A DFT Characterization of the Mechanism for the Cycloaddition Reaction between 2-Methylfuran and Acetylenedicarboxylic Acid

Diels–Alder reactions between hexafluoro-2-butyne and bis-furyl dienes: kinetic versus thermodynamic control

The tandem [4+2] cycloaddition between hexafluoro-2-butyne and bis-furyl diens can lead to “pincer” or “domino”-adducts.

Why trans- or cis-Dimethyl Fumarate Addition to 2,5-Dimethylpyrrole Gives Exclusively trans-7-Azanorbornane

The addition mechanism of dimethyl fumarate into 2,5-dimethylpyrrole is explored using density functional theory (DFT) methods. Our calculations find that TpW(NO)(PMe3)(η(2)-3H-2,5-dimethylpyrrole) prefers to undergo two TpW(NO)(PMe3) migrations, two 1,5-hydride migrations, and one reductive elimination to isomerize into TpW(NO)(PMe3)(η(2)-1H-2,5-dimethylpyrrole), in which TpW(NO)(PMe3) plays a proton-transfer role. trans-Dimethyl fumarate and TpW(NO)(PMe3)(η(2)-1H-2,5-dimethylpyrrole) tend to adopt a concerted cycloaddition manner to afford trans-7-azanorbornane with a free-energy barrier of 21.8 kcal/mol. cis-Dimethyl fumarate and TpW(NO)(PMe3)(η(2)-1H-2,5-dimethylpyrrole) are the most likely to experience a concerted cycloaddition → ring opening → ring closing process to provide trans-7-azanorbornane in which the concerted cycloaddition and the ring-opening process are in dynamic equilibrium (with similar energy barriers of 21.5 and 21.9 kcal/mol, respectively). The presence of TpW(NO)(PMe3) not only promotes the cycloaddition of trans- or cis-dimethyl fumarate with 2,5-dimethylpyrrole by donating d-electrons of the W atom into the diene system of the Diels-Alder reaction, but also is favorable for the ring-opening process of the formed cis-7-azanorbornane. Furthermore, trans-azanorbornane is 7.4 kcal/mol more stable than cis-azanorbornane. Our calculations provide a new explanation of the addition of dimethyl fumarate with 2,5-dimethylpyrrole exclusively giving trans-7-azanorbornane.

Understanding the domino reaction between 3-chloroindoles and methyl coumalate yielding carbazoles. A DFT study

The molecular mechanism of the reaction between N-methyl-3-chloroindole and methyl coumalate yielding carbazole has been studied using DFT methods at the MPWB1K/6-311G(d,p) level in toluene. This reaction is a domino process that comprises three consecutive reactions: (i) a polar Diels-Alder (P-DA) reaction between indole and methyl coumalate yielding two stereoisomeric [2 + 4] cycloadducts (CAs); (ii) the elimination of HCl from these CAs affording two stereoisomeric intermediates; and (iii) the extrusion of CO2 in these intermediates, finally yielding the carbazole. This P-DA reaction proceeds in a completely regioselective and slightly exo selective fashion. In spite of the highly polar character of this P-DA reaction, it presents a high activation enthalpy of 21.8 kcal mol(-1) due to the loss of the aromatic character of the indole during the C-C bond formation. Thermodynamic calculations suggest that the P-DA reaction is the rate-determining step of this domino reaction; in addition, the initial HCl elimination in the formal [2 + 4] CAs is kinetically favoured over the extrusion of CO2. Although the P-DA reaction is kinetically and thermodynamically very unfavourable, the easier HCl and CO2 elimination from the [2 + 4] CAs together with the strong exergonic character of the CO2 extrusion makes the P-DA reaction irreversible. An ELF topological analysis of the bonding changes along the P-DA reaction supports a two-stage one-step mechanism. An analysis of the global DFT reactivity indices at the ground state of the reagents confirms the highly polar character of this P-DA reaction. Finally, the complete regioselectivity of the studied reactions can be explained using the Parr functions.

Understanding the mechanism of polar Diels-Alder reactions

A good correlation between the activation energy and the polar character of Diels-Alder reactions measured as the charge transfer at the transition state structure has been found. This electronic parameter controls the reaction rate to an even greater extent than other recognized structural features. The proposed polar mechanism, which is characterized by the electrophilic/nucleophilic interactions at the transition state structure, can be easily predicted by analyzing the electrophilicity/nucleophilicity indices defined within the conceptual density functional theory. Due to the significance of the polarity of the reaction, Diels-Alder reactions should be classified as non-polar (N), polar (P), and ionic (I).

