Mechanism of Diels-Alder Reaction

The Diels-Alder reaction combines a diene and a dienophile (an alkene) to make rings and bicyclic compounds. The three double bonds existent in initial reagents are converted into two new single bonds and one new double bond.

Typically, the Diels-Alder reaction works best when either the diene is substituted with electron donating groups (like -OR, -NR2, etc) or when the dienophile is substituted with electron-withdrawing groups (like -NO2, -CN, -COR, etc).

Diels-Alder reaction takes place when diene is in the s-cis conformation. The s-cis conformation has both of the double bonds pointing on the same side of the carbon-carbon single bond that connects them. In solution, the carbon-carbon single bond in the diene is constantly rotating, so at equilibrium there is usually some mixture of dienes in the s-trans conformation and some in the s-cis conformation. The ones that are at that moment in the s-trans conformation do not react, while the ones in the s-cis conformation can react.

If dienophile is disubstituted (substituted twice), in the Diels-Alder reaction, the stereochemistry of initial compound is preserved. In other words, if the substituents started cis (on the same side) on the dienophile, they end up cis in the product. If they started trans (opposite sides) on the dienophile, they end up trans in the product.

PROPOSED MECHANISM

In proposed mechanism there is no necessary any mathematical apparatus for describing the diene –alkene interaction like in actual MO method.

As was presented at covalent bound an actual σ bound represent an interaction between two electron magnetic moments orientated after the line which pass through the nuclei and a π bound represent an electron magnetic moments interaction after a direction perpendicular on the line which pass through nuclei as in fig. 1

Fig. 1 σ and π bound formation

Carbon – hydrogen bounds does not participate at Diels Adler reaction, so they are omitted in order to simplify the representation.

The Diels Adler reaction supposes an attack of alkene at C₁ and C₄ diene atoms.

The electron magnetic moments from diene (C₁ and C₄) and alkene suffer a reorientation and two new simple bound are formed as presented in fig 2. The electron magnetic moments from C₂ and C₃ form a new double bound (π bound in actual notation) due to their orientation perpendicular on the hexagon plan.

In case of substituted alkene is very easy to observe the conservation of stereochemistry during reaction. More detail in the book …