Concerted thermal cycloadditions forming heterocycles

Concerted thermal [2 + 2] cycloadditions forming heterocycles have been reviewed (77ahc(21)245) and cross references to some examples discussed in the present book are given in Table 2. 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

More recent views on the theory of (2 + 2)-cycloaddition, in particular with respect to the question whether the two novel o-bonds are formed via a concerted or a stepwise mechanism, have been presented by Epiotis.28 He predicts that, if in the reaction of two n -electron systems one of the reactants has an electron-donating and the other an electron-accepting character, the activation energy of the concerted non-allowed 1 2 + n2s]-cycloaddition will be lowered, so that such a reaction may occur in a concerted manner under relatively mild conditions. As this condition is satisfied in most of the reported thermal (2 + 2)-cyclo-addition reactions of heterocyclic compounds, care must be taken in drawing any conclusions as regards the reaction pathways followed. 

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