Highly Selective Diels-Alder Reactions of Directly Connected Enyne Dienophiles

The paper describes the course of cycloadditions of Diels-Alder dienophiles containing linked enyne sites, each substituted with activating groups. Consistently, it was found that in the enyne cases the Diels-Alder reaction occurred specifically at the acetylenic center. Furthermore, it was found that the regiochemical sense of the cycloaddition was apparently determined by the remote activating group bound to the olefinic site. This remote acrylyl group totally dominated the course of the cycloaddition, relative to the activating group bound directly on the acetylene site. Explanations for these findings at the computational level are provided. The computations also rationalize the strong preference for cycloaddition to occur at the acetylene linkage and encompass the otherwise surprising regiochemical dominance by the remote ester on the olefinic site. The high selectivities available through such reactions provide important new opportunities in the synthesis of orsenillate type substructures that are found in a variety of natural products of contemporary interest.

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.

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?

The molecular mechanism for the cycloaddition reaction between 2-methylfuran and a masked o-benzoquinone has been characterized using quantum mechanical calculations at the B3LYP/6-31G theory level. An analysis of the results on the reaction pathway shows that the reaction takes place along a polar stepwise mechanism. The first and rate-determining step corresponds to the nucleophilic attack of the furan ring on the doubly conjugated position of the 2,4-dienone system present at the masked o-benzoquinone to give a zwitterionic intermediate. Closure of this intermediate affords the formally [2 + 4] cycloadduct. For the second step two reactive channels have been characterized corresponding to the formation of the formally [2 + 4] and [4 + 2] cycloadducts. Analysis of the energetic results indicates that while the first is the meta regiocontrolling and endo stereocontrolling step, the second one is responsible for the formation of the unexpected formally [2 + 4] cycloadduct. The global and local electrophilicity/nucleophilicity power of the reactants and intermediate have been evaluated to rationalize these results. Density functional theory analysis for these cycloadditions is in complete agreement with the experimental outcome, explaining the reactivity and selectivity of the formation of the formally [2 + 4] cycloadducts.

Aromatic dienophiles. 1. A theoretical study of an inverse-electron demand Diels-Alder reaction between 2-aminopyrrole and 1,3,5-triazine

This study is devoted to a detailed theoretical study of an inverse-electron demand Diels-Alder reaction (IDA) with 1,3,5-triazine as the diene and 2-aminopyrrole 1A(alpha) as the dienophile, which is a key step in a cascade reaction for the one-pot synthesis of purine analogues. Geometries were optimized with the B3LYP/6-31G* method and energies were evaluated with the MP2/6-311++G** method. This IDA reaction occurs through a stepwise mechanism, where the first step corresponds to the nucleophilic attack of 2-aminopyrrole to triazine to form a zwitterionic intermediate, which is in equilibrium with a neutral intermediate through a hydrogen transfer process, followed by a rate-determining ring-closure step. It is shown that the B3LYP method significantly overestimates the activation energy, whereas the MP2 method offers a reasonable activation barrier of 27.9 kcal/mol in the gas phase. The solvation effect has been studied by the PCM model. In DMSO, the calculated activation energy of the IDA reaction is decreased to 24.0 kcal/mol with a strong endothermicity of 17.4 kcal/mol due to the energy penalty of transforming two aromatic reactants into a nonaromatic IDA adduct. The possible stepwise [2+2] pathway is ruled out based on its higher activation and reaction energies than those of the [4+2] pathway. By comparing the IDA reactions of triazine to 2-aminopyrrole and pyrrole, we address two crucial roles of the alpha-amino substituent in lowering activation and reaction energies and controlling the reaction regiochemistry.

A theoretical study of the reaction between cyclopentadiene and protonated imine derivatives: a shift from a concerted to a stepwise molecular mechanism

The reaction between cyclopentadiene and protonated pyridine-2-carboxaldehyde imine derivatives has been studied by using Hartree-Fock (HF) and B3LYP methods together with the 6-31G basis set. The molecular mechanism is stepwise along an inverted energy profile. This results from the protonation on both nitrogen atoms of the imine group and the pyridine framework. The first step corresponds to the nucleophilic attack of cyclopentadiene on the electron-poor carbon atom of the iminium cation group to give an acyclic cation intermediate, and the second step is associated with the ring closure of this intermediate via the formation of a C-N single bond yielding the final cycloadduct. Two reactive channels have been characterized corresponding to the endo and exo approach modes of the cyclopentadiene to the iminium cation. The role of the pyridium cation substituent and the nitrogen position (ortho, meta, and para) along the reaction pathway has been also considered. Solvent effects (dichloromethane) by means of a continuum model have been taken into account to model the experimental environment.