# Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction

Frequently, even thermally allowed reactions only take place at very high temperatures. By increasing the reactivity of components through suitable substitution, conditions appropriate for laboratory experiments can be obtained. Examples:

Fig.1
Increased reactivity caused by suitable substituents

In praxis, this is done most easily with electron-withdrawing substituents at the dienophile. To a lesser degree, the same effect can be achieved with electron-rich substituents at the diene particularly in 1-position.

Fig.2
Electron-donating substituents on the dienophile
acroleine
acrylnitrile
maleic ester
fumaric ester
maleic anhydride
tetracyanoethylene

Addition of Lewis acids increases the electron-poor character of dienophiles.

Fig.3
Lewis acids

Electron-poor alkynes and heterodienes react also.

Fig.4
Acetylene dicarboxylate as a dienophile
Fig.5
Nitroso compunds as dienophiles

Likewise, electron-poor dienes and electron-rich dienophiles are reactive.

Fig.6
Diels-Alder reactions with inverse electron requirement

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An explanation for the substituent effects can be found in MO theory.

Fig.7
MO representation of Diels-Alder reactions with normal and inverse electron demand

Donor substituents D increase the energy of HOMO and LUMO while acceptor substituents A lower the energy of both frontier orbitals. Bonding interaction can only take place between the occupied orbitals of one and the unoccupied orbitals of the other component: either HOMO (diene) - LUMO (dienophile) or HOMO (dienophile) - LUMO (diene). The strength of the interaction increases with diminishing energy difference Δ$EHOMO-LUMO$. Minimization of Δ$EHOMO-LUMO$ by using various substituents increases the reactivity.

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