Density functional theory study of the cycloaddition reaction of furan derivatives with masked o-benzoquinones. Does the furan act as a dienophile in the cycloaddition reaction?

Electrophilicity index revisited

This review aims to be a comprehensive, authoritative, critical, and accessible review of general interest to the chemistry community; because the electrophilicity index is a very useful global reactivity descriptor defined within a conceptual density functional theory framework. Our group has also introduced electrophilicity based new global and local reactivity descriptors and also new associated electronic structure principles, which are important indicators of structure, stability, bonding, reactivity, interactions, and dynamics in a wide variety of physico-chemical systems and processes. This index along with its local counterpart augmented by the associated electronic structure principles could properly explain molecular vibrations, internal rotations and various types of chemical reactions. The concept of the electrophilicity index has been extended to dynamical processes, excited states, confined environment, spin-dependent and temperature-dependent situations, biological activity, site selectivity, aromaticity, charge removal and acceptance, presence of external perturbation through solvents, external electric and magnetic fields, and so forth. Although electrophilicity and its local variant can adequately interpret the behavior of a wide variety of systems and different physico-chemical processes involving them, their predictive potential remains to be explored. An exhaustive review on all these aspects will set the tone of the future research in that direction.

The Interplay between Kinetics and Thermodynamics in Furan Diels-Alder Chemistry for Sustainable Chemicals Production

Biomass-derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels-Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom-efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure-reactivity-stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron-rich furans and electron-deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.

An experimental and mechanism study on the regioselective click reaction toward the synthesis of thiazolidinone-triazole

An efficient procedure for the synthesis of novel thiazolidinone triazoles through 32 cycloaddition reactions in the presence of copper(I) species was described, and the molecular mechanism of this 32CA was investigated computationally. Different possible pathways for CA process have been studied to achieve this goal, including one-step pathways for both regioisomers 1,4- and 1,5-triazoles (uncatalyzed, mono-copper, di-copper) and also mono- and di-copper stepwise pathways for 1,4-disubstituted triazole. It was exhibited that the most convenient route in terms of energy barriers includes two copper ions. Based on the calculation, the reaction follows a di-copper stepwise mechanism involving the formation of a six-membered ring and then undergoes a ring contraction to a five-membered ring. The regiochemistry of the reaction was investigated based on local and global reactivity indices of reactants, the transition state stabilities calculation. The electron reorganization along the uncatalyzed one-step mechanism has been investigated by the ELF topological analysis of the bonding changes along with the CA reaction.

Diels-Alder Cycloadditions of Masked o-Benzoquinones with Alkenes

Diels-Alder cycloadditions of 3-oxobut-1-enyl substituted orthoquinone monoketals with olefinic dienophiles furnished functionalized ortho-endo bicyclo[2.2.2]octenone derivatives with high regio- and stereoselectivities. The competition between self-dimerization and Diels-Alder cycloaddition with an external dienophile usually exists, except in the case of 5-substituted orthoquinone monoketal.

An experimental and theoretical study on the regioselective synthesis of a new class of spiropyrrolothiazoles with quinoxaline motifs via a 1,3-dipolar cycloaddition reaction. An evaluation of DFT methods

Evaluation of B3LYP/6-31G(d,p), wB97xD/6-31G(d,p) and M06-2X/6-31G(d,p) methods revealed the importance of π/π interactions in regio- and stereoselectivity of cycloaddition reactions.

Density functional theory and RRKM calculations of the gas-phase unimolecular rearrangements of methylfuran and pyran ions before fragmentations

The potential energy profiles for the mutual conversion of the isomeric molecular ions [C5H6O]+* of 2-methylfuran, 3-methylfuran and 4H-pyran and the fragmentations that lead to [C(5)H(5)O](+) ions were obtained from calculations at the B3LYP/6-311G++(3df,3pd)//B3LYP/6-31G(d,p) level of theory. The various competing unimolecular processes were characterized by their RRKM microcanonical rate coefficients, k(E), using the sets of reactant and transition state frequencies and the kinetic barriers obtained from the density functional method. In either a high- or a low-energy regime, the pyrylium ion [C5H5O]+ is generated directly from the 4H-pyran molecular ion by a simple cleavage. In contrast, in the metastable kinetic window, the molecular ions of methylfurans irreversibly isomerize to a mixture of interconverting structures before dissociation, which includes the 2H- and 3H-pyran ions. The hydrogen atoms attached to saturated carbons of the pyran rings are very stabilizing at the position 2, but they are very labile at position 3 and can be shifted to adjacent positions. Once 4H-pyran ion has been formed, the C-H bond cleavage begins before any hydrogen shift occurs. According to our calculation, there would not be complete H scrambling preceding the dissociation of the molecular ions [C5H6O]+*. On the other hand, as the internal energy of the 2-methylfuran molecular ion increases, H* loss can become more important. These results agree with the available experimental data.

Computational investigations on the general reaction profile and diastereoselectivity in sulfur ylide promoted aziridination

Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches.

Condensed-to-atoms electronic Fukui functions within the framework of spin-polarized density-functional theory

A simple formalism devised to calculate the condensed-to-atoms Fukui function [R. R. Contreras, P. Fuentealba, M. Galván, and P. Pérez, Chem. Phys. Lett. 304, 405 (1999)] has been further analyzed within a spin-polarized density-functional theory framework. The model is based on a frozen-core approximation to these local reactivity indices [M. Galván, A. Vela, and J. L. Gázquez, J. Phys. Chem. 92, 6470 (1988)], giving us an extended local reactivity description of systems based on the frontier spin-up and spin-down molecular orbitals. Degenerate molecular spin orbitals have been explicitly included in our model equations. Computational results for the nitric oxide (NO) and some simple carbene systems are presented in order to test the model. These quantities have been discussed in the context of changes both in charge density and spin density within the context of electron charge transfer or spin-polarization processes.

Toward an Understanding of the Acceleration of Diels−Alder Reactions by a Pseudo-intramolecular Process Achieved by Molecular Recognition. A DFT Study

The pseudo-intramolecular Diels-Alder (DA) reaction between a 2-substituted furan (1) and a N-maleimide derivative (2) has been analyzed using DFT methods. Formation of two hydrogen bonds between the appendages on furan and maleimide derivatives favors thermodynamically the formation of a molecular complex (MC1) through an efficient molecular recognition process. The large enthalpy stabilization associated with the molecular recognition overcomes the unfavorable activation entropy associated with the bimolecular process. As a consequence, the subsequent DA reaction is clearly accelerated through a pseudo-intramolecular process.

Interplay of Structure and Reactivity in a Most Unusual Furan Diels-Alder Reaction

Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidly and in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the same conditions. These reactions have been explored using density functional theory (DFT) calculations. They reveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl(4) and by polar solvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding to the stepwise formation of the two new carbon-carbon bonds via transition states which have similar energies in all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predicted by our calculations. The calculated free energy barriers generally correlate well with measured reaction rates, supporting a reaction mechanism in which zwitterionic character is developed strongly. An in situ ring opening reaction of exo-cycloadduct ethyl exo-2-(N,N-diethylcarbamoyloxy)-3,3-difluoro-7-oxabicyclo[2.2.1]hept-5-enyl-2-endo-carboxylate, which results in the formation of cyclic carbonate ethyl 4,4-difluoro-5-hydroxy-2-oxo-5,7a-dihydro-4H-benzo[1,3]dioxole-3a-carboxylate by a Curtin-Hammett mechanism, has also been examined. Substantial steric opposition to Lewis acid binding prevents carbonate formation from 2-substituted furans.