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

Abstract

Density functional theory and the quantum theory of atoms in molecules approach were used to study two competing process: the Diels-Alder reaction (DA) and the 1,4-alkynylboration (AB) between dichloropropynylborane (1) and 2-tertbutylbutadiene (2) in dichloromethane. We analyzed several reaction pathways related with such reactions for both orientations (meta and para). The stepwise mechanisms for the two competitive reactions share the first step that leads to an intermediate zwitterionic structure. The second step is more favorable for the reaction occurring via TSC-m that leads to the meta enyne product 5, which is the kinetic product. The formation of the meta DA product cannot be explained through a direct cycloaddition, due to the higher activation free energy of the associated transition structure (TSB-m). An alternative transition structure with [4 + 3] character (TSD-m) that connects the meta enyne 5 with the meta cycloadduct 3 was found. We propose that at longer reaction times, 5 rearranges to the thermodynamic product 3 via TSD-m passing by a six-membered ring structure and a seven-membered ring structure. The topological analysis of the charge density along the selected reaction coordinates provided some understanding on the intriguing competitive reactions. ConclusionsThe reaction of dichloropropynylborane (1) with 2-tertbutylbutadiene (2) has been studied using density functional theory and the QTAIM approach in DCM in order to rationalize the mechanisms associated with the competing DA and AB reactions, and the origin of the experimental regioselectivity.

Theoretical Study of the BF3-Promoted Rearrangement of Oxiranyl N-Methyliminodiacetic Acid Boronates

“…Final-geometry optimizations were carried out using the MPWB1K global hybrid meta-GGA functional together with the 6-311G* basis set. The MPWB1K functional proposed by the Truhlar group has been found to provide an improvement in the accuracy of the thermodynamic data in excellent agreement with experimental results. − …”

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

“…Such analysis has been successfully applied to rationalize the mechanism of a wide variety of chemical transformations. 46,[50][51][52][53][54][55][56][57][58] The proles of topological properties evaluated at the selected bcps showed similar patterns for all reaction pathways. The evolution of the topological properties of the pairs of bcps C1-N1 and C3-N3, C1-C2 and C2-C3, and N1-N2 and N2-N3, are complementary demonstrating that a continuous electron transfer occurs.…”

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

“…The analysis of the variation of the topological properties such as ρ b and the change in atomic populations N (Ω) along the reaction coordinate provides interesting information about the bonding changes and allows us to establish the nature of the electronic rearrangement associated with a given molecular mechanism. , Therefore, a better understanding of the electron density changes of the boryl group, when it behaves as a nucleophile or an electrophile can be obtained using such an approach.…”

“…The MPWB1K functional proposed by the Truhlar group has been found to provide an improvement in the accuracy of the thermodynamic data in excellent agreement with experimental results. 37,38 Frequency calculations were performed to determine the nature of the stationary points: the transition structures (TSs) had one imaginary frequency, and the reactants, the intermediate structures, and the products had no imaginary frequencies. Solvent effects in DCM or acetonitrile (solvent used experimentally) were taken into account through full optimizations using the SMDcontinuum solvation method.…”