Cycloaddition Reactions between Cyclopentadiene and Ketene. Ab Initio Examination of [2 + 2] and [4 + 2] Pathways

The Diels−Alder [4 + 2] and Staudinger [2 + 2] reactions between ketene and cyclopentadiene were examined at the MP4/6-31G*//MP2/6-31G* and CCSD(T)/6-31G*//MP2/6-31G* levels. Both reactions are exothermic, 29 kcal mol -1 (Diels−Alder) and 23 kcal mol -1 (Staudinger). The activation energies are 31 kcal mol -1 (Diels−Alder) and 20 kcal mol -1 (Staudinger). The reaction is therefore kinetically controlled, consistent with experiments. The Diels−Alder reaction is concerted and synchronous, while the Staudinger reaction occurs in two stages, with partial formation of a C−C bond to the carbonyl carbon of ketene. The relative activation energies, regiocontrol, and mechanism are discussed in terms of frontier molecular orbital theory, natural population analysis, and topological electron density analysis.