Remarkable influence of mild Lewis acid catalysts on cycloadditions leading to tetrasubstituted isoxazolidines: DFT analysis augmented by X-ray and NMR studies

Solvent effects on cycloaddition reactions of potent spin-trapping probe N-tert-butylmethanimine N-oxide: A DFT study

[Formula: see text]-tert-butylmethanimine [Formula: see text]-oxide is a potent spin-trapping probe for biologically important radicals, and this nitrone undergoes complete regioselective cycloadditions to less electron-deficient monosubstituted olefins. In the present study, solvent effects on the cycloaddition of this nitrone to styrene have been theoretically studied in terms of the global properties of the reactants, electrophilic and nucleophilic Parr function analysis and the activation and reaction energies of located transition states and products. Formation energies of optimized radical adducts were computed to determine their stabilities in different solvents. The cycloaddition is predicted to be completely ortho regioselective which is in complete agreement with experiments and involved earlier C–C bond formation owing to the insufficient depopulation of [Formula: see text]-conjugated carbon atom in styrene and shows varying asymmetry indices in different solvents. Decrease in activation parameters and increase in stability of cycloadducts are predicted with decreasing solvent polarity. Aqueous media destabilize the radical adducts as predicted from the calculated formation energy, enthalpy and free energy of reaction. Hydroxyl as well as methyl radical adducts are predicted to be more stable than superoxide anion radical adducts. These predictions are in complete agreement with the experiments.

Acyclic and cyclic nitrone cycloadditions to 1-cinnamoyl-1-piperidine: A DFT study

DFT studies have been carried out for the cycloaddition reactions of a cyclic nitrone, 1-pyrroline-1-oxide and an acyclic nitrone, C,N-diphenyl nitrone to an unsymmetrically disubstituted dipolarophile, 1-cinnamoyl-1-piperidine. These reactions proved to be opposite to each other with respect to the electron demand character predicted by the electronic chemical potentials, electrophilicities and charge transfer at the transition states. The regio- and stereoselectivities have been predicted from DFT based reactivity indices, interaction energy calculations using a perturbative orbital independent theoretical model and the activation parameters of the located transition states. Two different concepts have been used for the evaluation of interaction energies. The selectivities were found to be in conformity with the experimental findings. The time gaps between the formations of C – C and C – O bonds were evaluated from single trajectory simulations. The asynchronicity of bond formation process was analyzed from the wiberg bond indices, atom–atom overlap weighted NAO bond orders and the calculated asymmetry indices of the transition states.

DFT interpretations for cycloadditions of an electron deficient C-aryl-N-phenyl nitrone

1,3-dipolar cycloaddition reactions of an electron deficient C-aryl-N-phenyl nitrone to benzylidene derivatives (with different electrophilicities) have been analyzed by density functional theory calculations. The transition states corresponding to the endo and exo approaches along the feasible regioisomeric reaction channels have been located for each cycloaddition. The reactions follow a concerted mechanism with asynchronous transition states. The asynchronicity along the regiochemical reaction modes depends on the β-carbon electrophilicities of the olefins. The regio and stereochemistries predicted from the calculated activation energy barriers (with solvent and higher basis set corrections) of the located transition states are in conformity with the experimental results. The local electrophilicities, softness matching indices and the interaction energies were then calculated to analyze how well these reactivity parameters can interpret the regioselectivities of such reactions. The electronic populations at the reactive sites computed from electrostatic potential-driven atomic charges provided correct and consistent predictions for each theoretical model contrary to the natural orbital based charges.

An experimental and theoretical study of the polar [2+3] cycloaddition reactions between 1-chloro-1-nitroethene and (Z)-C-aryl-N-phenylnitrones

Abstract Kinetic studies and B3LYP/6-31g(d) calculations indicate the polar nature of [2+3] cycloadditions between 1-chloro-1-nitroethene to (Z)-C-aryl-N-phenylnitrones. This is clearly confirmed by the activation parameters and the substituent and solvent effects. Graphical abstract