Asymmetric Olefins Hydroformylation. VI. Asymmetric Hydroformylation of Isomeric Butenes Using Platinum Catalysts

The asymmetric hydroformylation of the straight chain butenes with the [(?)-DIOP]PtCl₂-SnCl₂ catalytic system shows that asymmetric induction, contrary to the rhodium-(?)-DIOP catalytic system, takes place after the intermediate metal-alkyl-complex formation.     

Nuclear magnetic resonance investigation of intramolecular scrambling processes in meso-(2,3-butylenediamine)tetraacetate. Diamagnetic complexes of transition metal ions

Summary The acetate scrambling processes (, conversion and N inversion) occurring in the complexes formed from meso-(2,3-butylenediamine)tetraacetic acid (BDTA) with zinc(II), cadmium(II), mercury(II), scandium(III), yttrium(III) and lanthanum(III) ions have been investigated by line shape analysis of their n.m.r. spectra. Between 308 and 373 K fast conversion between and isomers and slow nitrogen inversion occur except for mercury(II) and yttrium(III) in the latter case. H^* = 2.3±0.1 kJ mol^–1 and S^* = –6.9±0.2 mol^–1 J K^–1 for the mercury complex; H^* = 2.6 ±0.2 kJ mol^–1 and S^* = –5.5±0.5 mol^–1 JK^–1 for the yttrium complex. Exchange mechanisms proceeding through different reaction intermediates are discussed on the light of the results obtained in this work as well as previously on other polyaminepolycarboxylate complexes of the same ions.     

The kinetics of the reaction of some chloroisoxazolo[4.5-c] and [5,4-b] pyridines with methoxide ion

Arrhenius parameters were measured for the methoxy-lechlorination reactions of some chloroderivatives of isoxazolo[4,5-c]- and [5,4-b]pyridine. A comparison of these results with the kinetic data for the corresponding chloropyridines shows that fusion of the isoxazole ring with the pyridine ring strongly increases the reactivity of the 4- and 6-positions toward nucleophilic substitution.     

All-trans 2,7-dialkylperhydrophenanthrene as a solvent for measurements of helical twisting power and linear dichroism

Abstract

The values of twisting power of a chiral bridged binaphthyl and (+)-trans-stilbene oxide in a rigid hydrocarbon perhydrophenanthrene nematic are much smaller than in conformationally flexible nematics, indicating the importance of the solvent structure for cholesteric induction. The perhydrophenanthrene phase constitutes an excellent solvent for linear dichroism measurements of anthracene but it is unable to orient 9,9′-spirobifluorene.     

On a dynamical system related to fluid mechanics

We introduce a dynamical system strictly related to fluid mechanics and similar to the classical N point vortex system. In the first part we analyze the qualitative behavior of the time evolution and in particular we show the properties of collapse and chaoticity. In the second part of the paper we investigate the relation of the dynamical system with a system of N concentrated large enough smoke rings in an incompressible and inviscid fluid, with axial symmetry and without swirl. We prove the rigorous connection between the two models at time zero for any N. The extension of the same result to any time is obtained only for a smoke ring alone, while for the general case it is just a matter of conjecture. Received June 1998; Revised November 1998     

A global condition for periodic Duffing-like equations

We study Duffing-like equations of the type ,with periodic. We prove that if the stable and unstable manifolds to the origin do not coincide, then the system exhibits positive topological entropy.

     

Synthesis, resolution, and absolute configuration of two novel and selective cyclooxygenase-2 inhibitors based on the 1,5-diarylpyrrole structure

Three racemic esters based on the 1,5-diarylpyrrole structure, which proved to be highly selective COX-2 inhibitors endowed with an appreciable anti-inflammatory activity in some animal models, were subjected to chiral resolution. Their absolute configurations were assigned by comparison of the CD spectrum measured on-line with a HPLC–CD detector, with that calculated by means of a TDDFT method.