by sulfurization of an iron wire was refined using single-crystal X-ray data. Troilite is known to be hexagonal, with space group P62c, which is non-centrosymmetric, allowing two non-identical inversely related spatial arrangements of atoms within the unit cell. All three samples represent the so-called inversion twins. They contain both inversely related atomic orientations instead of a single atomic arrangement. The inversion twinning may have developed as a result of a phase transition from the ideal centrosymmetric NiAs-type structure to troilite-type structure during cooling. In addition, all samples were found to be cation-deficient. The departure from ideal stoichiometry.up to almost 3.5 rel% of metal atoms are missing.is also possibly related to atomic ordering when the crystals cooled.

Stereocontrolled introduction of an amino group at C-6 of d-galactose via (3,3)-sigmatropic rearrangements—novel synthesis of lincosamine and 7-epi-lincosamine precursors

Abstract A new and stereoselective route to the aminoglycoside components of the antibiotics lincomycin and clindamycin is presented. The key step involves diastereoselective introduction of the amino group at C-6 of d -galactose by (3,3)-sigmatropic rearrangements of the corresponding allylic (Z)-trifluoroacetimidate and (Z)- and (E)-allylic thiocyanates. Epoxidation of the resulting trifluoroacetamide with m-CPBA led to the epoxide with high threo-selectivity.

Phase Transition in K3Na(MoO4)2 and Determination of the Twinned Structures of K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 at Room Temperature

The room-temperature phases of sodium potassium molybdates K 3 Na(MoO 4 ) 2 and K 2.5 Na 1.5 (MoO 4 ) 2 are isostructural with the monoclinic low-temperature phases of K 3 Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 , which are twinned distorted glaserite structures. In the molybdates there are two crystallographically independent potassiums and their environment slightly differs from those in K3Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 . The excessive Na in K 2.5 Na 1.5 (MoO 4 ) 2 occupies the position of the more firmly bound potassium. A reversible phase transition at 513 K was discovered in K 3 Na(MoO 4 ) 2 by DSC (differential scanning calorimetry), but no such transition in K 2.5 Na 1.5 (MoO 4 ) 2 was detected. Both samples used in the diffractometer experiment were found to be composed of six domains being related by twinning operations of the point group 6. The twinning may be considered as a combination of a merohedral and a pseudo-merohedral twinning with two- and threefold rotations as twinning operations, respectively. However, a reversible domain switching, which is observable in the related ferroelastic crystals of K 3 Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 , was not observed either in K 3 Na(MoO 4 ) 2 or in K 2.5 Na 1.5 (MoO 4 ) 2 , either due to semitransparency of the samples or high ferroelastic distortion. This distortion is manifested by the values of the atomic displacement vectors which are about twice as large as those in the selenate or the chromate.