Peri-, Site-, and Regioselectivity in Heterocumulene−Heterodiene Cycloaddition Reactions:  An ab Initio Study of the System Ketenimine + Acrolein

Geometries of the 12 possible cycloaddition products between ketenimine and acrolein as well as of the respective transition states have been obtained by ab initio calculations at the MP2/6-31G* level. Energies were obtained up to the MP4(SDTQ)/6-31+G* level. On the basis of product stability arguments, addition of the ketenimine C C double bond is more favorable than reaction of the C N double bond. Similarly, acrolein also preferentially should react via its C C rather than C O bond. In kinetically controlled reactions, however, participation of the ketenimine C N bond in [4 + 2] cycloadditions across the oxa-1,3-dienic system is comparable to ( 8 ) or far more feasible than ( 7 ) reaction of the C C bond.