What makes a dienophile reactivity

In order for a Diels-Alder reaction to occur, the diene molecule must adopt what is called the s-cis conformation :. The s-cis conformation is higher in energy than the s-trans conformation, due to steric hindrance. For some dienes, extreme steric hindrance causes the s-cis conformation to be highly strained, and for this reason such dienes do not readily undergo Diels-Alder reactions.

The result of a Diels-Alder reaction involving a cyclic diene is a bicyclic structure:. Here, we see another element of stereopecificity: Diels-Alder reactions with cyclic dienes favor the formation of bicyclic structures in which substituents are in the endo position.

The endo position on a bicyclic structure refers to the position that is inside the concave shape of the larger six-membered ring. As you might predict, the exo position refers to the outside position. The rate at which a Diels-Alder reaction takes place depends on electronic as well as steric factors.

A particularly rapid Diels-Alder reaction takes place between cyclopentadiene and maleic anhydride. We already know that cyclopentadiene is a good diene because of its inherent s-cis conformation. Maleic anhydride is also a very good dienophile, because the electron-withdrawing effect of the carbonyl groups causes the two alkene carbons to be electron-poor, and thus a good target for attack by the pi electrons in the diene.

In general, Diels-Alder reactions proceed fastest with electron-donating groups on the diene eg. The Diels-Alder reaction is just one example of a pericyclic reaction: this is a general term that refers to concerted rearrangements that proceed though cyclic transition states.

Two well-studied intramolecular pericyclic reactions are known as the Cope rearrangement. Notice that the both of these reactions require compounds in which two double bonds are separated by three single bonds. Pericyclic reactions are rare in biological chemistry, but here is one example: the Claisen rearrangement catalyzed by chorismate mutase in the aromatic amino acid biosynthetic pathway.

The study of pericyclic reactions is an area of physical organic chemistry that blossomed in the mids, due mainly to the work of R. Woodward, Roald Hoffman, and Kenichi Fukui. The Woodward-Hoffman rules for pericyclic reactions and a simplified version introduced by Fukui use molecular orbital theory to explain why some pericyclic processes take place and others do not.

A full discussion is beyond the scope of this text, but if you go on to study organic chemistry at the advanced undergraduate or graduate level you are sure to be introduced to this fascinating area of inquiry. While in NO2 the nitrogen attached to the benzene ring do not have extra lone pair of electrons hence it is -R effecting group which is meta directing.

NH2 attach to the benzene ring stable resonance structure is formed.. However, nitrogen also has a lone pair of electrons. It can donate this lone pair onto the carbon, and quite a few resonance structures can be drawn.

Resonance is much more significant than the inductive effect in aniline. The main concept here is the endo-exo isomerism. In the endo product, the substituents of the dienophile are pointing towards the larger bridge, while in the exo isomer, they are pointing away from the larger bridge:.

You can read more details such as the transition state and the endo-exo definition when acyclic dienes are reacted here. If the cyclic diene is reacted with a mono-substituted dienophile then the product contains one stereogenic center and the endo and exo products are formed as two enantiomers:. And any combination of an endo and exo product represents a pair of diastereomers:. If the diene is also unsymmetrical, then you need to consider the regiochemistry of the Diels-Alder as well.

It would be too much for today, so you can check this post. Notify me of followup comments via e-mail. You can also subscribe without commenting. However, these are conformations since they interconvert through a rotation abound the sigma bond between the two double bonds and we call them s- cis and s- trans : Acyclic dienes usually prefer the s- trans conformation. Cyclic dienes, on the other hand, are locked whether they are cis or trans since there is no free rotation about the sigma bond: These conformations are two extremes as the locked trans dienes do not react in the Diels-Alder while the cyclic cis dienes are so reactive that they may react with themselves.

Cyclic dienes produce bridged bicyclic compounds : Electronic effects The second factor is the electronic effect.