Search results for "Bases"

Amino Acid Esters as Chiral Auxiliary Groups in Lewis Acid Catalyzed Reactions of Electron-rich Siloxydienes with Imines

1990

Lewis Acid-Catalyzed Stereoselection on Carbohydrate Templates

1989

The chirality and the complexing abilities of carbohydrates are utilized for chemical stereoselection in different reactions. In this sense, the carbohydrates are investigated as the chiral auxiliaries in alkylation reactions and in Michael additions of ester enolates. Furthermore, carbohydrates are demonstrated to be efficient chiral matrices in Lewis acid-catalyzed Diels-Alder reactions. They also effect stereoselection in a new synthesis of β-branched carboxylic acid derivatives. O-Acylated glycosylamines are shown to be potent chiral templates in the Strecker synthesis and in the Ugi four-component condensation to give α-amino acid derivatives in high yield and diastereoselectivity. In …

Chloroplastic glutamine synthetase from Brassica napus.

1993

Understanding the formation of [3+2] and [2+4] cycloadducts in the Lewis acid catalysed reaction between methyl glyoxylate oxime and cyclopentadiene:…

2013

The formation of the formal [3+2] and [2+4] cycloadducts in the Lewis acid (LA) catalysed reaction of cyclopentadiene (Cp, 1) with methyl glyoxylate oxime (MGO, 2a) has been theoretically studied using DFT methods. Coordination of BF3 LA to the oxygen atom of MGO 2a not only increases the electrophilicity of the oxime, but also makes the corresponding tautomeric BF3:nitrone complex 8b the reactive species. The reaction is characterised by the nucleophilic attack of Cp 1 on the carbon atom of the corresponding BF3:nitrone complex 8b. The subsequent ring closure at the end of the reaction allows the formation of the [3+2] or [2+4] cycloadducts. ELF bonding analysis of selected points on the i…

Direct observation of a borane-silane complex involved in frustrated Lewis-pair-mediated hydrosilylations.

2014

Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interactio…

A Study of the Effects of the Lewis Acid Catalysts on Oxa‐Diels‐Alder Reactions through Molecular Electron Density Theory

2020

A DFT study of the role of the Lewis acid catalysts in the [3 + 2] cycloaddition reaction of the electrophilic nitrone isomer of methyl glyoxylate ox…

2015

The molecular mechanism and stereoselectivity of the BF3 Lewis acid catalyzed [3 + 2] cycloaddition (32CA) reaction between C-methoxycarbonyl nitrone and cyclopentene has been theoretically studied using DFT methods at the MPWB1K/6-31G(d) computational level. The BF3 catalyst accelerates the 32CA reaction by decreasing the activation energy leading to the formation of the trans cycloadduct as the kinetic product, in agreement with the experimental data. Inclusion of solvent effects slightly increases the activation energy and decreases the exothermic character of the 32CA reaction as a consequence of a better solvation of nitrone than the transition state and the cycloadduct. The use of the…

Halogen bonded supramolecular complexes and networks

2008

In spite of some controversy of the true nature of the interaction between polarized halogen atoms and neutral or charged Lewis bases, termed “halogen bonding”, as a primary interaction, it is a very useful new tool/way to construct supramolecular complexes and networks. This is especially true in solid state supramolecular chemistry where utilization of weak intermolecular interactions such as halogen bonding opens up new insights to materials design and supramolecular synthesis.

The influence of branching isomerization on the product distribution obtained during cracking of n-heptane on acidic zeolites

1985

Abstract The initial selectivities for the primary products occurring during the cracking of n-heptane on a rare-earth (RE)-HY ultrastable zeolite up to 470°C have been calculated. The activation energies for the cracking of n-heptane and its branched isomers in the fraction C4 + C3 have been obtained by molecular orbital calculations, and these values and the predicted product distribution have been compared with those obtained experimentally. It is concluded that the cracking of n-heptane on large-pore zeolites may take place by different parallel routes involving protolytic and β-cracking of n-heptane and β-cracking of the readily formed branched carbenium ions. The relative importance o…

ChemInform Abstract: Synthesis of Functionalized Indoles with an α-Stereogenic Ketone Moiety Through an Enantioselective Friedel-Crafts Alkylation wi…

2010

Chiral complexes of BINOL-based ligands with hafnium tert-butoxide catalyze the enantioselective Friedel-Crafts alkylation of indoles with (E)-1,4-diaryl-2-butene-1,4-diones at room temperature, with good yields and ee up to 94%. Hafnium(IV) was found to be a more effective Lewis acid than other frequently used metal ions such as titanium(IV) or zirconium(IV). Unlike the enantioselective Friedel-Crafts alkylation of indoles with α,β-unsaturated compounds where the stereogenic center is generated in the β-position to a carbonyl group, the Friedel-Crafts alkylation with 2-butene-1,4-diones described here generates an α-stereogenic center with respect to one of the carbonyl groups. This can be…