1,720,994 research outputs found
Parity violation energy of 5-pyrimidyl alkanol, a chiral autocatalytic molecule
No full textThe energy of parity-violation due to weak-nuclear interactions between molecular electrons and nucleons associated with three molecules that participate in highly enantioselective autocatalytic reactions is estimated with ab initio techniques. The molecules, derivatives of (S)-5-pyrimidyl alkanol, are found to have very small parity-violation energy. Although our results are not conclusive, the computed parity-violation energy is not expected to have any effect on the direction of spontaneous symmetry breaking reported in the literature and leading to a preferential formation of the (S) form
Generalization possibilities of autocatalytic absolute enantioselective synthesis
No full textA simple empirical formula enables the quantitative description of chiral autocatalysis. This formula was used for the prediction of the number of consecutive autocatalytic reaction cycles needed to obtain a high enantiomeric excess without chiral auxiliary or an asymmetric physical field (absolute enantioselective synthesis). The results show, that even less selective Soai-type systems can be used for absolute enantioselective synthesis, which, therefore, appears to be a fairly general phenomenon. (c) 2006 Published by Elsevier Ltd
Stochastic and empirical models of the absolute asymmetric synthesis by the Soai-autocatalysis
No full textAbsolute asymmetric synthesis (AAS) is the preparation of pure (or excess of one) enantiomer of a chiral compound from achiral precursor(s) by a chemical reaction, without enantiopure chiral additive and/or without applied asymmetric physical field. Only one well-characterized example of AAS is known today: the Soai-autocatalysis. In an attempt at clarification of the mechanism of this particular reaction we have undertaken empirical and stochastic analysis of several parallel AAS experiments. Our results show that the initial steps of the reaction might be controlled by simple normal distribution ("coin tossing") formalism. Advanced stages of the reaction, however, appear to be of a more complicated nature. Symmetric beta distribution formalism could not be brought into correspondence with the experimental observations. A bimodal beta distribution algorithm provided suitable agreement with the experimental data. The parameters of this bimodal beta function were determined by a Polya-urn experiment (simulated by computer). Interestingly, parameters of the resulting bimodal beta function give a golden section ratio. These results show, that in this highly interesting autocatalysis two or even perhaps three catalytic cycles are cooperating. An attempt at constructing a "designed" Soai-type reaction system has also been made
Variation of volumic properties with temperature and composition of 2-butanone+1,2 propanediol binary mixtures
No full textIn this work we present experimental values of the density (rho), and some related quantities such as excess molar volumes (V-E), of the 2-butanone + 1,2-propanediol binary mixtures at various temperatures in the -10 less than or equal to t / degreesC less than or equal to 80 range and as a function of mole fraction. The experimental results have been fitted to some relationships of the type rho = rho (T), rho = rho (x(i)), and rho = rho (T,x(i)) to estimate the property in correspondence of the experimental data gaps. The observed behaviour has been interpreted on the basis of specific interactions and molecular features of the components. (C) 2000 Elsevier Science B.V. All rights reserved
O-SILYLATION OF ACYLCOBALT TETRACARBONYLS - SYNTHESIS OF A NEW TYPE OF DINUCLEAR MU-2-HYDROXYCARBENE COBALT CARBONYL DERIVATIVES
No full textO-Silylation of acylcobalt tetracarbonyls, RC(O)Co(CO)4, gives the dinuclear carbene complexes [{mu-2-RC(OSiR'3)}(mu-2-CO)Co2(CO)6], of which one derivative (R = Me, R' = Ph) was characterized by X-ray diffraction
