Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Activity and Stereoselectivity

Preparing the active catalytic system in situ from [Rh(diene)Cl]2 and the corresponding tertiary phosphines, mainly covalent rhodium-phosphine complexes are formed. Their activity is highly influenced by the quality of the ligand used. Best results have been achieved with phosphines of high basicity and low steric requirements.RhCl(PPh3)3 is completely inactive under such conditions. [Pg.319]

Addition of small quantities of EtsN transformed the inactive RhCl(PPh3)3 into an active catalyst for ketone reduction. Catalytic systems of similar activity were prepared in situ from rhodium diene derivatives and also other aryl phosphines in presence of bases like EtsN. [Pg.320]

These latter transformed the catalyst into a hydridorhodium(I)phosphine complex and enabled the catalytic reduction of different dialkyl-, alkyl-aryl-, diaryl-, and cyclic ketones with acceptable reaction rates under mild conditions (50 , 1 bar). [Pg.321]

It was shown that by changing the phosphine/rhodium ratio in catalytic systems prepared in situ several active species are formed, but while increasing the P/Rh ratio in systems prepared from [Rh(NBD)Cl]2 + [Pg.321]

Polymer-bound heterogenized homogeneous rhodium catalysts have also been successfully used in ketone reduction. Italian scientists studied ionic rhodium complexes supported on a Merrifield resin (4), but the Wilkinson-type analog proved to be active only in the presence of [Pg.321]


The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. [Pg.165]

Ziegelman-Fjeld, K., Musa, M., Phillips, R., Zeikus, J. and Vieille, C., A Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase mutant derivative highly active and stereoselective on phenylacetone and benzylacetone. Protein Eng. Des. Set, 2007, 20, 47-55. [Pg.286]

The above-mentioned complexes are the sole iridium derivatives applied to DCR, and the cycloaddition of nitrones to enals or methacrylonitrile, the unique process studied. We think that iridium-based catalysts are underrepresented in 1,3-dipolar cycloaddition chemistry. For example, no iridium (1) systems have been developed to this end. It can be anticipated that the (bidentate ligand)lr(l) fragment could be active (and stereoselective if chiral bidentate ligands are used) in DCR such as those involving azomethine ylides. [Pg.228]

A suspension of the recycled polyamino acids in toluene is stirred vigorously with 4.0 M aqueous sodium hydroxide solution for 16 h. The procedure may be repeated to ensure optimum regeneration. The polyamino acid recovered from this procedure shows catalytic activity and stereoselectivity comparable to the freshly made polyamino acid. [Pg.129]

Table 6.1 Effect of fluoride on activity and stereoselectivity in the Ir-catalyzed addition of aniline to norbornene (Equation 6.5). ... Table 6.1 Effect of fluoride on activity and stereoselectivity in the Ir-catalyzed addition of aniline to norbornene (Equation 6.5). ...
The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. [Pg.40]

Another method for assaying the activity and stereoselectivity of enzymes at in vitro concentrations is based on surface-enhanced resonance Raman scattering (SERRS) using silver nanoparticles (116). Turnover of a substrate leads to the release of a surface targeting dye, which is detected by SERRS. In a model study, lipase-catalyzed kinetic resolution of a dye-labeled chiral ester was investigated. It is currently unclear how precise the method is when identifying mutants which lead to E values higher than 10. The assay appears to be well suited as a pre-test for activity. [Pg.30]

The group I III metal component increases activity and stereoselectivity by alkylation (often together with reduction) (Eqs. 8-28 through 8-32) of the transition metal component to form more active and stereoselective reaction sites. The group I—III metal component may also be involved in stabilizing the active transition metal sites by complexation and may be... [Pg.647]

Asymmetric hydrogenation-based processes using a highly active and stereoselective catalyst generate relatively little waste. The asymmetric hydrogenation of readily preparable a-hydroxycarbonyl- or tz-alkoxycarbonyl-suInstituted enamrdes has frequently been applied to the preparation of unnatural a-amrno acids with a wide variety of side chains since the successful application of L-Dopa (Scheme 9.4) [13h]. [Pg.187]

