A New Look at the Diels-Alder Transition State

The chronology of π-orbital mixing in the Diels–Alder cycloaddition of butadiene and ethylene differs for the two main interactions, the HOMOdiene–LUMOdieneophile and HOMOdienophile–LUMOdiene interactions. The C2−C3 π bond of the cycloadduct is already well developed at the level of the transition state, which is reminiscent of a late transition state.