(tarek H. Musslimani)
Diels-Alder reaction of phosphole with ethylene under acidic conditions proceed via an inverse electron demand. This is because the energy gap between the HOMO (highest molecular orbital) of the diene and the LUMO ( Lowest unoccupied molecular orbital) of the dienpohile is greater than the energy gap between the LUMO of the diene and the HOMO of the dienophile under acidic conditions. Phosphole, in contrast to other five membered heterocyclic aromatic rings adds an acid to the phosphorus atom. The other heterocyclic aromatic compounds add an acid to the double bond instead of adding it to the heteroatom.