Conversion of edge-to-loop and loop-to-edge technique used to study [π² + π²] and [π² + π⁴] chemical reactions

The criterion of the conversion of edge to loop and loop to edge proposed by the Bratislava group is being presented with some chemical insight. The loops are generated inside the reaction or are supplied from outside to facilitate the reaction. These loops are termed nucleophilic, electrophilic, carbene (singlet/triplet), or biradical. The supplied loops induce the various centers of the reactants following frontier orbital theory (FOT). In a [π² + π²] reaction system, when one of the centers of a reactant is induced by a single loop, a linear polyene is obtained. But, when two centers of a reactant are induced, a three- or a four-membered ring is produced. A [π² + π⁴] reaction system gives six-, five-, and three-membered ring products. In order to have some chemical insight in these model reactions, more realistic quantum chemical AM1 calculations have been done to characterize one analogous five-membered transition state structure by considering the reaction between SO₂ and 1,3-butadiene.