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:
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.
Addition of Lewis acids increases the electron-poor character of dienophiles.
Electron-poor alkynes and heterodienes react also.
Likewise, electron-poor dienes and electron-rich dienophiles are reactive.
An explanation for the substituent effects can be found in.
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. Minimization of Δ by using various substituents increases the reactivity.