Benzo thiophene, Diels-Alder reactions structure

This bond order deviation from an ideal transition state structure to an example of cyclopropene added to a benzo-fused heterocycle may now be applied. Before we exEunine bond order deviation from an ideal transition state, we can take a look at the sums of rings bond order in the transition state structures. To simplify this picture, we will focus only on exo transition state structures between cyclopropene and benzo[c]heterocycles. Previously, we mentioned that the Diels-Alder reaction with benzo[c]heterocycles as dienes is a HOMO controlled diene reaction, therefore, an electron rich (higher sum of bond order) transition state structure should be energetically preferred. If this is the case, the order of reactivity should be benzo[c]furan, benzo[c]thiophene, and then benzo[c]pyrrole, which is exactly the same as determined on the basis FMO energy change (Table 23). 

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