Diels-Alder Cycloaddition
The reaction between a conjugated diene and an olefin leads to a cyclohexene. This is an amazing reaction in which two C–C σ bonds are formed at the expense of two π bonds. The double C=C bonds of the diene cooperate, acting like tweezers, to attack the olefin.
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For instance, 1,3-pentadiene reacts with propenoic acid to yield 2-methylcyclohex-3-ene-1-carboxylic acid:
This reaction was discovered by the German organic chemists Diels and Alder who were awarded the Nobel Prize in 1950.
Otto P. H. Diels (1876-1954): German chemist, professor at the University of Kiel, co-author together with Kurt Alder of the "Diene Synthesis", a procedure to obtain six-membered rings. In addition to the theoretical interest of these reactions, the method allowed for the synthesis of many molecules like camphor, vitamin D and cortisone. Nobel Prize in Chemistry shared with Alder in 1950.
Kurt Alder (1902-1958): German chemist, professor at the University of Cologne. Disciple of Otto Diels, he collaborated in Diels' research. They shared the Nobel Prize in Chemistry in 1950.
The reaction proceeds by a single TS in which the π clouds of diene and olefin interact. Three π bonds break, giving rise to two σ bonds and a new π bond:
In the single TS the π electrons circulate leading to the new bonds (in black). The resulting cyclohexene has been colored for you to easily identify which carbon comes from which molecule.
The interaction of the π clouds in the TS can be easily understood by means of a simple molecular orbital diagram:
The Highest Occupied Molecular Orbital (HOMO) of one species has to interact (donate electrons) with the Lowest Unoccupied Molecular Orbital (LUMO) of the other species: "the nucleophile attacks the electrophile", as always.
One can then have two situations. Diene's HOMO attacks olefin's LUMO or Diene's LUMO is attacked by olefin's HOMO.
Please, be aware that the symmetry of the molecular orbitals in both HOMO-LUMO situations is appropriate for a bonding setup and the formation of two σ bonds.
In general, the most common situation is that of the diene acting as the electron donor from its HOMO and the olefin, the so-called dienophile, as the electron receptor to its LUMO.
This situation is considered as normal electronic demand.
The reaction is thus favored if the diene bears electron-releasing groups and the dienophile electron-withdrawing groups.
These are examples of very common dienes:
These are instances of usual dienophiles:
