Search results for "solvent effects"

Molecular dynamics simulation in aqueous solution of N -methylazetidinone as a model of β -lactam antibiotics

1999

In this article, we analyze the results of a molecular dynamics simulation in aqueous solution of the N-methylazetidinone molecule, often used to model β-lactam antibiotics. The radial distribution functions (RDFs) corresponding to the most interesting atoms, in terms of reactivity, are presented. We focus our study on the effect of a polar environment on the molecule. The solvent structure around the system is compared to the structure of β-lactam-water complexes, as obtained in a previous study of reaction mechanisms for the neutral and alkaline hydrolyses of N-methylazetidinone. Two types of complexes have been considered which are related to different hydrolysis mechanisms having simila…

Step growth polymerization via tandem ene and diels-alder reactions

1997

In contrast to other electron deficient enophiles or dienophiles which react with 1,4-cyclohexadiene (1,4-CHD) under aromatization, 1,2,4-triazoline-3,5-diones react in a defined reaction sequence. In a first step the 1,4-cyclohexadiene reacts via an ene-reaction. The intermediately formed substituted 1,3-cyclohexadiene immediately reacts in a Diels-Alder reaction. The regiochemistry of the Diels-Alder reaction is controlled by the solvent polarity. This reaction sequence can be used to synthesize polymers. Low-molar-mass model compounds were synthesized to elucidate the stereochemistry of the polymers. By varying the bistriazolinedione, polymers with backbones of different rigidity are ava…

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…

Understanding the Participation of Quadricyclane as Nucleophile in Polar [2σ + 2σ + 2π] Cycloadditions toward Electrophilic π Molecules

2008

The formal [2sigma + 2sigma + 2pi] cycloaddition of quadricyclane, 1, with dimethyl azodicarboxylate, 2, in water has been studied using DFT methods at the B3LYP/6-31G** and MPWB1K/6-31G** levels. In the gas phase, the reaction of 1 with 2 has a two-stage mechanism with a large polar character and an activation barrier of 23.2 kcal/mol. Inclusion of water through a combined discrete-continuum model changes the mechanism to a two-step model where the first nucleophilic attack of 1 to 2 is the rate-limiting step with an activation barrier of 14.7 kcal/mol. Analysis of the electronic structure of the transition state structures points out the large zwitterionic character of these species. A DF…

A Theoretical Study of the Favorskii Rearrangement. Calculation of Gas-Phase Reaction Paths and Solvation Effects on the Molecular Mechanism for the …

1997

The molecular mechanism of the α-chlorocyclobutanone transposition to yield cyclopropanecarboxylic acid, as a model of the Favorskii rearrangement, has been theoretically characterized in vacuo by means of ab initio molecular orbital procedures at the Hartree−Fock (HF) level of theory with the 6-31G* and 6-31+G* basis sets. The electron correlation has been estimated at the MP2/6-31G* level and calculations based on density functional theory, BLYP/6-31G*. The solvent effects are included at HF/6-31G* level by means of a polarizable continuum model. The questions related to the two accepted molecular mechanisms, the semibenzilic acid and the cyclopropanone transpositions, as well as the comp…

The influence of solvent on conformational properties of peptides with Aib residue—a DFT study

2017

The conformational propensities of the Aib residue on the example of two model peptides Ac-Aib-NHMe (1) and Ac-Aib-NMe2 (2), were studied by B3LYP and M06-2X functionals, in the gas phase and in the polar solvents. To verify the reliability of selected functionals, we also performed MP2 calculations for the tested molecules in vacuum. Polarizable continuum models (PCM and SMD) were used to estimate the solvent effect. Ramachandran maps were calculated to find all energy minima. Noncovalent intramolecular interactions due to hydrogen-bonds and dipole attractions between carbonyl groups are responsible for the relative stabilities of the conformers. In order to verify the theoretical results,…

Efficient Resolution of Menthylamine with Inexpensive (R,R)-Tartaric Acid by Dielectrically Controlled Resolution (DCR)

2013

A practical procedure for the resolution of menthylamine 2 with (R,R)-tartaric acid [(R,R)-3] as resolving agent is presented. Variation of the solvent system allows both enantiomers of 2 to be selectively crystallized. Performing the resolution in methanol containing 6 % water leads to (–)-2·(R,R)-3·MeOH. The other, less-soluble diastereomeric salt is obtained by applying a solvent system consisting of methanol with 19 % water with a yield of 14 %. Subsequent basic workup with aqueous sodium hydroxide gave the free menthylamine compounds. Further workup of the mother liquors and an additional recrystallization step allowed the (–)-2·(R,R)-3·MeOH salt to be obtained in an overall yield of 2…

Solvent dependent photochemical reactivity of 3-allyloxy-1,2,4-oxadiazoles

2009

Structural study of oxalamide compounds: 1H, 13C, and DFT calculations

2011

Abstract The conformational properties of some N-alkyl, N,N′-dialky, and tetraalkyloxalamides have been investigated, in vacuo and in solvent using DFT methods at the B3LYP/6-31G∗∗ computational level. Special emphasis has been given on oxalamides with substituents of the type –CH2CH2OH. In oxalamides with the N–H group (N-alkyl and N,N′-dialky), the most stable conformations are those in which the oxalamide moiety adopts a planar s-trans arrangement and the amide bonds are trans. A different situation appears in the case of tetraalkyloxalamides, in which the oxalamide moiety always adopts a skewed arrangement and there are conformations with similar energy. A careful study of 13C and 1H NM…

Heterogeneous Gold Catalyst: Synthesis, Characterization, and Application in 1,4-Addition of Boronic Acids to Enones

2015

The new 1 wt % Au/TiO2–UVM-7 catalyst was prepared and fully characterized. This heterogeneous catalyst proved to be active, selective and recyclable for the unprecedented gold-catalyzed 1,4-addition of various functionalized arylboronic acids to 2-cyclohexen-1-one and other selected enones using toluene as a solvent. The gold-based catalyst was recycled two times and played an active role in this reaction, and the nature of the solvent determined a remarkable change in the products’ selectivities.