Explanation of why two dienophiles can't react together
Diels alder rxn occurs when HOMO (diene) reacts with LUMO (dienophile)
ethylene + ethylene LUMO and HOMO do not "line up"
with a photon of light an electron is excited to a higher energy level, which redefines LUMO and HOMO for that species AND CAUSES THEM TO LINE UP!
2+2 and 4+4 will work in light! 2+4 will stop working!