Regioselectivity in Diels-Alder reactions
Click the structures and reaction arrows in sequence to view the 3D models and animations respectively
What is important here is the potential for different regioisomers (regio: position), due to both the diene and dieneophile being unsymmetrical.
What is interesting is that only one regioisomer predominates, which can be explained by partnering the largest coefficients (biggest same phase lobes) of the HOMO and LUMO. Click the button to see trimmed down versions of these frontier orbitals. The reaction itself becomes unsymmetrical, but still concerted, with the bond resulting from the largest coefficients being mostly formed in the transition state.
Notice the lower energy s-trans diene must rotate to s-cis conformation.
Did you spot this is the exo transition state? That is not significant in this example.
Richard Windsor - Undergraduate Final Year Project 2008