A density functional theory study of secondary orbital overlap in endo cycloaddition reactions. An example of a Diels-Alder reaction between butadiene and cyclopropene

Density functional theory (B3LYP, BLYP) and ab initio (HF, MP2, and QCISD(T)) methods support the hypothesis that transition state structures for the addition of cyclopropene to butadiene is stabilized through interactions between the hydrogen of cyclopropene and the pi-bond of butadiene. This finding is supported by the computions of bond orders and charge separations and by the differences in frontier orbital energies. Because the interactions between the hydrogen of cyclopropene and the pi-bond of butadiene is possible only in an endo transition state structure, it is conceivable that the formation of an endo cycloadduct should be dominant. This is confirmed by the computation of a lower activation barrier, similar to 2.0 kcal/mol, for the endo transition state structure than for the exo transition state structure.