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

A DFT study of the reactivity, regio- and stereoselectivity of Diels–Alder reactions between 3-bromo, 5-bromo, and 3,5-dibromo-2 H -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-2 H -pyran-2-one were lower than those for 3-bromo-2 H -pyran-2-one and 5-bromo-2 H -pyran-2-one, which is in line with experimental observations. DFT-based reactivity indices clearly predicted the regiochemistry of the isolated cycloadducts