The Diels-Alder reaction is a versatile tool in synthetic organic chemistry and is widely used in the synthesis of natural products. The reaction can proceed with remarkable regio and stereoselectivity and up to four stereocenters can be created in one step in the formation of a six-membered ring.
Asymmetric Diels-Alder reactions can moreover proceed with high enantioselectivity when catalyzed by chiral Lewis acids. Diels-Alder reactions are also ideal in respect of atom utilization. Although many Diels-Alder reactions proceed at room temperature or at slightly elevated temperatures, when less reactive reactants are used, or the reaction involves thermally unstable reactants or products, catalytic methods can be used to enable the reactions to be conducted under mild conditions.
Diels Alder ReactionBack to Top
The 4+2 cycloaddition reaction between a conjugated diene (4$\pi$-electron system) and a compound containing a double or triple bond, called the dienophile (2$\pi$-electron system), to form an addition product (adduct) is known as Diels-Alder reaction. Bettet yields are obtained if the dienophile is activated by introducing to it electron withdrawing groups such as carbonyl, notro or nitrile.
The normal Diels-Alder reaction is as follows.
EDG = electron donating group
EWG = electron withdrawing group
It is the reaction between a conjugated diene and another activated unsaturated molecule called the dienophile to give a ring compound was discovered by Diels and Alder and is popularly known as the Diels-Alder reaction.
Diels Alder Reaction MechanismBack to Top
The mechanism of the Diels-Alder reaction has still sustained interest because of the controversy which centers around whether the reaction is truly concerted or step-wise. These two pathways differ in the timing of bond-making and bond breaking. The reaction is of second order, the transition state thus consists of only the two reactants. In the concerted mechanism the two $\sigma$-bonds are formed simultaneously, but step wise mechanism (involving radicals or ions) envisages the rate-determining formation of one bond followed by a closure of the second bond more rapidly than the internal rotation.
Diels Alder Reaction ExamplesBack to Top
Some of the examples of Diels-Alder reaction is given below.
Another example is given below.