Diels alder and stability of conjucated dienes

diels alder reactions and stability of conjugated dienes depending factors

1. STABILITY OF CONJUGATED DIENES M.Maruthamuthu , M .Pharm ., Assistant professor , Department Of Pharmaceutical Chemistry , svcp .

2. Conjugated dienes are characterized by alternating carbon-carbon double bonds separated by carbon-carbon single bonds. Cumulated dienes are characterized by adjacent carbon-carbon double bonds. While conjugated dienes are energetically more stable than isolated double bonds. Cumulated double bonds are unstable Introduction

3. Conjugated diene stability Conjugated  dienes  are more stable than non conjugated  dienes  (both isolated and cumulated) due to factors such as delocalization of charge through resonance and hybridization energy. This stability can be seen in the differences in the energies of hydrogenation between isolated and conjugated alkenes . Since the higher the heat of hydrogenation the less stable the compound, it is shown below that conjugated  dienes  (~54 kcal) have a lower heat of hydrogenation than their isolated (~60 kcal) and cumulated  diene  (~70 kcal) counterparts.

4. Conjugated double bonds are separated by a single bond. 1,3-dienes are an excellent example of a conjugated system. Each carbon in 1,3 dienes is  sp 2   hybridized and therefore has one  p  orbital. The four  p  orbitals in 1,3-butadiene overlap to form a conjugated system. Conjugated double bonds are separated by a single bond. 1,3-dienes are an excellent example of a conjugated system. Each carbon in 1,3 dienes is  sp 2  hybridized and therefore has one  p  orbital. The four  p  orbitals in 1,3-butadiene overlap to form a conjugated system.

5. The resonance structure shown below gives a good understanding of how the pi electrons are delocalized across the four carbons in this conjugated diene . This delocalization of electrons stablizes the conjugated diene

6. The Diels–alder Reaction

7. The concepts we have been discussing come into focus in an important reaction in synthetic organic chemistry , the Diels–Alder reaction. In this reaction, a diene with its 4 π electrons reacts with an alkene with its two π electrons to give a cyclohexene ring. Since the product is a ring, it is a cyclo addition reaction . Since the 4 π electrons of the diene and the two π electrons of the alkene participate in the reaction.The simplest example of a Diels-Alder reaction is the cyclo addition reaction of 1,4-butadiene and ethene to give cyclohexene. In every case, a cyclohexene ring is a product of a Diels–Alder reaction. The Diels–Alder reaction has three important characteristics. 1. It is concerted . Thus, there are no intermediates. 2. It is a thermal reaction ; that is, it is initiated by heat. 3. The transition state for the reaction contains 6 π electrons.

8. we saw that butadiene can exist in either an s- cis or an s- trans conformation. For open-chain , conjugated dienes , the diene must be in an s- cis conformation for the reaction to occur. Reactants that contain conjugated double bonds in a ring, such as cyclopentadiene , react much faster than open-chain conjugated dienes . The alkene that reacts with the diene is called the dienophile . Ethene is a poor dienophile , and the reaction is much faster when an electron withdrawing is conjugated to the dienophile .

9. Since the Diels–Alder reaction is concerted, the stereochemistry of the dienophile is retained in the product . The groups that are trans in the dienophile are trans in the product, and groups that are cis in the dienophile are cis in the product.

10. And, since the reaction is concerted, groups that are trans in the diene are also trans in the product, and groups that are cis in the diene are cis in the product.

11. Reactants that contain conjugated double bonds in a ring, such as cyclopentadiene , react much faster than open-chain conjugated dienes . Reactions of a dienophile with a cyclic diene give bicyclic prod ucts . The products in bicyclo [2.2.1] ring systems have substituents on the opposite side of one-carbon bridge; that is, they are endo .

12. Cylopentadiene reacts with itself over the course of a few hours in a Diels–Alder reaction. This reaction is reversible, and if the dimer is heated, cyclopentadiene is regenerated.

13. Electrophilic Addition to Conjugated Dienes 1,2- and 1,4-Electrophilic Addition Reactions The addition of HBr to a conjugated diene is strikingly different. Adding 1M equivalent of HBr at a low temperature yields two products.

14. The energy of the transition state for 1,2-addition of HBr to 1,3-butadiene is lower than the energy of the transition state for 1,4-addition. Energy Profile for 1,2- and 1,4-Electrophilic Addition Reactions

15. 1,2-Addition predominates at low temperature because there is not enough energy for the system to reach the transition state for 1,4-addition. This energy difference is responsible for kinetic control of the addition reaction. The 1,4-addition product is more stable than the 1,2-addition product , but it forms more slowly. At higher temperatures, the transition state leading to the more stable product can be attained and leads to 1,4 addition. Thus, at high temperatures, the product composition reflects the relative stabilities of the products, not the relative energies of the transition states leading to them.

