[4 + 3] and [4 + 2] mechanisms of the Diels-Alder reactions of vinylboranes: an analysis of the electron charge density distribution

Understanding the Fate of the Banert Cascade of Propargylic Azides: Sigmatropic versus Prototropic Pathway

The Banert cascade is an efficient synthetic strategy for obtaining 4,5-disubstituted 1,2,3-triazoles. The reaction can proceed via a sigmatropic or prototropic mechanism depending on the substrate and the conditions. In this work, the mechanisms of both pathways from propargylic azides with different electronic features were investigated using density functional theory, quantum theory of atoms in molecules, and natural bond orbital approaches. The calculated energy barriers were consistent with the experimental data. Three patterns of electron density distribution on the transition structures were observed, which reflected the behaviors of the reactants in the Banert cascade. The stronger conjugative effects were associated with lower/higher free activation energies of sigmatropic/prototropic reactions, respectively. A clear relationship between the accumulation of the charge at the C³ atom of propargylic azides with the energy barriers for prototropic reactions was found. Thus, the obtained results would allow the prediction of the reaction's course by evaluating reactants.

Ability of Boron to Act as a Nucleophile and an Electrophile in Boryl Shift Reactions Unveiled by Electron Density Distribution Analysis

The behavior of the tetracoordinate boron of N-methyliminodiacetic acid (MIDA) boronates as a nucleophile and an electrophile during the 1,2-boryl migration promoted by a Lewis acid and the 1,4-boryl migration promoted by a neighboring atom, respectively, have been investigated using density functional theory and the quantum theory of atoms in molecules. We found that when boron acts as a nucleophile, the electron density of the B-N interaction of the BMIDA moiety maintains the charge concentration over the boron atom, facilitating its transport toward the electron-deficient center. In this process, the BMIDA remains as a tetracoordinate. On the other hand, the B-N weakening generates a charge depletion region over the boron, allowing it to interact with the electron-rich center of O₁, developing the boron atom, a pentacoordinate form. Then, the B-N bond breaking triggers a series of changes in the electronic structure of the boron atom. Our results explain the role of the MIDA ligand upon the remarkable susceptibility of the boron atom for switching its structural and electronic characteristics in the migration processes. In addition, the dichotomous behavior was evaluated with a different scenario, considering tricoordinate pinacol boronate as a boryl migrating group.

Stereoselective [4+2]-Cycloaddition with Chiral Alkenylboranes

A method for the stereoselective [4+2]-cycloaddition of alkenylboranes and dienes is presented. This transformation was accomplished through the introduction of a new strategy that involves the use of chiral N-protonated alkenyl oxazaborolidines as dieneophiles. The reaction leads to the formation of products that can be readily derivatized to more complex structural motifs through stereospecific transformations of the C-B bond such as oxidation and homologation. Detailed computation evaluation of the reaction has uncovered a surprising role of the counterion on stereoselectivity.

Experimental and theoretical studies of the [3,3]-sigmatropic rearrangement of prenyl azides

[3,3]-Sigmatropic rearrangement of isoprenyl azides has been extensively investigated in an experimental and theoretical level.

α-Hydroxyacids accelerate the Diels–Alder reaction of dibutyl vinylboronate with cyclopentadiene: experimental results and mechanistic insights

Dibutyl vinylboronate reacts with cyclopentadiene under mild conditions in the presence of (S)-mandelic acid probably via an activated dioxaborolane intermediate.

A DFT study on PBu3-catalyzed intramolecular cyclizations of N-allylic substituted α-amino nitriles for the formation of functionalized pyrrolidines: mechanisms, selectivities, and the role of catalysts

The detailed mechanisms and stereoselectivities of PBu₃-catalyzed intramolecular cyclizations for the formation of functionalized pyrrolidines have been investigated using a DFT method.

Organocatalyzed asymmetric Michael addition by an efficient bifunctional carbohydrate–thiourea hybrid with mechanistic DFT analysis

A series of thiourea based bifunctional organocatalysts having d-glucose as a core scaffold were synthesized and examined as catalysts for the asymmetric Michael addition reaction of aryl/alkyl trans-β-nitrostyrenes over cyclohexanone and other Michael donors having active methylene.

