Reactivity and Selectivity of Boron-Substituted Alkenes in the Diels–Alder Reaction with Cyclopentadiene. A Study of the Electron Charge Density and Its Laplacian

Abstract

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.

Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers via ROMP

“…Our calculations indicate that the reaction favors the endo product 5c with Ts-energies of endo-BpinBdan-TS, which is 2 Kcal /mol less than that of exo-BpinBdan-TS (Figure 4E). 21,29,33 This is in a good agreement with the fact that the aromaticplanar Bdan group is less bulky than the Bpin group. 13,27,32,42 Therefore, it would appear that the diastereoselectivity in this case is driven primarily by sterics.…”

“…28 To answer these questions, we first conducted computational studies on the DA reaction to predict whether gem-diborylalkenes (2) could be used as dienophile reactive partners for the DA reaction. 29,33 According to our theoretical calculations of the energy profile of the cycloaddition reaction of diene-Cp 3e with dienophiles 1, 2 at room temperature-rt, the transition state of gemBpinBpin-TS is likely to be more energetically stabilized compared to vinyboronate TS's Bpin-exo-TS by 1.4 Kcal/mol. 30,33,34 This supports the fact that 2 is slightly more "dienophilic" toward the DA reaction than 1, albeit more bulky (Figures 2C-D).…”

“…29,33 According to our theoretical calculations of the energy profile of the cycloaddition reaction of diene-Cp 3e with dienophiles 1, 2 at room temperature-rt, the transition state of gemBpinBpin-TS is likely to be more energetically stabilized compared to vinyboronate TS's Bpin-exo-TS by 1.4 Kcal/mol. 30,33,34 This supports the fact that 2 is slightly more "dienophilic" toward the DA reaction than 1, albeit more bulky (Figures 2C-D). Cylview structures of the TSs of Bpin-exo-TS, Bpin-endo-TS and gemBpinBpin-TS, the calculations were performed with Gaussian 16 software using the M06-2X.…”

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

“…24,25,[28][29][30] To further elucidate the electron density redistribution during the two competing processes and in an attempt to explain how the zwitterionic six-membered ring rearranges to the seven-membered ring structure leading to the DA cycloadduct from AB product, we carried out a topological analysis of charge density along the IRC coordinate associated with TSC-m and TSD-m. We analyzed the variations of several topological properties (the charge density ρ b , its Laplacian ∇ 2 ρ b and the ellypticity ε) at the bond critical point (bcp) in some bonds along the reaction path as well as the delocalization index (DI). 24,25,[28][29][30] To further elucidate the electron density redistribution during the two competing processes and in an attempt to explain how the zwitterionic six-membered ring rearranges to the seven-membered ring structure leading to the DA cycloadduct from AB product, we carried out a topological analysis of charge density along the IRC coordinate associated with TSC-m and TSD-m. We analyzed the variations of several topological properties (the charge density ρ b , its Laplacian ∇ 2 ρ b and the ellypticity ε) at the bond critical point (bcp) in some bonds along the reaction path as well as the delocalization index (DI).…”

“…However, the formation of the meta enyne as the major product at short reaction times in dichlorometane observed with highly substituted systems and why the cycloadduct became the major product when the reaction was left for longer could not be completely rationalized. The analysis based on QTAIM along the reaction paths connecting the stationary points has been successfully applied to rationalize the mechanism of chemical reactions, such as the DA reactions of unsaturated organoboron dienophiles, 25 the dimerization of cyclopentadiene, 26 the Cope rearrangement of 1,5hexadiene, 27 pericyclic and pseudopericyclic reactions, [28][29][30][31][32][33][34] among others. This study indicated that in the initial stage of the reaction the σ-overlap between the 2p atomic orbitals is more stabilizing for the C-B secondary interaction than for the C-C counterpart, favoring the [4 + 3] pathway.…”