Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions: Introduction to the Diels-Alder Reaction
The Woodward-Hoffmann reaction not only represents the prototype of cycloadditions but the whole class of pericyclic reactions. Besides the Grignard reaction, it is the most cited name reaction in chemical literature. The representative of all Diels-Alder reactions as well as its most elementary one is the reaction between 1,3-butadiene and ethylene.
The reaction principle was discovered in 1928 by Otto Diels and his student Kurt Alder. Both were honored with the Nobel Prize for Chemistry in 1950.
Some 1000 variations that are derived formally by substitution or isoelectronic replacement of C-atoms by N, O or other heteroatoms have been published in the literature so far. The four-atom component is called diene and the two-atom part dienophile. The diene provides four and the dienophile 2π electrons to the formation of the bonds. The Diels-Alder reaction belongs to [4+2] cycloadditions according to the Woodward-Hoffmann nomenclature.