Search results for "Selectivity"

The role of Zn2+ in enhancing the rate and stereoselectivity of the aldol reactions catalyzed by the simple prolinamide model

2011

Abstract The aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by single l -prolinamide and its zinc complexes has been studied. An increase in the rate and the stereoselectivity of the reaction has been shown by using zinc derivatives. A mechanistic proposal, based on NMR and ESI studies, has been put forward to explain the experimental data: zinc–prolinamide complexes catalyze the reaction following the general mechanism of stereoselective enamine nucleophilic addition to the acceptor aldehyde. Zn2+ prevents the nonspecific base-catalyzed reaction by diminishing the basicity of the amine nitrogen of prolinamide.

The nucleophilic addition of nitrones to carbonyl compounds: insights on the nature of the mechanism of the l-proline induced asymmetric reaction fro…

2004

Abstract The mechanism of the l -proline induced asymmetric nitrone–aldol reaction of N -methyl- C -ethylnitrone with dimethyl ketomalonate has been studied by using density functional theory at B3LYP/6-31G** level. The reaction was initialized by the addition of l -proline to the nitrone to form an aminal, which by elimination of the hydroxylamine gave a chiral enamine. The nucleophilic addition of this chiral enamine to dimethyl ketomalonate corresponds to stereoselective C–C bond-formation step. Further nucleophilic addition of hydroxylamine to the zwitterionic intermediate formed in the enamine addition gave a second aminal, which by l -proline elimination afforded the corresponding β-h…

Chemodiscrimination of Olefin Bonds Through Cross‐Metathesis Reactions – Synthesis of Functionalized β‐Lactam and β‐Amino Acid Derivatives

2019

Synthesis and Physicochemical Characterization ofmeso-Functionalized Corroles: Precursors of Organic-Inorganic Hybrid Materials

2005

Cobalt(III) corroles exhibit an infinite selectivity for the coordination of carbon monoxide towards dioxygen and dinitrogen. This peculiar property thus allows their use as sensing devices for CO detection. Here are described the syntheses and physico-chemical characterization of meso mono-, bis- and tris(triethoxysilyl)-functionalized corroles, precursors of organic–inorganic materials. The corrole ring formation was achieved in every case using the “2+1” method involving the reaction of two equivalents of an encumbered dipyrromethane with one equivalent of an aromatic aldehyde in the presence of a catalytic amount of trifluoroacetic acid. The functionalization of the corrole by triethoxy…

Carbohydrates as chiral templates: asymmetric ugi-synthesis of alpha-amino acids using galactosylamines as the chiral matrices

1988

Abstract In the presence of Lewis acid catalysts O-acetyl- (1) and O-pivaloyl- (2) protected β-D-galactopyranosylamines react with aldehydes, isocyanides and carboxylic acids in Ugi-four-component-condensations to give the corresponding N-galactosyl-amino acid amide derivatives 3,5 in almost quantitative yields. Zinc chloride is the most effective Lewis acid catalyst. At 0°C or even at room temperature the (2R,β-D)-diastereomers of the amino acid derivatives 3,5 are formed with high diastereoselectivity. If the sterically more demanding O-pivaloyl galactosylamine 2 is used at -78°C to -25°C the stereoselectivity often exeeds 20:1 favouring the (2R,β-D) diastereomers 5. After one recrystalli…

A calixarene-based chromoionophore for the larger alkali metals

1995

Abstract A bis(arylazo)calix[5]arene crown ether derivative displays large spectral changes upon complexation of cesium and rubidium salts, with good selectivity over the smaller alkali metals.

Alkylation of lithium dienediolates of butenoic acids. Regioselectivity effects of structure and leaving group of the alkylating agent

1998

Abstract Regioselectivity of alkylation of but-2-enoic acids 1 and 2 by alkyl halides strongly depends on the reactivity of the electrophile. High α selectivity results for saturated alkyl halides, whereas poor α-selectivity is obtained for highly reactive allyl and benzyl halides. For reactive alkylating halides selectivity is partly governed by the ion pairing aggregates of the dienediolates. Lithium bromide and the carboxylate generated in the ongoing reaction cause opposite effects on regioselectivity.

Studien zum Vorgang der Wasserstoffübertragung 46. Zur kathodischen Spaltung von Alkyl- und Cycloalkyl(triphenyl)-phosphoniumsalzen; Abhängigkeit der…

1977

Die kathodische Spaltung von Methyl-, Butyl- und einiger Cycloalkyl(triphenyl)phosphoniumsalze wurde potentiostatisch und galvanostatisch durchgefuhrt. Beim Methyl(triphenyl)-phosphoniumsalz hangt die prozentuale Zusammensetzung der Spaltprodukte: Triphenylphosphin und Methyl(diphenyl)phosphin sowohl vom Potential als auch von der Temperatur ab, die sich ihrerseits wieder gegenseitig beeinflussen. Zusatze unterschiedlich strukturierter quartarer Ammoniumsalze als Leitsalz verandern das prozentuale Verhaltnis (C6H5)3P/(C6H5)2PCH3 nicht, wohl aber der Zusatz von Essigsaure, welche die Bildung von Methyl(diphenyl)phosphin begunstigt. - Bei den Cycloalkyl(triphenyl)phosphoniumsalzen mit Cyclohe…

Oxidative cyclization of some aldehyde semicarbazones induced by metallic salts

1993

The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.

ChemInform Abstract: Oxidative Cyclization of Some Aldehyde Semicarbazones Induced by Metallic Salts.

2010

The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.