Malcolm Rivera
The Diels-Alder reaction is a chemical reaction that combines a conjugated diene and a substituted alkene (dienophile) to form a cyclohexene derivative. This reaction was discovered by German chemists Otto Diels and Kurt Alder in 1928, and they were awarded the Nobel Prize in Chemistry in 1950 for their work. The Diels-Alder reaction is a powerful tool in organic chemistry, allowing for the introduction of chemical complexity in the synthesis of natural products and new materials. When non-symmetrical dienes react with non-symmetrical dienophiles, two constitutional isomers, also known as regioisomers, can be formed. This occurs due to the two possible orientations that the diene and dienophile can adopt during the reaction. The regioselectivity of the Diels-Alder reaction is influenced by the electron distribution in the diene and dienophile, with the most electron-rich carbon of the diene reacting with the most electron-deficient carbon of the dienophile.
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What You'll Learn
The Diels-Alder reaction
During the Diels-Alder reaction, two pi bonds are converted to two sigma bonds, and three pi bonds are broken. Specifically, the C1-C2, C3-C4, and C5-C6 pi bonds are broken, and two sigma bonds and one pi bond, C2-C3, are formed. This results in the formation of a new six-membered ring. The reaction occurs in a single mechanistic step, where carbons 1 and 4 of the diene and both alkene carbons of the dienophile undergo rehybridization from sp2 to sp3, and electrons rearrange to create two new sigma bonds in the cyclic product.
For the Diels-Alder reaction to occur, the diene must be conjugated and in the s-cis conformation. The reaction rate is faster when there is an electron-withdrawing group on the dienophile and electron-donating groups on the diene. However, substituents on the diene or dienophile do not affect the bond-forming or bond-breaking pattern of the reaction, but they do influence the reaction rate. The regioselectivity of the reaction typically follows the ortho-para rule, where the cyclohexene product has substituents in positions analogous to the ortho and para positions of disubstituted arenes.
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Stereochemistry
The Diels-Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile. The reaction results in the formation of a new six-membered ring, with the breaking of three pi bonds and the formation of two sigma bonds and a pi bond. This reaction is of great importance in organic chemistry as it allows for the introduction of chemical complexity in the synthesis of natural products and new materials.
The Diels-Alder reaction is stereospecific, meaning that the stereochemistry of the starting materials is preserved in the product. The two "outside" groups on the diene, when drawn in the s-cis conformation, end up on one face of the new six-membered ring, while the two "inside" groups end up on the other face. This is known as Diels-Alder Stereochemistry Rule #2. The relationship between the substituents about the double bond in the dienophile is also preserved in the product.
The endo and exo stereochemistry of the Diels-Alder reaction is another important consideration. Endo is the kinetic product and is typically considered the major product. However, at higher temperatures, the thermodynamic exo product may dominate as it is the more stable stereoisomer. The endo product is formed when the electron-withdrawing group of the dienophile interacts favourably with the non-bonding orbitals of the diene, resulting in a decrease in the energy of the transition state.
The Diels-Alder reaction exhibits regioselectivity, which refers to the preference of a chemical reaction to yield one structural isomer over others. In the case of the Diels-Alder reaction, the most electron-rich carbon of the diene reacts with the most electron-deficient carbon of the dienophile. This regioselectivity results in the formation of two possible constitutional isomers, which differ in the orientation of the diene and dienophile.
The stereochemistry of the Diels-Alder reaction can be manipulated by introducing substituents to either the diene or the dienophile. These substituents do not affect the bond-forming or bond-breaking patterns but can influence the reactivity of the components and the stability of the products formed. For example, a bulky substituent at the C2 or C3 position can increase the reaction rate by forcing the diene into the reactive s-cis conformation.
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Regioselectivity
The Diels-Alder reaction is a powerful and widely applied tool in organic chemistry. It involves a reaction between a conjugated diene and a substituted alkene (commonly termed the dienophile) to form a new six-membered ring. This reaction is named after its discoverers, Diels and Alder, who were awarded the Nobel Prize in Chemistry in 1950.
The regioselectivity of the Diels-Alder reaction can be understood through the ortho-para rule. This rule states that the cyclohexene product bears substituents in positions analogous to the ortho and para positions of disubstituted arenes. The "ortho" product is formed when a diene bears an electron-donating group (EDG) at C1, while the dienophile has an electron-withdrawing group (EWG) at C1. On the other hand, the "para" product is formed when the diene is substituted at C2, resulting in the largest HOMO coefficient at C1.
The regioselectivity of the Diels-Alder reaction can also be influenced by steric effects. In certain cases, steric effects can override the typical endo selectivity and favour the formation of the exo isomer. This occurs when the diene component has bulky substituents at the C2 or C3 position, which increases the reaction rate by forcing the diene into the reactive s-cis conformation.
