The first density functional study on the [4+2]-cycloaddition reactions of1,2-diaza-1,3-butadiene with alkenes

The hetero-Diels-Alder reactions of 1,2-diaza-1,3-butadiene with ethylene, methyl vinyl ether, and methyl acrylate have been investigated theoreticall y with the aid of density functional theory using the Becke3LYP/6-31G(d) co mputational level. In the reactions with substituted alkenes, the transitio n states are concerted but asynchronous; the shortest bond-forming distance involves the nonsubstituted carbon of the alkene. In agreement with the ex perimental results, the reaction with methyl vinyl ether proceeds with high endo stereoselectivity and with complete regioselectivity to form the 6-me thoxy cycloadduct. The conformational study of the transition states shows a sharp s-trans preference. In contrast, the [4 + 2]-cycloaddition of 1,2-d iaza-1,3-butadiene with methyl acrylate have been found to occur with low r egio- and stereoselectivity but with a s-cis preference in the transition s tructures.