Stereochemistry of Diels-Alder Reaction -endo, exo product-Chem Article
Welcome to Chem Article. Today we will discuss Diels-Alder Reaction Stereochemistry. How to form a product for using different types of reactant for Diels-Alder reaction. For the Diels-Alder reaction, we have to use electron-rich diene and electron-deficient dienophile. Which are interactions between HOMO of diene and LUMO of dienophile. And the formation of cyclic products during this cycloaddition reaction. They are already seen in the previous class. We hope that you have seen previous class notes. This is the pericyclic reaction class part-5.
We are already discussing the electrocyclic reaction, the mechanism of the electrocyclic reaction, molecular orbital symmetry concept, cycloaddition reaction, the mechanism of cycloaddition reaction. All topics are discussed in pericyclic reaction class part-1, pericyclic reaction part-2, pericyclic reaction part-3. And we have to discuss the mechanism for Diels-Alder Reaction in pericyclic reaction part-4.
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Now, Today we will discuss the stereochemistry of Diels-Alder Reaction, Regio-selective of Diels-Alder Reaction, the example of Diels-Alder Reaction.
What is the stereochemistry of Diels-Alder Reaction?
The great organic synthesis of the Diels-Alder reaction not only depends on regioselectivity but is also dependent on stereoselectivity. The variety of six-member rings substitutes have different types of stereochemistry. To Achieve a better yield of the product we have to have half of the stereoselectivity- like the synthesis of complex natural products. For stereoselectivity, we have to help kinetically control reaction and thermodynamic control reaction, the epimerization of starting materials or reactants or products.
The cis principle
Due to the stereochemistry of the Diels-Alder reaction, change the stereochemistry of reactant formation of different types of stereoselective product. In case according to the cis principle, we have to use cisoid diene and cisoid dienophile. Because using the cisoid diene and dienophile they have the formation of primary and secondary interaction. Therefore, the product of the Diels-Alder reaction, which is the most stable configuration. On the other hand, we have seen that transoid dienophiles, their substituent groups, do not participate in secondary interaction.
|The cis principle of Diels-Alder reaction
The application of the cis principle
The mechanism of the Diels-Alder reaction is most common, and it provides strong evidence. In the mechanism of the Diels-Alder reaction, during the 1,4-addition of diene and dienophile formation of two new σ-bonds at approximately the same time or simultaneously. Which depends on the rate of the reaction, it is a two-step process. In the case of the second bond formation using the diradical or zwitterion intermediate, which case rate is faster than the rate for the rotation of carbon-carbon bond. That means this type of mechanism is kinetically controlled.
The endo addition rule
In Diels-Alder reaction formation two types of products are formed, they all are different one is exo-product and another is endo product. First of all, we consider which transition states are more stable in this Diels-Alder reaction. That's why we have to consider that thermodynamically and kinetically stable product. According to endo addition rules, we have to see the diene and dienophile which are arranged by parallel plane. Which configuration is more stable is the maximum accumulation of dienophile double bonds. Mainly in cases, the dienophile has an active group including π-bond. That's why we have to see that the end addition is the most stable configuration. In case of extra secondary interaction between diene orbitals and active group of dienophile orbitals.
|endo addition rule: where endo product is kinetically stable and exo-product is a thermodynamically stable product. The cycloaddition of cyclopentadiene and maleic anhydride, the formation of stable endo product.
We have to see Stereoselective addition of maleic anhydride and cyclopentadiene and formation of endo product. Which is a kinetically stable product. This is the major product. On the other hand, a small amount of formation produces a thermodynamically stable product. In case this is a minor product.
Cycloaddition of diene furan and maleic anhydride, finally the formation of exo-product. which is thermodynamically stable. This is the major product. In Incase see that does not obey endo rules. On the other hand a small amount of product from in endo configuration. This is a kinetically stable product. And this product is minor.
What is the reason for stable endo product formation?
The factor of determination of cycloaddition product- In the die in a file has an extra activate group ( C=O) which interacts with the HOMO of diene other orbitals. This interaction is called secondary interaction. Due to these types of interactions, it is a stronger cycloaddition product. That's why the Endo form is a stable transition state.
On the other hand, the rotation of the dienophile Substituent group or activate group (C=O). Due to the rotation of the active group of dienophiles, That's no possible secondary interaction with HOMO orbitals of diene. In the case of dienophile activate groups is the Formation of electrostatic interaction with diene hydrogen. Due to the formation of Electrostatic interaction between diene and dienophile- the transition state becomes weaker. That's why the exo-form is in an unstable transition state.
Reason of temperature factor in Diels-Alder reaction
Due to the cycle addition of diene and maleic anhydride the temperature will be room temperature that's why this is a thermodynamically stable product. On the other hand, the cycloaddition of maleic anhydride and cyclopentadiene in case the temperature will be high that's why the product is from a kinetically stable endo product.
- That's why we have to Conclude that the end addition rule does not obey anywhere. Most of the case endo addition rule is obeyed in Cycloaddition of cyclic diene and cyclic dienophile.
- When we have to use the cyclic diene and cyclic dienophile, then the formation of an endo-product. This type of cycloaddition reaction follows the end addition rules.
- The formation of endo product and exo-product depends on the reaction temperature.
|Reason of temperature factor in Diels-Alder reaction: Chem Article
A trick of Stereochemistry in Diels-Alder Reaction
- First of all, we are considered diene substitutes. There are two parts, one is the left-hand side(L) and one is the right-hand side(L).
- Then we have to consider the dienophile substituent. There are two parts, one is the left-hand side(L) and one is the right-hand side(L).
- In case two left-hand side substituents have the same stereochemistry ( wedge or dash position)
- In case two right-hand side substituents have the same stereochemistry ( dash or wedge position).
- Two substituents side, one is left-hand side and another is right-hand side, They are shown opposite Stereochemistry Comparatively.
In case we have to see that triple bond dienophiles participate in the Diels-Alder reaction. In the case of sp2 hybridized carbon of diene and sp hybridized carbon of dienophile ( which is attached with a most electron-withdrawing group -COOEt) participate in Diels-Alder reaction. After the reaction, the formation of sp2 and sp3 hybridized carbon- six-membered cyclic rings (where the formation of two new σ-bonds and extended the two π-bonds. In This six-membered cyclic Transition four sp2 carbon and two sp3 carbon.
- We know that the sp2 hybridization carbons are Planner that's why it does not show any stereochemistry.
- In case sp3 is non-planner that's why this is shown in different stereochemistry.
|A trick of Stereochemistry in Diels-Alder Reaction: Chem Article
Finally, we have to conclude that the cis principle and endo addition principle of the Diels-Alder reaction depends on temperature. That means it depends on the thermodynamics and kinetic rate of the reaction.