A theoretical study of the mechanism and stereoselectivity of the Diels–Alder cycloaddition between difluoro-2-methylencyclopropane and furan

DFT Investigation of the Stereoselectivity of the Lewis-Acid-Catalyzed Diels-Alder Reaction between 2,5-Dimethylfuran and Acrolein

Density functional theory (DFT) has been enacted to study the Diels-Alder reaction between 2,5-dimethylfuran (2,5-DMF), a direct product of biomass transformation, and acrolein and to analyze its thermodynamics, kinetics, and mechanism when catalyzed by a Lewis acid (LA), in comparison to the uncatalyzed reaction. The uncatalyzed reaction occurs via a typical one-step asynchronous process, corresponding to a normal electron demand (NED) mechanism, where acrolein is an electrophile whereas 2,5-DMF is a nucleophile. The small endo selectivity in solvents of low dielectric constants is replaced by a small exo selectivity in solvents with larger dielectric constants, such as DMSO. In the catalyzed process, the LA interacts with acrolein, forming a O-LA coordinating bond, that enhances its electron-acceptor character, further favoring the NED mechanism and reducing the activation energy. When AlCl3 and GaCl3 catalyze the reaction, the bond formations of both the endo and exo pathways occur via a two-step asynchronous process. Thus, these processes involve the formation of two transition states and a stable intermediate. The second transition state is the critical one and it dictates the increase of the exo selectivity, in comparison to the uncatalyzed reaction. The DFT calculations have also unraveled that the LA plays additional roles, i.e. it forms stable complexes with the carbonyl group of acrolein while AlCl3 and GaCl3 form dimers, which also impact the different equilibria.

A computational investigation of the selectivity and mechanism of the Lewis acid catalyzed oxa-Diels-Alder cycloaddition of substituted diene with benzaldehyde

The selectivity and the mechanism of the uncatalyzed and AlCl₃ catalyzed hetero-Diels-Alder reaction (HDR) between ([E]-4-methylpenta-2,4-dienyloxy)(tert-butyl)dimethylsilane 1 and benzaldehyde 2 have been studied using density functional theory at the MPWB1K/6-31G(d) level of theory. The uncatalyzed HDR between diene 1 and alkene 2 is characterized by a polar character and proceeds via an asynchronous one-step mechanism for the meta paths and synchronous for the ortho ones. In the presence of AlCl₃ catalyst, the mechanism changes to be stepwise, while the first step is the rate-determining step. The activation energies widely decrease, and the polar character increases dramatically. A large analysis of the mechanism is performed using the activation strain model/energy decomposition analysis (ASM/EDA) model, the natural bond orbital (NBO) and state specific dual descriptors (SSDDs). The obtained results indicate that the combined interaction energy associated with the distortion of the reactants in these HDR are at the origin of the observed kinetics. NBO analyses were applied to estimate the Lewis-acid catalyst donor-acceptor interaction with the molecular system. The SSDD analysis shed light into the orientation effects on the reaction kinetics by providing important information about charge transfer interactions during the chemical reaction. It indicates that the more favorable HDR pathway have the lowest excitation energies, facilitating the interaction between diene 1 and benzaldehyde 2 moieties. Non-covalent interaction (NCI) and QTAIM analyses of the meta-endo structure indicate that the presence of several weak NCIs formed at this approach is at the origin of the meta-endo selectivity.

The Role of the Catalyst on the Reactivity and Mechanism in the Diels–Alder Cycloaddition Step of the Povarov Reaction for the Synthesis of a Biological Active Quinoline Derivative: Experimental and Theoretical Investigations

An experimental and theoretical study of the reactivity and mechanism of the non-catalyzed and catalyzed Povarov reaction for the preparation of a 4-ethoxy-2,3,4,4a-tetrahydro-2-phenylquinoline as a biological active quinoline derivative has been performed. The optimization of experimental conditions indicate that the use of a catalyst, namely Lewis acid with an electron-releasing group, creates the best experimental conditions for this kind of reaction. The chemical structure was characterized by the usual spectroscopic methods. The prepared quinoline derivative has been also tested in vitro for antibacterial activity, which displays moderate inhibitory activity against both Escherichia coli and Staphylococcus aureus. The antioxidant activity was investigated in vitro by evaluating their reaction with 1,1-diphenyl-2-picrylhydrazyl DPPH radical, which reveals high reactivity. The computational study was performed on the Diels–Alder step of the Povarov reaction using a B3LYP/6-31G(d,p) level of theory. The conceptual DFT reactivity indices explain well the reactivity and the meta regioselectivity experimentally observed. Both catalysts enhance the reactivity of the imine, favoring the formation of the meta regioisomers with a low activation energy, and they change the mechanism to highly synchronous for the Lewis acid and to stepwise for the Brønsted acid. The reaction of imine with allyl alcohol does not give any product, which requires high activation energy.

Theoretical investigation of the effect of hydrogen bonding on the stereoselectivity of the Diels–Alder reaction

Here, we report the computational prediction of high exo selectivities in a series of Diels–Alder reactions with H-bonding interaction.

Effects of Solvent and Side-Chain Length on the Cycloaddition of Cyclopentadiene to N-alkylmaleimides: A Dft Study

Quantum chemistry calculations have been performed to investigate the kinetics and mechanism of the cycloaddition reaction between cyclopentadiene and N-alkylmaleimides in the gas phase and different solvents. To investigate the effects of side-chain length on the cycloaddition reaction, the rate constants and kinetic parameters of the reaction between cyclopentadiene with N-methyl-, N-ethyl-, N-propyl- and N-butyl-maleimide were calculated. The results obtained indicate that the reaction in solvents is faster than in the gas phase. Moreover, the dipole moments of the transition states are larger than those of the reactants. Therefore, the reactions in the most polar solvent (water) are faster than in ethanol, n-hexane, 2,2,2-trifluoroethanol (TFE) and acetonitrile. Quantum mechanics-molecular mechanics (QM/MM) calculations on the reactions using the explicit solvent model for water and TFE indicate that hydrogen bond interactions of the solvents have a key role in the rate of the reaction and these are more important than the polarity of the solvent. Natural bond orbital analysis reveals that the charge transfer between the reactants in solvents is more than in the gas phase. Finally, HOMO–LUMO analysis indicates that solvents increase the reactivity of the reactants in comparison to the gas phase.

Non-classical CH⋯O hydrogen-bond determining the regio- and stereoselectivity in the [3 + 2] cycloaddition reaction of (Z)-C-phenyl-N-methylnitrone with dimethyl 2-benzylidenecyclopropane-1,1-dicarboxylate. A topological electron-density study

Formation of a non-classical CH⋯O hydrogen-bond involving the nitrone C–H hydrogen is responsible for the selectivity experimentally found in this non-polar zw-type 32CA reaction.