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Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction

Cycloadditions: Reactivity

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:

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.

Electron-donating substituents on the dienophile
maleic ester
fumaric ester
maleic anhydride

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

Lewis acids

Electron-poor alkynes and heterodienes react also.

Acetylene dicarboxylate as a dienophile
Nitroso compunds as dienophiles

2-D Animation of the Diels-Alder reaction with acetylene dicarboxylate

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

Diels-Alder reactions with inverse electron requirement

Dieldrin was used in the past as an insecticide; however, its use is now illegal .

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

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 ΔEHOMO-LUMO. Minimization of ΔEHOMO-LUMO by using various substituents increases the reactivity.

Electrostatic potential of dienes and dienophiles at normal and inverse electron demand

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