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[2+2] cycloaddition contd..
correlation diagram for 4n
A [4+2] cyclo addition
Diels-Alder reaction
The Theory

When 1,3-Butadiene and ethene molecules add on forming cyclohexene the vertical(σv) plane of symmetry (also called mirror plane)is maintained through out the process which includes the product. (see the diagram). Hence the analysis is based on correlation of this symmetry element.

The MO diagram of the interacting orbitals and their symmetry properties

D&A orbsym

The MO with letter "S" is symmetric with respect to mirror plane, and "A" indicates anti-symmetric.

From the diagram: p1 p p2 the bonding MO of the reactants correlate with the bonding MO s1 s2 p of the product.

Similarly the anti-bonding MO p3* p* p4*of the reactants correlate with the antibonding MO p* s1* s4* of the product.

Clearly this process is symmetry allowed and therefore thermally feasible.

Back to cycloadditions and symmetry consideratioons
Cycloaddition-The FMO method
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