DFT study on the reaction mechanisms and stereoselectivities of NHC-catalyzed [2 + 2] cycloaddition between arylalkylketenes and electron-deficient benzaldehydes

In this paper, two possible mechanisms (mechanisms A and B) on the stereoselective [2 + 2] cycloaddition of aryl(alkyl)ketenes and electron-deficient benzaldehydes catalyzed by N-heterocyclic carbenes (NHCs) have been investigated using density functional theory (DFT). Our calculated results indicate that the favorable mechanism (mechanism A) includes three processes: the first step is the nucleophilic attack on the arylalkylketene by the NHC catalyst to form an intermediate, the second step is the [2 + 2] cycloaddition of the intermediate and benzaldehyde for the formation of a β-lactone, and the last step is the dissociation of the NHC catalyst and the β-lactone. Notably, the [2 + 2] cycloaddition, in which two chiral centers associated with four configurations (SS, RR, SR and RS) are formed, is demonstrated to be both the rate- and stereoselectivity-determining step. Moreover, the reaction pathway associated with the SR configuration is the most favorable pathway and leads to the main product, which is in good agreement with the experimental results. Furthermore, the analysis of global and local reactivity indexes has been performed to explain the role of the NHC catalyst in the [2 + 2] cycloaddition reaction. Therefore, this study will be of great use for the rational design of more efficient catalysts for this kind of cycloaddition.

Enantioselective synthesis of 1,2-dihydronaphthalene-1-carbaldehydes by addition of boronates to isochromene acetals catalyzed by tartaric acid

Tartaric acid is an ideal asymmetric catalyst as it is abundant, cheap, and environmentally friendly. (+)-Tartaric acid was found to catalyze a novel enantioselective [4 + 2] cycloaddition of isochromene acetals and vinylboronates. A variety of substituted isochromene acetals were tolerated, furnishing the desired dihydronaphthalenes and dihydrobenzofluorene products in good yields. High enantiomeric ratios (up to 98.5:1.5) and excellent diastereoselectivities (all >99:1) were observed employing 10 mol % of (+)-tartaric acid as the catalyst, in combination with 5 mol % of Ho(OTf)3.

Competing mechanisms for the reaction of dichloropropynylborane with 2-tert-butylbutadiene. Diels–Alder reaction versus alkynylboration

Density functional theory and the quantum theory of atoms in molecules approach were used to study two competing process: the Diels–Alder reaction and the 1,4-alkynylboration.

Reactivity and selectivity of boron-substituted alkenes in the Diels-Alder reaction with cyclopentadiene. A study of the electron charge density and its Laplacian

The effect of the nature of the boron moiety upon the reactivity and the selectivity of a variety of vinylboron dienophiles (1-12) in the Diels-Alder (DA) reaction was investigated using density functional theory and the quantum theory of atoms in molecules. The calculated reactivity of the dienophiles decreases in the order vinylborane (1) > dihalovinylboranes (2-4) > dialkylvinylboranes (5-7) ≈ vinyl boronic acid (8) > vinylboronates (9, 10) > vinyl MIDA boronate (11) ≈ vinyltrifluoroborate (12). The DA reactions of 1-7 were slightly endo-selective due to the stronger C6-B secondary orbital interaction in the endo transition structures (TSs) evaluated by the C6|B delocalization index. In the TSs of 5 and 7, a combination of electronic and steric factors reduce the endo selectivity. The moderate exo selectivity calculated for the DA reactions of boronates 8-11 was attributed mainly to the hydrogen bond between the oxygen atom of boronate moieties and one of the acidic hydrogens of the methylene of cyclopentadiene in the exo TSs, which also reduces the ability of the oxygen lone pairs to donate electron density into the vacant boron orbital. Interestingly, the cooperative effect between the two hydrogen bonds in the exo TS of the DA reaction of vinyltrifluoroborate (12) determines the almost exclusive exo selectivity predicted for this DA reaction. We propose that the relative reactivities of the dienophiles can be estimated by the charge density (ρr) and its Laplacian (∇(2)ρr) at the (3,+1) critical point in the topology of ∇(2)ρr, evaluated at the reactant molecules in the ground state. The profiles of the several topological parameters along the reaction are affected by the nature of the substituents attached to the boron atom and by the mode of addition (endo and exo) in the DA reactions.