Ambimodal Dipolar/Diels-Alder Cycloaddition Transition States Involving Proton Transfers.

Quantum mechanical computations and molecular dynamics simulations have been used to elucidate the factors that control reaction outcomes in ambimodal transition states leading to both dipolar and Diels-Alder cycloaddition products, which can interconvert via α-ketol rearrangements. The dipolar cycloaddition pathways were found to be disadvantaged due to the persistence of charge separation after the second C-C formation en route to the dipolar cycloaddition adducts. Structural modifications that result in the stabilization of the charge-separated species lead to an increase in the amount of dipolar cycloadducts formed.