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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

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