These reports have accelerated research investigations into improving the asymmetric alkylation of 1 in terms of catalytic activity and stereoselectivity, the result being the emergence of a series of appropriately modified cinchona alkaloid-based catalysts. The performance of the representative monomeric catalysts in the asymmetric benzylation and allylation of 1 are summarized in Table 2.1, in order to provide an overview of the relationship between the structure, activity and enantioselectivity. [Pg.11]

Consideration of Activation and Stereoselectivity. Studies on triadimefon, one of the EBI compounds, provide thoughts to consider In developing new compounds as well as addressing resistance problems. In evaluating the effectiveness of triadimefon applied to plants for pest control, the following must be taken Into consideration (33) ... [Pg.42]

The use of ILs contributes many advantages in biocatalysis, including enhanced stability, activity, and stereoselectivity [41, 44, 45], When combined with SC-CO2, ILs showed a promising media for enhanced stereoselectivity and stabiUty of biocatalysts [9, 46-48]. The use of SC-CO2 and room temperature ILs as a combined bioreaction media has been widely demonstrated [5, 7, 8]. There has been much interest in research on an efficient enzymatic system in IL/CO2 media for race-mates kinetic resolution [49, 50]. Due to the wide spectmm of their physicochemical properties, several ILs, based on the N, N -dialkyUmidazoUum cations were used for testing the enantioselectivity of CALB [34—36]. [Pg.116]

The most active and stereoselective catalysts for hydrosilylation of cyclic and acyclic dienes are analogs of [PPFA]PdCl2 C containing fluoroalkyl groups in the R side chain15. [Pg.1241]

Z)-Vinylsilanes are also accessible by using [RuCl2(p-cymene)]2 precatalyst [112]. High activities and stereoselectivity have been achieved with very good tolerance to functional groups such as chloro, alkoxy, or ester in the alkyl chain. It must be mentioned that this complex selectively hydrosilylates triple bonds in the presence of an olefmic bond (Scheme 8.40). [Pg.212]

Growing awareness of the relevance of drug stereochemistry has not only greatly stimulated the development of methods for asymmetric synthesis of enantiomerically pure drugs as well as the preparative separation of racemic pharmaceuticals, but also initiated the development of methods for precise and sensitive determination of enantiomeric proportions. On the other hand, access to pure stereoisomers has enabled scientists to study physiological activity and stereoselective metabolism of enantiomerically pure drugs. [Pg.108]

As a logical extension of the discrete bifunctional catalytic systems described above, Lectka et al. developed the homogeneous salicylate indium complex 22, containing the chiral nucleophile (alkaloid) and the Lewis acid (In(OTf)3) in a single unit, which exhibited excellent catalytic activity and stereoselectivity (90% yield, 99% ee, and 10 1 diastereomeric ratio (dr)) (Scheme 4.20) [47, 48]. Mechanistic studies revealed that the chiral nucleophiles form zwitterionic enolates that react with the metal-coordinated imines to form a ternary complex 23 in which C—C bond formation occurs. [Pg.91]


See other pages where Activity and Stereoselectivity is mentioned: [Pg.139]    [Pg.47]    [Pg.200]    [Pg.77]    [Pg.543]    [Pg.31]    [Pg.490]    [Pg.311]    [Pg.53]    [Pg.121]    [Pg.647]    [Pg.656]    [Pg.657]    [Pg.658]    [Pg.141]    [Pg.142]    [Pg.232]    [Pg.247]    [Pg.272]    [Pg.109]    [Pg.751]    [Pg.20]    [Pg.94]    [Pg.213]    [Pg.241]    [Pg.239]    [Pg.31]    [Pg.4258]    [Pg.156]    [Pg.267]    [Pg.307]    [Pg.455]    [Pg.949]    [Pg.972]    [Pg.30]   


SEARCH



Stereoselectivity activity

Stereoselectivity and

© 2024 chempedia.info