Understanding the stereo- and regioselectivities of the polar Diels-Alder reactions between 2-acetyl-1,4-benzoquinone and methyl substituted 1,3-butadienes: a DFT study
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The polar Diels-Alder (DA) reactions of 2-acetyl-1,4-benzoquinone (acBQ) with methyl substituted 1,3-butadienes have been studied using DFT methods at the B3LYP/6-31 G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3-dienes to the beta conjugated position of the acBQ followed by ring-closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions.