Addition of benzyne to furan, pyrrole, and thiophene

As mentioned above, a major problem in using heterocyclic aromatic compounds as dienes in the Diels-Alder reaction is their highly aromatic character which hinders their involvement in reactions that include direct participation of their 7c-bonds. Some heterocycles such as pyrrole would, in many instances, rather participate in Michael-addition type of reactions than in Diels-Alder 

BO = bond order of ring s bond SBO = sum of bond orders BOD = sum of bond order deviation from an average ring bond order. The second re-bond is not a part of the ring re-bonds and should be excluded if BOi = 1.359 then SBO = 8.292 and BOD = 0.19 

By examining bond order uniformity, it is clear that benzyne is quite a reactive species if a triple bond is involved in the reaction, but not very reactive if a double bond is involved in the reaction. The furan ring had the highest bond order deviation and, as a result, it should be the most reactive five-membered heterocycle with one heteroatom studied here. 

Let us now explore the reactivity of these compounds as dienes (furan, pyrrole, thiophene) with a dienophile (benzyne) in Diels-Alder reactions. One approach that, for a long time, has been widely employed by chemists, is the use of Frontier Molecular Orbital (FMO) [19] energy gap between two of the reactants. According to this theory, the most reactive reactant pair will be the one that has a lower FMO energy gap. The reaction is predicted to be HOMO diene-controlled. If 

Reactant HOMO LUMO AEi AEii AEiii AEiv AEv AEvi 

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