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

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• 作者：Margarita M. Vallejos ; Nicol谩s Grimblat ; Silvina C. Pellegrinet
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：July 24, 2014
• 年：2014
• 卷：118
• 期：29
• 页码：5559-5570
• 全文大小：577K
• ISSN：1520-5215

文摘

The effect of the nature of the boron moiety upon the reactivity and the selectivity of a variety of vinylboron dienophiles (1鈥?b>12) in the Diels鈥揂lder (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鈥?b>4) > dialkylvinylboranes (5鈥?b>7) 鈮?vinyl boronic acid (8) > vinylboronates (9, 10) > vinyl MIDA boronate (11) 鈮?vinyltrifluoroborate (12). The DA reactions of 1鈥?b>7 were slightly endo-selective due to the stronger C₆鈥揃 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鈥?b>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 (蟻_r) and its Laplacian (²蟻_r) at the (3,+1) critical point in the topology of ²蟻_r, 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.