New Class of Chiral Diphosphine Ligands for Highly Efficient Transition Metal-Catalyzed Stereoselective Reactions: The Bis(diphenylphosphino) Five-membered Biheteroaryls

The synthesis and application of three examples of a new class of chiral (C(2)) atropisomeric diphosphines characterized by two interconnected five-membered heteroaromatic rings, with hindered rotation around the interanular bond, are described. Optically pure (+)- and (-)-2,2'-bis(diphenylphosphino)-4,4',6,6'-tetramethyl-3,3'-bibenzo[b]thiophene (tetraMe-bitianp) (1a) and the parent unsubstituted system (+)- and (-)-bitianp (1b) were synthesized. They were found to be optically stable at 100 degrees C and were successfully employed as ligands in the Ru(II)-catalyzed hydrogenation of alpha- and beta-oxo esters to the corresponding alpha- and beta-hydroxy esters and in the hydrogenation of olefinic substrates. The optical and chemical yields were comparable with those reported for the same Ru(II)-binap-catalyzed reactions carried out under the same experimental conditions. The 2,2'-bis(diphenylphosphino)-3,3'-bibenzo[b]furan (1c), the oxygenated analogue of bitianp, was found to be configurationally unstable at room temperature. Complete structural X-ray elucidation of the Pd complexes of 1a-c is reported. The advantages of these biheteroaromatic ligands over the classical biaryl systems are discussed.     

Teicoplanin-Based Enantiomeric Separations in CZE Using a Partial Filling Technique

Baseline separation of the enantiomers of a number of negatively charged amino and mandelic acid derivatives was achieved in less than 10 min by capillary electrophoresis in a polyacrylamide coated capillary, using the “partial filling method” (PFM) with submillimolar concentration of Teicoplanin (TE) as the chiral selector. The influence of the charge and concentration of TE, electrolyte solution composition and pH, on the enantioresolution was examined. Further proofs were brought to corroborate the hypothesis that the enantiorecognition takes place at the D-Ala-D-Ala binding site, whose blockade is responsible for the antibacterial activity of glycopeptide antibiotics. While the dependence of the chiral recognition capabilities of TE on electrolyte solution composition and pH could limit its applicability, improved sensitivity, reduction of TE wall adsorption, resulting in a good efficiency, and high cost reduction, due to the very small amount of chiral selector required, were shown as advantages of the PFM adopted in this study.     

Effect of intramolecular hydrogen bonding on the empty level structure of gas-phase salicylaldehyde:electron affinities and anion yield upon electron impact

The gas-phase empty level structures of γ-butyrolactone, α-hydroxy-γ-butyrolactone, benzaldehyde, 3-hydroxybenzaldehyde and 2-hydroxybenzaldehyde (salicylaldehyde) are studied by means of electron transmission spectroscopy (ETS) and ab initio 6-31G^** calculations. The most stable conformer of salicylaldehyde is characterised by intramolecular hydrogen bonding which causes relevant geometrical variations and, in particular, a lengthening of the C=O double bond. The empty π^* molecular orbitals of salicylaldehyde with large contribution from the carbonyl group are significantly stabilised with respect to their counterparts in benzaldehyde and in the 3-hydroxy derivative. Dissociative electron attachment spectroscopy (DAS) did not reveal formation of negative fragments at low energy, that is, the presence of dissociative decay channels for the π^* resonances observed in the ET spectra. The total anion current measured at the walls of the collision chamber (close to the electron beam), as a function of the incident electron energy, displays signals peaking at zero energy and at about 0.8 eV in benzaldehyde and in the two hydroxy derivatives. However, the only signal detected using a quadrupole mass filter (which requires a lifetime ≥10⁻⁶ s) is a zero energy peak in salicylaldehyde, due to the molecular anion.     

Formation and compatibilizing effect of the grafted copolymer in the reactive blending of 2-diethylsuccinate containing polyolefins with poly-ϵ-caprolactam (nylon-6)

The intermolecular reaction and its role in determining the partial compatibility between diethylsuccinate containing linear low-density polyethylene or ethylene propylene copolymer and poly-ϵ-caprolactam (PA6) has been investigated in the melt using a Brabender mixer. The reaction product has been submitted to selective solvent extraction with formic acid and n-heptane; the characterization of the two extracted fractions and the insoluble residue has demonstrated the formation of a polyolefin–nylon (PO–PA6) grafted copolymer. The formation of grafted copolymer has an evident effect on the compatibilization of the two original polymers, indeed the differential scanning calorimetry analysis shows a remarkable decrease of temperature and enthalpy of PA6 crystallization. Moreover scanning electron microscopy micrographs show clear evidence of size reduction of PA6 domains associated with improved interface interactions. © 1998 John Wiley & Sons, Ltd.     

