An ab Initio Study of Facial Selectivity in the Diels−Alder Reaction

Abstract

Facial selectivities in the Diels-Alder reactions of 5-substituted 1,3-cyclopentadienes with a variety of dienophiles are predicted reliably at the ab initio HF/6-31G level. The ranges of activation energies for syn addition are large relative to those for anti addition, which are all similar to the activation energy for cyclopentadiene itself. Partitioning the activation energy into diene deformation, dienophile deformation, and diene-dienophile interaction energies shows that the major factor in determining facial selectivity is in the energy required to deform the diene into its transition state geometry. Deformation of the 5-fluoro-, 5-hydroxy-, and 5-amino-1,3-cyclopentadienes into their syn transition state geometries is predicted to require less energy than deformation of cyclopentadiene itself, which is in accord with experimental observation of syn addition with these dienes. The first definition of an ab initio steric factor is presented which correlates very well with syn activation energies. This indicates that facial selectivity with these dienes is primarily due to steric hindrance between the dienophile and the plane-nonsymmetric groups on the diene. However, we have also identified a significant lone pair-lone pair interaction with the reacting nitrogens when the dienophile is 1,2,4-triazoline-3,5-dione.

Transition state analysis on regioselectivity in [2+2] photocycloaddition reactions of substituted 2-cyclohexenone with cycloalkenecarboxylates

“…Such calculations were shown in explanations of the facial selectivity in the Diels -Alder reaction. 15 Consequently, we have calculated the energy resulted from the interaction which was some repulsion between the alkenes and the enones (DE int .0) and it can be calculated as follows: DE int ¼E TS 2(E def-enone þE def-alkene ) (E TS were taken from Table 1). These energies are presented in Tables 4 and 5.…”

Hyperconjugative Aromaticity and Antiaromaticity Control the Reactivities and π-Facial Stereoselectivities of 5-Substituted Cyclopentadiene Diels–Alder Cycloadditions

“…There is a correlation between the electronegativity of the C 5 – X substituent and the activation barriers, as observed earlier by Burnell. 11,12 Electron-withdrawing substituents accelerate the reactivity, and electron-donating substituents decrease the reactivity, with a range in activation energies of 10 kcal/mol.…”

Molecular Simulations of Photoaddition Selectivity and Chirality in Challenging Photochemical Reactions

“…21,38 The E df is the energy required to change the reactants' geometry into the transition state geometry. The TS energy differences (¦E act ) are partitioned as follows:…”