Theoretical study on the Diels-Alder reaction of bromo-substituted 2H-pyrane-2-ones and some substituent vinyls

Abstract

A DFT study of the reactivity, regio- and stereoselectivity of Diels-Alder reaction between 3-bromo, 5-bromo, and 3,5-dibromo-2(H)-pyran-2-ones and some weakly activated and unactivated alkenes has been carried out using density functional theory (DFT). Four possible reaction channels, which are related to the formation of meta- and para- and endo- and exo-cycloadducts have been explored and characterized. The energy and natural bond orbital analysis shows that the meta-regioselectivity on the exo pathway is preferred and follows an asynchronous concerted mechanism with a polar nature in all Diels-Alder cycloadditions. Moreover, the activation free energies of Diels-Alder cycloadditions of 3,5-dibromo-2(H)-pyran-2one are lower than 3- bromo-2(H)-pyran-2one and 5-bromo-2(H)-pyran-2one, which are in line with experimental observation. DFT-based reactivity indices clearly predict the regiochemistry of the isolated cycloadducts.

Keywords

• bromo-2(H)-pyran-2-ones
• DFT study
• Reaction mechanism
• reactivity indices
• regio- and stereo selectivity