Search results for "addition"

1993

The influence of lithium tert-butoxide (tBuOLi) and of lithium chloride on the oligomerization of tert-butyl acrylate (tBuA) initiated by tert-butyl α-lithioisobutyrate (tBiB-Li) was investigated. These additives affect both the kinetics and the product distribution. Whereas the addition of LiCl leads to a narrower molecular-weight distribution (MWD) the presence of tBuOLi induces broader MWD's, characterized by a very high fraction of the dimer. Both additives decrease the rates of propagation to different degrees. These effects are discussed on the basis of the formation of aggregates and adducts, the lithiated dimer having a higher tendency to form aggregates than the other oligomers. Th…

A quantum mechanics/molecular mechanics study of the acylation reaction of TEM1 β-lactamase and penicillanate

2000

The acylation step in β-lactamase catalyzed hydrolysis of β-lactams has been explored by means of a quantum mechanics/molecular mechanics approach (AM1/CHARMM). The TEM1 enzyme, a class A β-lactamase, and the penicillanate constitute the system employed in our study. The entire molecular system is divided into a quantum and a classical region: the quantum part is composed by the substrate, the serine Ser70 and the essential moieties of key active site residues, Lys73, Ser130 and Glu166, as well as a water molecule present in the active site region, while the classical part is formed by the remaining residues and structural waters of the enzyme. In particular, the sequence of steps proposed …

Die Herstellung einer molekulareinheitlichen Tri-, Tetra- und Pentamethacrylsäure. V. Mitt. Modelle für Matrizenreaktionen

1968

Vom p-Kresol sowie einer phenolischen Zwei- und einer Dreikernverbindung wurden mit verschiedenen Acylierungsmethoden Methacrylsaureester erhalten; die Zweikernverbindung enthielt danach zwei und die Dreikernverbindung drei Methacrylreste. Loste man diese Methacrylsuureester in groser Verdunnung in siedendem Benzol und lies gleichzeitig einen grosen Uberschus an Radikalen aus α.α′-Azoisobuttersauredinitril einwirken, so wurde beim Methacrylsaure-p-kresylester eine Polymerisation unterdruckt. Bei den mehrfachen Estern der Mehrkernverbindungen wurde die polyrmerisationsahnliche Additionsreaktion auf die einzelnen Molekule begrenzt (intramolckulare Polymerisation); sie ergab neben einer cyclis…

Cure behaviour of epoxy resin matrices for carbon fibre composites

1999

The cure behaviour of two resin formulations (with high and low curing agent content respectively) of an epoxy resin system, used as matrix for carbon fibre composites, was studied through calorimetric analysis. The aim of this work is to investigate the kinetics of this specific epoxy system in order to be able to choose a proper set of processing parameters which will give good composite material properties. The shape of the conversion curves gives evidence of the differences in the cure kinetics of the two systems. Furthermore, the values of the activation energies were determined both for formulation in the conversion range where vitrification occurs, following a phenomenological approa…

The identification of vicinally substituted cyclohexane isomers in their mixtures by 1H and 13C NMR spectroscopy.

2000

The radical addition reactions of organobromine compounds, XBr (X = CH2COOMe, PhCH2, CHBr2 and CCl3) with cyclohexene afforded mixtures of cis/trans isomer pairs of 1-X-2-Br-cyclohexanes. In addition to benzyl benzoyloxy derivatives are formed also, when benzoyl peroxide is used as an initiator. Owing to the great difficulties in separating these cis/trans isomer pairs, they are identified directly in their mixtures by NMR spectroscopy. In addition to one-dimensional (ID) 1H, proton decoupled 13C and DEPT-135, also two-dimensional (2D) 13C-13C INADEQUATE as well as 1H-13C HMQC experiments have been used in assigning the signals of each compound in their mixtures. The identification of each …

Asymmetric Addition and Cycloaddition Reactions with Ylidene‐Five‐Membered Heterocycles

2021

Five-membered heterocycles bearing an exocyclic double bond have been successfully used as substrates in asymmetric addition and cycloaddition reactions. Ylidene-heterocycles are attractive substrates due to their high functionalization and the presence of an electrophilic conjugated exocyclic double bound that can participate in nucleophilic addition reactions as well as cycloaddition reactions, which may be triggered by the formation of aromatic intermediates or products in many cases. During the last decades, catalytic methodologies have been developed using ylidene-heterocycles as substrates in order to synthesize useful optically active heterocyclic derivatives. 4-Ylidene-pyrazol-5-one…

Enantioselective Addition of Dimethylzinc to Aldehydes Catalyzed by N-Substituted Mandelamide-Ti(IV) Complexes.

2005

Abstract Amides derived from ( S )-(+)-mandelic acid in the presence of titanium isopropoxide catalyze the enantioselective addition of dimethylzinc to aromatic aldehydes with good yields and ee up to 90%.

2014

Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical’s SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta react…

Synthesis of new polydentate oxalamide-based ligands as chiral catalysts for the enantioselective addition of diethylzinc to benzaldehyde

2006

New polydentate oxalamide-based ligands have been studied as chiral catalysts in the enantioselective addition of diethylzinc to benzaldehyde

ChemInform Abstract: One-Pot Synthesis of Polysubstituted Indolizines by an Addition/Cycloaromatization Sequence.

2013

Indolizines carrying various substituents in positions 5-8 were obtained from readily available 2-(1H-pyrrol-1-yl)nitriles and α,β-unsaturated ketones or aldehydes in a one-pot procedure. Michael addition of the deprotonated aminonitriles to the acceptors followed by acid-catalyzed electrophilic cyclization produces 5,6-dihydroindolizine-5-carbonitriles. From these stable intermediates, substituted indolizines were obtained via base-induced dehydrocyanation.