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

Abstract

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