Reactivity and Stereoselectivity in the Inverse-Electron-Demand Diels–Alder Reaction of 1-Aza-1,3-Butadiene

The reactivity and stereoselectivity in the inverse-electron-demand Diels–Alder reaction between 4-methoxycarbonyl-N-(phenylsulfonyl)-1-aza-1,3-butadiene and methoxyethene was examined using density functional theory (DFT) calculations at the M06-2X level. The formation of the two bonds in this reaction was calculated to be asynchronous. The formation of the C−C bond occurs first and is driven by electron delocalization from the dienophile to the diene, a process which simultaneously governs the regioselectivity. Moreover, the endo selectivity of the reaction was found to arise from non-bonding-orbital interactions, electrostatic attractions, and dispersion interactions. The sulfonyl group attached to the diene influences the selectivity and the reactivity. In contrast, when a methoxycarbonyl group is attached to the diene, it affects the selectivity in a different way depending on the position where it is attached.