16. The ratio of products depends on the temperature at which the reaction is carried out. In a separate experiment , when either of the two compounds is heated at 45 °C, an equilibrium mixture with a ratio of 85:15 of 1 -bromo-2-butene to 3 -bromo-1-butene forms.

17. The reactions of 1,3-butadiene are reasonably typical of conjugated dienes . The compound undergoes the usual reactions of alkenes, such as catalytic hydrogenation or radical and polar additions, but it does so  more readily  than most alkenes or dienes that have isolated double bonds. Furthermore, the products frequently are those of 1,2  and   1,4 addition

18. Formation of both 1,2- and 1,4-addition products occurs not only with halogens, but also with other electrophiles such as the hydrogen halides.. The first step, as with alkenes, is formation of a carbocation. However, with 1,3-butadiene, if the proton is added to C1 (but not C2), the resulting cation has a substantial  delocalization energy , with the charge distributed over two carbons. Attack of  Cl ⊖  as a nucleophile at one or the other of the positive carbons yields the 1,2- or the 1,4- addition produc

19. An important feature of reactions in which 1,2 and 1,4 additions occur in competition with one another is that the ratio of the products can depend on the 1.Temperature 2. The solvent, and also on the  3. Total time of reaction . The reason for the dependence on the reaction time is that the formation of the carbocation is  reversible , and the ratio of products at equilibrium need not be the same as the ratio of the rates of attack of  Cl ⊖  at C 1  and C 3  of the carbocation. This is another example of a difference in product ratios resulting from kinetic control  versus  equilibrium control.

20. The fact is that at low temperatures the 1,2 product predominates because it is formed more rapidly, and the back reactions, corresponding to k−1 or k−3, are slow . However, at equilibrium  the 1,4 product is favored because it is more stable, not because it is formed more rapidly.

21. Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism

22. Markovnikov's rule allows us to predict the products of most addition reactions, but constitutional isomers form in some reactions. For example, the addition of HCl to 3-methyl-1-butene gives not only the expected product , 2-chloro-3-methylbutane, but also 2-chloro-2-methylbutane. Allylic rearrangement

23. What is the origin of this second, apparently unexpected product? The answer is: the carbocation formed in the first step of the reaction subsequently rearranges to a more stable species, which then reacts with chloride . The expected product forms from the reaction of the nucleophilic chloride ion with the secondary cation that forms when a proton adds to the double bond by Markovnikov addition. The isomeric product forms when a nucleophilic chloride ion reacts with a tertiary carbocation This carbocation forms when the hydrogen atom at C-3 moves, with its bonding pair of electrons, to the adjacent secondary carbocation center This rearrangement is called a 1,2-hydride shift because a hydride ion (H– ) moves between adjacent carbon atoms.

24. Some of the secondary carbocation also reacts with chloride ion without rearranging to give the expected product . The relative amounts of rearranged and un rearranged products depend on how efficiently the carbocation is captured by the nucleophile versus the rate of the rearrangement process. The reaction of 3,3-dimethyl-1-butene with HCl gives us an example of this type of rearrangement.

25. Adding a proton to the less substituted carbon atom of the double bond ( Markovnikov’s rule again) gives a secondary carbocation. This secondary carbocation reacts with the nucleophilic chloride ion to give the expected product. The rearranged product forms by reaction of chloride ion with a tertiary carbocation that forms by a shift of a methyl group, with its bonding pair of electrons, from the quaternary center to the adjacent secondary carbocation center. This rearrangement is called a 1,2-methide shift because a CH 3 unit moves between adjacent carbon atoms.

26. Sample Solution Write the structures of all of the possible addition products of 3,3-dimethylcyclohexene with HBr . Adding a proton at C-2 gives a secondary carbocation. Capture of this carbocation by bromide ion yields 1-bromo-3,3-dimethylcyclohexane.

27. Adding a proton at C-1 gives a secondary carbocation at the original C-2 atom. Capture of the car bocation by bromide ion can give 1-bromo-2,2-dimethylcyclohexane. However, the secondary carbocation can rearrange to two possible tertiary carbocations . Migration of methyl followed by capture of the carbocation gives 1-bromo-1,2-dimethylcyclohexane.

28. A 1,2-shift of a methylene group of the ring can also occur to give a tertiary carbocation. Capture of the carbocation by bromide gives a product containing a cyclopentane ring. We can say, however, that when we consider carbocation chemistry we should more or less " expect the unexpected."