Reactions between Benzene and 1,3-butadiene

4-additions occur between benzene and butadienes, +jt cycloadditions occur 1,3-Butadiene adds to benzene and p-addition product containg cis- and trans-double bonds along with ortho-, meta- and other products are formed. Addition product of benzene and trans-alkene undergoes thermal addition with butadiene to give dimeric product (A) and monomeric product Y). 

Dimeric product is supposed to be formed by thermally allowed n +ita cycloaddition of initial products [A] and (B). Correlation diagram for n +n cycloaddition of benzene and trans-1, 3-butadiene and benzene with cis-1, 3-buta- diene are shown in Fig. 5.12 and 5.13, respectively. 

Product from cycloaddition of benzene and trans-butadiene is predominates. To benzene trans-butadiene adds more readily, it +n cycloaddition is photochemically allowed. Formation of charge transfer complex between two ground state molecules or an exciplex between excited state of one molecule and ground state of other may lower the energy barrier to this pericyclic reaction, it +7c cycloaddition may occur is case of two aromatic substrates also. For examples, dimerization of naphthalene and anthracene is very common (shown on next page). 

Frontier molecular orbital (FMO) approach to predict course of cycloaddition reaction takes into account the symmetry properties of HOMO of one reactant and LVMO of other reactant. Reaction is favourable when signs of cofficient of HOMO and LVMO are same. Cycloaddition of ethylene is 

Diels-Alder Reaction In the manner similar to cycloaddition of ethylene Diels-Alder reaction can be analysed which involves Jt-molecular orbitals of butadiene and ethylene. 

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