Conjugated diene, 1,2-addition molecular orbitals

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 - . 

The principles of conservation of orbital synunfetry can be applied to inter-molecular cycloaddition reactions and to what can be thought of as their reverse, the fragmentation of one molecule into two or more smaller fragments. Let us consider first the Diels-Alder reaction, a reaction of great synthetic usefulness as well as the subject "oT many theoretical and mechanistic speculations and studies. In its simplest form, this reaction is the addition of an olefin to a conjugated diene to give a cyclohexene. It is termed a [4 -I- 2]-cycloaddition reaction, since it involves a four-7r-electron system and a two-7r-electron system. All available data indicate that the reaction is a concerted one that proceeds stereospecifically by cis addition to the double bond. This stereospecificity is exemplified by the formation of stereoisomeric products from the reaction of butadiene with ethyl maleate and ethyl fumarate ... 

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