PHOTOBEHAVIOR OF EUGLENA GRACILIS: ACTION SPECTRUM FOR THE STEP-DOWN PHOTOPHOBIC RESPONSE OF INDIVIDUAL CELLS

Abstract—Light-induced behavioral responses of Euglena gracilis have been investigated in single cells by means of a video system coupled to an optical microscope. Light intensity-effect curves at different wavelengths in the near UV and visible range have been determined. From these curves the action spectrum for the step-down photophobic response of Euglena has been calculated. From a comparison with the results obtained using a population method by means of a phototaxigraph, it is concluded that a single photomotile reaction is responsible for cell accumulation, brought about by trapping in the light spot and possibly by phototaxis towards scattered light from organisms already in the light field.     

2-Methylisoxazolin-5-ones III. Curboxylic. Acids and their Derivatives

Carbethoxy-substituted 12-methylisoxazolin-5-ones and several functional derivatives have been prepared. Spectral properties of the corresponding carboxylic acids are discussed. These acids ant stronger when the carboxy group is in position 3 than when in position 4 of the heterocycle.     

Reactivity of 2-amino-1,3,4-thiadiazoles. Nucleophilic behaviour of some 2-amino-1,3,4-thiadiazoles: Model compounds

The ambident nucleophilic behaviour of some 2-amino-5-H-1,3,4-thiadiazoles in alkylation, acylation and nitrosation reaction has been verified. The structures assigned to the 2-amino-1,3,4-thiadiazoles ( 1a-i ) and to the Δ²-1,3,4-thiadiazolines ( 2a-e ) agree with the spectral data.     

NMR stereospecific long-range coupling and preferred conformations in some (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl] acrylic acids

NMR data have been used to assign the stereochemistry to some new (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl]acrylic acids. The (E)-molecules are biased in the s-cis conformation showing NMR spectroscopic features strictly depending on the conformation.The analysis of the NMR spectra reveals that the (Z)-isomers exist also in the s-cis conformation.     

The reactivity of the A-CH=N-NR-CX-B system. 1,3,4-Oxadiazoles and 2,3,4,5-tetrahydro-1,2,4-triazin-3-ones through the 1–5 and 1–6 cyclization reactions of α-thienylglyoxal monosemicarbazones

The factors influencing the reactivity of α-thienylglyoxal monosemicarbazones when treated with cyclizing reagents (bromine/sodium acetate and hydrobromic acid in acetic acid) were investigated. Depending on the experimental conditions, on the position of the substituent on the semicarbazide residue, and on the cyclizing agent, the substrates 1a-e give the semicarbazone bromides 2a-b, 5 , the 1,3,4-oxadiazoles 3a-c , the 1,2,4-triazine 11 and the 2,3,4,5-tetrahydro-1,2,4-triazine-3-ones 6, 8 and 9 . Compound 6 by thermolysis undergoes ring contraction in the Δ²-1,3,4-oxadiazoline 12 , while treatment with base involves the conversion of 6 into 1,2,4-triazol-5-one 13 . Ir, nmr and mass spectra support the reported structures.     

N,N′-dialkylsubstituted Imidazolidine-2-thionecopper(I) complexes. X-ray analysis of chloro-bis(N,N′-dimethylimidazolidine-2-thione)copper(I)

Summary New complexes of copper(I) withN,N-dialkylsubstituted imidazolidine-2-thione ligands were prepared by reduction of CuX₂ (X = Cl or Br). The i.r. spectra show that in all the complexes the ligand coordinates through the sulphur atom. The crystal structure of chloro-bis(N,N-dimethylimidazolidine-2-thione)copper(I) has been determined from x-ray diffraction data. Crystals are monoclinic. space groupC ₂, with unit cell dimensions:a = 16.022(15),b = 9.942(10),c = 15.112(15) A, = 139.84(10)², Z = 4. The final R index is 5.2%. The copper coordination is trigonal, involving sulphur atoms of the two ligands and One chlorine atom. The steric effect of the two methyls imposes a rotation of the imidazolidine rings with respect to the coordination plane. The dihedral angle between the mean plane of thiourea moieties. parallel one with the other. and the coordination plane is 119.3°.This work was supported by the National Research Council (C.N.R.) of Rome.