Additionally, the Diels-Alder reaction exhibits regioselectivity in both normal and inverse electron-demand scenarios. By considering the interaction energy between the most energetically well-matched HOMO-LUMO pairs, it is possible to predict the main regioisomer resulting from a given diene-dienophile combination. This prediction holds true for all available experimental data.
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Endo and exo products
The Diels-Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly known as the dienophile. The reaction results in a substituted cyclohexene derivative and the formation of a new six-membered ring.
The Diels-Alder reaction is stereospecific, meaning that the substituents' cis or trans configuration on the dienophile will be preserved in the new six-membered ring. The two "outside" groups on the diene in the s-cis conformation will end up on the same face of the new ring, as will the two "inside" groups.
When both the diene and dienophile are substituted, two types of products, known as "exo" and "endo," can be formed. These are diastereomers, which are stereoisomers that are not mirror images of each other. The exo and endo products differ in the relative positions of the substituents on the new six-membered ring. In the exo product, the substituent on the dienophile is positioned on the opposite side of the ring compared to the "outside" groups on the diene. In contrast, in the endo product, the substituent on the dienophile is on the same side as the "outside" groups on the diene.
The preference for endo versus exo products is an intriguing aspect of Diels-Alder reactions. Despite appearing more sterically hindered, endo products are generally favored over exo products. For instance, in the Diels-Alder reaction of cyclopentadiene with maleic anhydride, the ratio of endo to exo products is approximately 4:1. The endo product tends to form faster, while the exo product is often more stable due to steric factors.
The Alder Endo Rule provides insight into the preference for endo products. According to this rule, if two isomeric products are possible, the preferred product is the one with an unsaturated substituent(s) on the alkene oriented toward the newly formed cyclohexene double bond. Additionally, Lewis acids can influence the endo:exo selectivity by accelerating the rate of the Diels-Alder reaction and increasing the stability of the endo transition state.
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Reaction conditions
The Diels-Alder reaction is a versatile chemical reaction in organic chemistry, widely applied in synthesising natural products and new materials. It involves a conjugated diene and a substituted alkene, known as the dienophile, combining to form a new six-membered ring structure. This reaction is classified as a thermally allowed [4+2] cycloaddition, with the simultaneous formation of two new carbon-carbon bonds.
For the Diels-Alder reaction to occur, specific reaction conditions must be met, primarily concerning the nature of the diene and dienophile. Here are the key reaction conditions:
Conjugated Diene
The diene must be conjugated for the reaction to proceed. This means that the two carbon-carbon pi bonds in the diene must be adjacent and in the same plane, allowing for the required s-cis conformation. In other words, the diene must be flat, with the two pi-bonds oriented cis to the central carbon-carbon single bond. An example of such a diene is 1,3-butadiene, which readily undergoes the Diels-Alder reaction.
Dienophile
The dienophile is typically an alkene with a pi bond. The reaction rate is influenced by the presence of an electron-withdrawing group on the pi bond of the dienophile. The dienophile's stereochemistry is preserved in the product, with substituents on the dienophile remaining cis or trans in the new six-membered ring.
Stereochemistry
The Diels-Alder reaction is stereospecific, meaning the relative positioning of atoms in the reactants is reflected in the product. The two "outside" groups on the diene in the s-cis conformation will end up on one face of the new ring, while the two "inside" groups will be on the opposite face. This arrangement results in the formation of diastereomers, termed "exo" and "endo".
Regioselectivity
The Diels-Alder reaction exhibits regioselectivity, favouring the formation of one constitutional isomer over another. The reaction is influenced by the electron distribution in the diene and dienophile, with the most electron-rich carbon of the diene reacting with the most electron-deficient carbon of the dienophile. This regioselectivity leads to the formation of either "ortho" or "meta" products, with the “ortho” product being the major isomer.
Temperature
Temperature plays a role in the Diels-Alder reaction, with higher temperatures favouring the reverse reaction, known as the retro-Diels-Alder reaction. Additionally, at higher temperatures, the thermodynamic "exo" product may dominate due to its greater stability.
Intramolecular Reactions
In certain cases, the Diels-Alder reaction can occur intramolecularly, where the diene and dienophile are part of the same molecule. This results in the formation of two new rings.
In summary, the Diels-Alder reaction is a powerful tool in organic chemistry, with its versatility stemming from the various reaction conditions that influence the stereochemistry and regiochemistry of the resulting product. Adhering to these conditions allows for the controlled formation of six-membered rings, making it a valuable synthetic reaction.
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Frequently asked questions
The Diels-Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative.
The Diels-Alder reaction produces two regioisomers (constitutional isomers) that are formed as a result of the two possible orientations that the diene and dienophile can have.
The Diels-Alder reaction is influenced by the stereochemistry of the starting diene and dienophile, the regioselectivity of the reaction, and the presence of electron-donating or electron-withdrawing groups on the diene and dienophile.
