This simple Diels-Alder reaction illustrates the interacting molecular orbitals of the Diene (bottom) and the Dienophile (top). Note that the former has four p-electrons [C=C-C=C]and the latter two [C=C]. The sign (+ or -)(here the colour) of the coefficients is the same and these frontier orbitals can overlap with no impediment. Thus in the Diels-Alder reaction the developing overlap is bonding at both sites where new bonds are being formed. As always, to form a bond a highest occupied molecular orbital (p) interacts with a lowest unoccupied molecular orbital (p*). There is no unfavourable antibonding here, where opposite poles interact!
In this demonstration, the favoured approach to afford the major diastereomer of the dienophile (tautomeric form in situo is on top of the diene as shown by the blue arrow; the minor diastoreomer results from attack below. Click on the green button to change speed:
