Cycloaddition molecular orbital treatment

Earlier theoretical treatments based on molecular orbital interactions revealed that very polar intermediates or transition states are involved in both reactions, but these studies could not differentiate between the two cycloaddition modes [13]. However, a more recent treatment suggests that a common unpolar biradical intermediate exists [14]. The distinction between the ortho and the meta mode than occurs by dynamic effects mainly influenced by the substituents. 

I- 2] and [3 -I- 2] photocycloadditions are often competitive, and it has been proposed that the difference of the redox potentials of the reaction partners should play an important role for the control of the modes of cycloaddition [84]. However, treatments based on molecular orbital interactions could not differentiate between the two cycloaddition modes [85]. 

Theoretical quantitative treatments of cycloadditions mainly concern the Diels-Alder reaction. A possible approach is that of calculating the para-localisation energy, that is the variation of Tr-electron energy of the conjugated diene system, when two Tr-electrons are localised upon the atoms, in 1,4-relation to each other, which must form the new tr-bonds. This has been done by Brown , using the molecular orbital method some successful predictions of reactivity and of the positions of addition for polycyclic aromatic hydrocarbons and polyenes were made. This method has also been used by other authors - . 

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