Ab initio and semiempirical calculations of the hetero‐Diels‐Alder reaction of 1‐aza‐1,3‐butadiene with ethene

The transition structures of the hetero‐Diels‐Alder reaction of ( E )‐ and ( Z )‐1‐aza‐1,3‐butadiene ( 8 and 11 ) were studied by means of ab initio (UHF/6‐31+G* and RHF/6‐31+G*) and semiempirical (AM1, PM3) methods. The energy surface of the title reaction was calculated with AM1/CI. Two competing reaction channels exist, a concerted and a two‐step path. All methods used show a preference for the two‐step mechanism in agreement with experimental observations.