Diels-Alder reaction, the Diels-Alder reaction required by the anti-electron and the Hetero-Diels–Alder reaction are all [4+2] cycloaddition reactions, which are all synergistic reactions. It is a classic reaction of constructing cyclohexene skeleton by conjugated diene and dienophile. The reaction has a rich stereochemical presentation, including stereoselectivity, stereospecificity, and regioselectivity. This reaction is reversible, and the reaction temperature of the forward ring-forming reaction is relatively low, while the reverse ring-opening reaction requires a higher temperature. This reaction can synthesize six-membered rings with very little energy and is one of the very important cyclization methods for preparing unsaturated six-membered rings, which has played an important role in promoting the development of organic chemical reaction theory.
Compared with general dienes, Danishefsky dienes have better reactivity and site selectivity. Therefore, it is a well-known organic synthesis reagent.
The 1950 Nobel Prize in Chemistry was awarded to German chemist Otto Paul Hermann Diels and his student Kurt Alder for their discovery in 1928 of the famous "Diels-Alder diene synthesis reaction".
Reaction Mechanism
The reaction mechanism is generally believed that during the reaction, the two reactants are close to each other and interact with each other to form a cyclic filtration state. Then it is gradually transformed into product molecules, that is, the cleavage of old bonds and the formation of new bonds are coordinated in the same step-cooperative reaction, and there is no intermediate formation. Dienes with electron-donating groups react with dienophiles with electron-absorbing groups. Because the smaller the energy difference between the frontier orbitals (HOMO of Dienes and LUMO of dienophiles), the interaction between orbitals can be stabilized and the reaction can be carried out more easily (electronically required). Similarly, the reaction of Dienophile with electron donor group and conjugated Diene with an electron-withdrawing group is also easy (anti-electron requirement type).
The reaction is carried out according to the synergistic reaction cis-addition, giving priority to the formation of endo-addition products (the endo rule). In the electron-required Diels-Alder reaction, the secondary orbital interaction can roughly explain this rule, but the endo/exo selectivity is also affected by stereoselectivity. Examples of complete selection of exo products according to different substrates have also been reported. In addition, the intramolecular Diels-Alder reaction is not fully applicable to the endo rule because the ring structure is fixed and the configuration degree of freedom is low.
According to organic electron theory, the addition products of the Diels-Alder reaction are more likely to make the substitution group in ortho-position or para-position (ortho-para rule). It can be explained by frontier orbital theory, that is, the reaction points with a large coefficient of HOMO-LUMO are easy to overlap and add.
The cyclic transition diene can be added when it is an s-cis (cisoid) structure, while Diels-Alder reaction can not take place in the s-trans (transoid) structure. In the following reaction, it is very difficult for Z-1,3-pentadiene to transform into an s-cis structure, and the reactivity is obviously lower than that of E type.