Carbohydrate-derived alkylcobalt carbonyl: {[(1,2 : 5,6-di-O,O-isopropylidene-alpha-D-glucofuranos-3-yl)oxycarbonyl]methyl}cobalt tricarbonyl triphenylphosphine
No full textA carbohydrate-derivative alkylcobalt carbonyl, {[(1,2:5,6-di-O,O-isopropylidene-alpha -D-glucofuranos-3-yl) oxycarbonyll methyl I cobalt tricarbonyl triphenylphosphine (3) was prepared and characterized by IR, NMR, and CD spectra, as well as by X-ray diffraction. The supramolecular chemistry in the crystalline P2(1)2(1)2(1) (chiral) phase and in solution was analyzed and compared. The bulky carbohydrate-based ligand stabilized one chiral conformation, which, however, is less ordered than for analogous compounds with more flexible and less bulky chiral groups. Intermolecular, H-bond interactions are more important in the P2(1)2(1)2, phase of complex 3 than at other analogous compounds
Alkylcobalt Carbonyls, Part 13. Preparation and Molecular Structures of Benzyl- and Phenylacetylcobalt Carbonyls
No full textThe benzyl-type Co carbonyl complexes (para-t-BuC6H4CH2)Co(CO)3PPh3 (I) and [para-ClC6H4CH2C(O)]Co(CO)3PPh3 (II) were prepd. and characterized by analyses, spectra and x-ray single-crystal diffraction. The overall structures both of the alkylcobalt-type I and the acylcobalt-type II display trigonal bipyramidal geometry, with the two noncarbonyl ligands in the two axial positions. The relevance of the stereochem. of complexes I and II to the supposed mechanism of the CO insertion/deinsertion on Co is discussed
Electrochemical behaviour of oligometallic sandwich complexes of cyclosiloxanolate ligands
No full textThe conductometric and voltammetric behaviour of 16 oligocyclosiloxanolate transition metal complexes in DMF is reported and discussed. The results of the conductometric measurements indicate that in DMF most of the compounds are electrolytes and the conductivity is due prevalently to the mobile (free) Na+ cation, the contribution of the large cluster anion being negligible, The cyclic voltammetric curves show only one reversible peak couple with E-p values very close to each other. These results suggest a high extent of electronic delocalisation in the cluster core of these sandwich type complexes and permit these compounds to be regarded as heteronuclear clusters of higher-valent transition metals
Infrared spectra of isotopically substituted XCo(CO)(4) (X = H, EtC(O), EtOC(O) and EtOC(O)CH2) molecules
No full textIn this paper we present a series of observations on the infrared spectra of DCo(CO)(4) (in comparison with HCo(CO)(4)) as well as of the (CO)-C-13-exchanged derivatives of HCo(CO)(4), EtOC(O)CH2Co(CO)(4), EtOC(O)Co(CO)(4) and EtC(O)Co(CO)(4). Calculated frequencies and intensities for HCo((CO)-C-12)(n)((CO)-C-13)(4-n) based on the C-O factored force field approximation predict data in excellent agreement with that observed experimentally. Splitting of the E band in the complexes R-Co(CO)(4) is shown to depend on reduction of local site symmetry and appears to be greatest for R groups containing a local dipole capable ́of through-space ́ interaction with the three equatorial carbonyls. The analysis of the infrared absorption bands for the (CO)-C-13-substituted compounds sheds light on the nature of bonding and symmetry of these molecules, and the extent of interaction between the organic substituent and the four CO groups in the cobalt tetracarbonyl compound
Asymmetric synthesis of amino acids by Cr(II) complexes of natural amino acids
No full textStoichiometric reduction of the C,N double bond of oxime precursors of a-amino acids was performed in aqueous media by Cr(II) complexes of natural amino acids. The reduction of oximes of alpha-ketophenylacetic, alpha-keto-beta-phenylpropionic and alpha-ketopropionic acids proceeded up to > 90% conversion and 2-30% enantiomeric excess. 1:2 complexes of Cr(II) with L-alanine, L-valine, L-aspartic acid, L-histidine and L-phenylalanine were used as reducing agents. The reduction of alpha-(oximino)phenyl acetic acid showed increasing e.e. (and decreasing conversion) with increasing temperature
- …
