Exploring the impacts of the vinylogous anomeric effect on the synchronous early and late transition states of the hydrogen molecule elimination reactions of cis-3,6-dihalocyclohexa-1,4-dienes

Abstract

LC-ωPBE, B3LYP, and M06-2X methods with the 6–311+G** basis set on all atoms and natural bond orbital (NBO) interpretation were performed to investigate the roles and contributions of the effective factors on the potential energy surfaces of the hydrogen molecule elimination reactions of cyclohexa-1,4-diene (1) and its cis-3,6-dihalo derivatives [halogen=F (2), Cl (3), Br (4)] to hydrogen molecule and their corresponding aromatic rings. The ring puckering in compound 2 (which results from the repulsive electrostatic interactions between the natural bond orbital dipole moments of two C-F bonds) shortens the allylic hydrogen atoms’ distance, leading to the smaller barrier height in compound 2 compared to that in compound 1. The barrier heights of the hydrogen molecule elimination reactions increase from compounds 2 to 4 while their corresponding exothermic characters decrease. The variations of the advancements of transition state structures (δB av) reveal that the hydrogen molecule elimination reactions of compounds 2–4 do not obey the Hammond-Leffler postulate. In compound 2, the ring puckering shortens the allylic hydrogen distance (d H8-H10) while d H8-H10 values increase going from compounds 2 to 4, leading to the increase of their corresponding hydrogen molecule elimination reactions barrier heights. Interestingly, the variations of the vinylogous hyperconjugative anomeric effects justify the directions of the rings puckering going from compounds 2 to 4. The increase of the activation exchange components [PETR (TS)-PETR (GS)] going from compounds 2 to 4 correlates well with their corresponding hydrogen molecule elimination process barrier heights.