Search results for "Reaction"
showing 10 items of 6134 documents
Lewis acid induced [4+3] cycloadditions of 2-silyloxyacroleins. Insights on the mechanism from a DFT analysis
2005
The mechanism for the Lewis acid induced [4+3] cycloadditions of 2-(trimethylsilyloxy)acrolein with furan has been examined here through DFT calculations at B3LYP/6-31G* level. The mechanism is a three-step process initialized by the nucleophilic attack of furan to the β-conjugated position of acrolein yielding a zwitterionic intermediate. The key step on the formation of the seven-membered ring is the electrophilic attack of the furan residue to the carbonyl carbon in this intermediate. The endo selectivity experimentally observed is reproduced by the calculations.
Towards an Understanding of the Polar Diels–Alder Reactions of Nitrosoalkenes with Enamines: A Theoretical Study
2006
The polar Diels–Alder reactions of nitrosoalkenes with enamines have been studied using DFT methods at the B3LYP/6-31G* level of theory. These Diels–Alder reactions are characterized by a nucleophilic attack of the enamine at the conjugated position of the nitrosoalkene with concomitant ring-closure. The reactions present a total regioselectivity and a large endo selectivity. The analysis, based on the global electrophilicity of the reagents in the ground state, the natural bond orbital (NBO), and the topological analysis of the electron localization function (ELF) in the endo transition state (TS) and “halfway” along the IRC between the TS and cycloadduct, correctly explain the polar natur…
Theoretical study of phosphodiester hydrolysis in nucleotide pyrophosphatase/phosphodiesterase. Environmental effects on the reaction mechanism.
2010
We here present a theoretical study of the alkaline hydrolysis of methyl p-nitrophenyl phosphate (MpNPP(-)) in aqueous solution and in the active site of nucleotide pyrophosphatase/phosphodiesterase (NPP). The analysis of our simulations, carried out by means of hybrid quantum mechanics/molecular mechanics (QM/MM) methods, shows that the reaction takes place through different reaction mechanisms depending on the environment. Thus, while in aqueous solution the reaction occurs by means of an A(N)D(N) mechanism, the enzymatic process takes place through a D(N)A(N) mechanism. In the first case, we found associative transition-state (TS) structures, while in the enzyme TS structures have dissoc…
Enzyme Promiscuity in Enolase Superfamily. Theoretical Study of o-Succinylbenzoate Synthase Using QM/MM Methods
2015
The promiscuous activity of the enzyme o-succinylbenzoate synthase (OSBS) from the actinobacteria Amycolatopsis is investigated by means of QM/MM methods, using both density functional theory and semiempirical Hamiltonians. This enzyme catalyzes not only the dehydration of 2-succinyl-6R-hydroxy-2,4-cyclohexadiene-1R-carboxylate but also catalyzes racemization of different acylamino acids, with N-succinyl-R-phenylglycine being the best substrate. We investigated the molecular mechanisms for both reactions exploring the potential energy surface. Then, molecular dynamics simulations were performed to obtain the free energy profiles and the averaged interaction energies of enzymatic residues wi…
Chain-extending of Hydroxytelechelic Polybutadiene: Synthesis and Characterization
2010
Extension of hydroxytelechelic polybutadiene oligomers ( 3200 g/mol) is performed in mild conditions, in order to avoid backbone modifications, using succinic anhydride or methylenedicyclohexyl diisocyanate. The two routes present significative extension of the initial oligomers. Extension through the anhydride route, in the presence of DCC and DMAP, leads to new hydroxytelechelic oligomers ( 8000 g/mol). Extension through the diisocyanate route, catalyzed by DBTL at 65°C leads to hydroxytelechelic oligomers of higher average molecular weight ( 20000 g/mol). New materials are characterized by FTIR/1H-NMR and changes in their Tg according to , are discussed. The influence of reaction time on…
Ab initio study on the mechanism of the HCO+O2→HO2+CO reaction
2003
Abstract The gas-phase reaction HCO+O 2 →CO+HO 2 has been investigated by means of ab initio calculations. The mechanism can proceed through either a direct hydrogen abstraction or addition of O 2 to the formyl radical. The energy barriers calculated at the QCISD(T)/6-311G(2df,2p) level of theory upon QCISD/6-311G(d,p) optimized structures are, respectively, of 2.98 kcal mol −1 for the direct abstraction and of 2.26 kcal mol −1 for the addition. Thus, the results obtained show that there is not a dominant pathway in the HCO+O 2 reaction under atmospheric conditions of temperature and pressure.
Catalytic aminomercuration of olefins in a tandem aminomercuration-deoxymercuration; One-step synthesis of secondary n-arylallylamines from allylalco…
1990
Abstract Allyl alcohols react with primary aromatic amines and stoichiometric amounts of mercury(II) tetrafluoroborate to give mixtures of mono- and diallyl anilines. However, the use of the tandem aminomercuration-deoxymercuration promoted by catalytic mercury(II) tetrafluoroborate allows to perform regiospecifically the monoallylation reaction with very high yields. A mechanism is proposed to account for the observed results.
Oxygenation of Alkane C−H Bonds with Methyl(trifluoromethyl)dioxirane: Effect of the Substituents and the Solvent on the Reaction Rate
2005
[Chemical reaction: See text] The mechanism of the oxygenation of alkane C-H bonds with methyl(trifluoromethyl)dioxirane (1a) is studied through the effect of the substituent and solvent on the rate of oxygenation of 2-substituted adamantanes (2). The results suggest a remarkable electron deficiency at the reacting carbon atom in the transition state leading to the regular oxygenation products. The linearity of the Hammett plot reveals that the reaction mechanism does not change within a range of 0.15-0.67 units of sigma(I). A change in the solvent does not affect the distribution of the products, indicating a through-bond transmission of the substituent effect as the origin of the deactiva…
An efficient probe for sensing different concentration ranges of glutathione based on AIE-active Schiff base nanoaggregates with distinct reaction me…
2018
Abstract A novel tetraphenylethene-diketopyrrolopyrrole Schiff base (TPE-DPP) was synthesized by one-pot protocol. Based on thiol group and acid-base property of glutathione, nanoaggregates of TPE-DPP for sensitive and selective detection of diff ;erent concentration ranges of glutathione by two distinct reaction mechanisms was reported. It showed a turn-on mode of fluorescence signals towards low concentration of GSH by hydrolysis of TPE-DPP, but a ratiometric mode of fluorescence signal was elicited towards high concentration of GSH by addition reaction of thiol with TPE-DPP. What’s more, the probe was successfully utilized for monitoring GSH in living cells.
A QM/MM study of the reaction mechanism for the 3′-processing step catalyzed by HIV-1 integrase
2009
Integrase (IN) is one of the three human immunodeficiency virus type 1 enzymes (HIV-1) essential for effective viral replication. This viral enzyme is involved in the integration of HIV DNA into host chromosomal DNA. In this work we have carried out molecular dynamics simulations using a hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) approach to study the reaction mechanism for the 3′-processing step of DNA integration using a model substrate. The results obtained by QM(AM1)/MM and QM(PM3)/MM simulations have been improved by single-point corrections using an ab initio method to describe the quantum subsystem. The results obtained within this computational model can be used to obtain …