An experimental study on the effect of substituents on aromatic-aromatic interactions in dithia[3,3]-metaparacyclophanes

Simple model systems based on the 2,11-dithia[3,3]-metaparacyclophane skeleton were synthesized to study the effects of substituents on the intramolecular aromatic-aromatic interactions between benzene rings. X-ray crystallography established that, in their more stable conformations, these metaparacyclophanes featured partially overlapping aromatic rings (interplanar distances of about 3.5??), with the planes of the aromatic systems arranged in a slightly tilted disposition (interplanar angles in the range 5-19°). Calculations showed that these derivatives underwent topomerization by flipping of the meta-substituted ring over the para-substituted one, a process in which the two rings adopted a continuum of edge-to-face dispositions, including an orthogonal one, which were less stable than the starting face-to-face arrangement. The energy barriers to the isomerization process were experimentally determined by variable-temperature NMR spectroscopy, by using an internal temperature standard to assess even minor differences in energy (relative experimental error: (±0.1?kJ?mol(-1)). The variation in the barriers as a function of the different substituents on the interacting ring was small and apparently unrelated to the effect of the substituents on the polarity of the π-systems. An explanation based on the charge-penetration effect seemed more-suitable to rationalize the observed trends in the barriers.     

The barrier to carbon-phosphorus bond rotation in tribenzoylphosphine. An experimental reinvestigation

An examination of the ¹³C NMR spectrum of tribenzoylphosphine over the temperature range ?75 to +70°C fails to disclose any evidence for hindered rotation about the PC bonds, contrary to a previous claim.     

Enantioselective catalysis in water: Mukaiyama-aldol condensation promoted by copper complexes of bisoxazolines supported on poly(ethylene glycol)

(S)-3-Phenyl-2-aminopropanol-derived bisoxazolines supported on a modified poly(ethylene glycol) were shown to be effective Cu(II) ligands for the enantioselective Mukaiyama-aldol condensation of various aldehydes with the trimethylsilyl keteneacetal of methyl isobutyrate carried out in water. Enantiomeric excesses comparable to those obtained with nonsupported ligands in the same solvent were observed. The solubility of the ligand in water, ensured by the presence of the polymeric support, allowed a very convenient catalyst-recycling procedure involving simple removal of the reaction product by extraction in Et(2)O and addition of fresh reagents to the catalyst-containing aqueous solution. The chemical and stereochemical efficiency of the catalyst was only marginally eroded over its use in three reaction cycles.     

Through-space interactions between face-to-face,center-to-edge oriented arenes: importance of polar-pi effects

Two series of conformationally restricted polycyclic compounds (1-3 and 4-7) have been synthesized as model systems for studying the through-space interactions between face-to-face, center-to-edge (parallel-offset) oriented arenes. These compounds feature different X substituents on one of the interacting rings. By monitoring the variation of the delta Gz for the rotation around the aryl-aryl bond in 1-7 as a function of X by 2D [1H,1H] EXSY NMR spectroscopy, it was found that the barriers increase on passing from electron-donating to electron-withdrawing substituted derivatives. Quantum mechanical calculations [MP2/DVZ (2d,p)//B3LYP/DVZ(2d,p)] gave barrier values and variations in agreement with the experimental data. The results are consistent with a repulsive arene-arene interaction dominated by electrostatic effects.     

Cr(Salen)] as a 'bridge' between asymmetric catalysis,Lewis acids and redox processes

An overview focused on the synthesis, structural features and catalytic applications of chiral [Cr(Salen)] complexes is presented. Key aspects of modern organic chemistry such as Lewis acids, asymmetric catalysis and redox processes are strictly connected with chronium-Schiff base complexes. Among the asymmetric transformations mediated by [Cr(Salen)] complexes, Diels-Alder and hetero-Diels-Alder reactions, ARO of epoxides, kinetic resolution of meso-epoxides and Nozki-Hyama reactions are taken into account.     

InBr3-catalyzed Friedel-Crafts addition of indoles to chiral aromatic epoxides: a facile route to enantiopure indolyl derivatives

Aromatic optically active epoxides can be opened in a regioselective and clean way with indoles in the presence of catalytic amount of InBr3 (1 mol %). The reaction takes place with a SN2 pathway affording the 2-aryl-2-(3'-indolyl)ethan-1-ols with excellent enantioselectivity (ee up to 99%).     

Efficient synthesis of an enantiopure beta-lactam as an advanced precursor of thrombin and tryptase inhibitors

A new and efficient synthesis of a beta-lactam that is an advanced precursor of inhibitors of thrombin and tryptase is reported. The reaction sequence is based on the use of an inexpensive enantiomerically pure starting material and is designed to allow access to both enantiomers of the target molecules by epimerization of a side-product obtained along the synthesis. An improved procedure for the epimerization step that takes advantage of the use of a polymer-supported and recyclable phase-transfer catalyst is described.     

Die Oxydoreduktionsreaktionen des Germaniums an der Quecksilbertropfelektrode

Zusammenfassung Die Reduktionsvorgänge bzw. Oxydationsvorgänge des zweiwertigen Germaniums zum Metall bzw. zur Germaniumsäure imph-Bereich 0 bis 6 wurden untersucht. Es handelt sich in allen Fällen um nicht reversible Prozesse, teils weil das Germanium kein Amalgam bildet, teils wegen des gleichzeitigen Vorhandenseins vieler verschiedener Ionenformen. Reduktion und Oxydation werden beiph > 1 von der Acidität in gewohnter Weise beeinflußt, weil — wie wir annehmen — das Germanium-II-Ion unter diesen Umständen als basisches Ion vorliegt. Beiph <0,5 ist die Acidität ohne Einfluß auf das Halbwellenpotential des Reduktionsvorganges. Darin ist eine Bestätigung für die Anwesenheit von in stark saurer Lösung beständigen GeCl ₃ ^- -Ionen zu erblicken.

---

Summary A study was made of the redox reactions of bivalent germanium leading to the metal or to germanic acid in theph range 0–6. All cases dealt with irreversible reactions, partly because germanium does not form an amalgam, and partly because of the simultaneous presence of many different ionic forms. Atph > 1, the reduction and oxidation are affected in the usual way by the acidity, because — as we assume — the germanium(II) ion is present under these conditions as a basic ion. Atph < 0.5, the acidity is without influence on the half wave potential of the reduction process. This may be viewed as a confirmation of the presence of the stable GeCl ₃ ^- ions in strong acid solution.

---

Résumé On a expérimenté les processus de réduction et d'oxydation du germanium bivalent en métal ou en acide germanique dans le domaine deph de 0 à 6. Il ne s'agit pas de processus réversibles dans tous les cas parce que le germanium ne donne pas d'amalgame et à cause de la présence simultanée de différentes formes ioniques variées. La réduction et l'oxydation sont influencées pourph > 1, comme d'usage, par l'acidité car, comme nous le supposons, dans ces conditions, l'ion Germanium bivalent se trouve à l'état d'ion basique. Pourph < 0,5, l'acidité est sans influence sur le potentiel de demi-palier et sur le processus de réduction. Il faut supposer en solution fortement acide la présence d'ion GeCl ₃ ^- instable.

---

---

Mit 3 Abbildungen.