Search results for "Lewis acids"
showing 10 items of 165 documents
A DFT Study of the Domino Inter [4 + 2]/Intra [3 + 2] Cycloaddition Reactions of Nitroalkenes with Enol Ethers
2000
The molecular mechanism of the domino inter [4 + 2]/intra [3 + 2] cycloaddition reactions of nitroalkenes with enol ethers to give nitroso acetal adducts has been characterized using density functional theory methods with the B3LYP functional and the 6-31G* basis set. The presence of Lewis acid catalyst and solvent effects has been taken into account to model the experimental environment. These domino processes comprise two consecutive cycloaddition reactions: the first one is an intermolecular [4 + 2] cycloaddition of the enol ether to the nitroalkene to give a nitronate intermediate, which then affords the final nitroso acetal adduct through an intramolecular [3 + 2] cycloaddition reacti…
ChemInform Abstract: (S)-Proline Benzyl Ester as Chiral Auxiliary in Lewis Acid-Catalyzed Asymmetric Diels-Alder Reactions.
1989
Asymmetric Diels-Alder reaction of cyclopentadiene (I) with (S)-proline benzyl ester (II) yields a mixture of the diastereomeric cycloadducts (R)-(III) and (S)-(IV).
Bonding Trends in Lewis Acid Adducts of S4N4 — X-Ray Structure of TeCl4×S4N4.
2006
Tetrasulfur tetranitride and tellurium tetrachloride react in dichloromethane to form a 1:1 adduct TeCl4·S4N4 (1). The crystal structure of 1 shows that TeCl4 is bonded to the S4N4 ring through a Te–N linkage. As a consequence, the transannular S···S bonds in S4N4 are broken and the molecule assumes an open, monocyclic conformation. The Te–N bond of 2.16(1) A is slightly longer than the single bond. The S–N bonds span a range of 1.55(1)–1.67(1) A. The adduct 1 was also characterized by mass spectrometry and Raman spectroscopy. The bonding and spectroscopic properties of 1 are compared by DFT calculations at the B3PW91/(RLC ECP) level of theory with those of BF3·S4N4 (2), SO3·S4N4 (3), AsF5·…
G2(MP2) study of the substituent effects in the H3BXHnMe3−n (X=N, P; n=0–3) donor–acceptor complexes
1998
Abstract The complexation energies of H 3 BXH n Me 3− n (X=N, P; n =0–3) donor–acceptor complexes have been investigated at the G2(MP2) level of theory. MP2(Full)/6-31G(d) optimized geometries and G2(MP2) calculated complexation energies are in good agreement with experiment. Increasing methyl substitutions on `X' donor atom augments both the basicity of XH n Me 3− n Lewis bases and the stability of complex. The NBO partitioning scheme suggests that there is no correlation between the charge transfer and the complexation energies.
An Efficient Method for Controlled Propylene Oxide Polymerization: The Significance of Bimetallic Activation in Aluminum Lewis Acids
2009
DFT Study of the Molecular Mechanism of Lewis Acid Induced [4 + 3] Cycloadditions of 2-Alkylacroleins with Cyclopentadiene
2009
The mechanism of the Lewis acid (AlCl(3)) induced [4 + 3] cycloaddition of 2-methylacrolein with cyclopentadiene (Cp) [ J. Am. Chem. Soc. 2004, 126, 2692] has been examined here through DFT calculations at the MPW1K(DCM)/6-31+G** level. Formation of these seven-membered carbocycles is a domino process that comprises three consecutive reactions. The first one is a polar Diels-Alder reaction that is initialized by the nucleophilic attack of Cp to the beta-conjugated position of acrolein, yielding the formation of the endo and exo [4 + 2] cycloadducts. The corresponding LA-[4 + 2] cycloadduct complexes equilibrate through a skeleton rearrangement with a low free activation energy with two seve…
Über die wirksamkeit von lewis-basen als initiatoren der anionischen polymerisation von formaldehyd. 27. Mitt. über polyoxymethylene
1966
Die anionische Polymerisation des monomeren Formaldehydes mit LEWIS-Basen wurde bei −78°C in Ather, Aceton und Toluol untersucht. Die Basizitat der LEWIS-Basen ist ein wichtiger Faktor for ihre Initiatoraktivitat, ist jedoch nicht allein entscheidend; es spielen auch strukturelle und steren rische Faktoren eine Rolle. Primare Amine sind wahrscheinlich deshalb schlechte Initiatoren, weil sie mit Formaldehyd Methylol- und Dimethylolamine bilden. Verzweigungen aliphatischer Substituenten wirken sich um so starker retardierend aus, je naher sich die Verzweigungsstelle am Stickstoffatom befindet. Die Polymerisationsgeschwindigkeit steigt bei den meisten Aminen mit zunehmender Polaritat des Losun…
Why Do Five-Membered Heterocyclic Compounds Sometimes Not Participate in Polar Diels–Alder Reactions?
2013
The reactions of bicyclic enone (BCE, 1) with cyclopentadiene (Cp, 2) and the five-membered heterocyclic compounds (FHCs) furan 3 and N-methyl pyrrole 4 for the construction of polycyclic heterocyclic compounds have been studied at the B3LYP/6-31G* level. No reaction takes place in the absence of Lewis acid (LA) catalysts as a consequence of the high activation energy associated with these reactions. Electrophilic activation of BCE 1 by formation of a complex with the BF3 LA, 1-BF3, and solvent effects favor the reactions. However, a different reactivity is manifested by Cp 2 and FHCs 3 and 4. Thus, while the reaction of 1-BF3 with Cp 2 yields the expected exo [4 + 2] cycloadduct, the react…
ChemInform Abstract: Low-Loading Asymmetric Organocatalysis
2012
Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Bronsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).