Organic Chemistry William H. Brown & Christopher S. Foote

Conjugated Dienes Chapter 23

Conjugated Dienes  Dienes are divided into three groups from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes

Conjugated Dienes.

Conjugated Dienes conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol

Conjugated Dienes the pi system of butadiene is derived from the combination of four 2p atomic orbitals; there are two bonding MOs and two antibonding MOs

Conjugated Systems systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds

,2- and 1,4-Addition  Addition of 1 mole of HBr to butadiene at -78°C gives a mixture of two constitutional isomers we account for these products by the following two- step mechanism

,2- and 1,4-Addition the key intermediate is a resonance-stabilized allylic carbocation

,2- and 1,4-Addition  Addition of 1 mole of Br 2 to butadiene at -15°C also gives a mixture of two constitutional isomers we account for the formation of these 1,2- and 1,4- addition products by a similar mechanism

Additional Exptl Info for addition of HBr at -78°C and Br 2 at -15°C, the 1,2- addition products predominate; at higher temperatures (40° to 60°C), the 1,4-addition products predominate if the products of low temperature addition are warmed to the higher temperature, the product composition becomes identical to the higher temperature distribution; the same result can be accomplished using a Lewis acid catalyst, such as FeCl 3 or ZnCl 2 if either pure 1,2- or pure 1,4- addition product is dissolved in an inert solvent at the higher temperature and a Lewis acid catalyst added, an equilibrium mixture of 1,2- and 1,4-product forms; the same equilibrium mixture is obtained regardless of which isomer is used as the starting material

,2- and 1,4-Addition  We interpret these results using the concepts of kinetic and thermodynamic control of reactions  Kinetic control:  Kinetic control: the distribution of products is determined by their relative rates of formation in addition of HBr and Br 2 to a conjugated diene, 1,2- addition occurs faster than 1,4-addition

,2- and 1,4-Addition  Thermodynamic control:  Thermodynamic control: the distribution of products is determined by their relative stabilities in addition of HBr and Br 2 to a butadiene, the 1,4- addition product is more stable than the 1,2-addition product

,2- and 1,4-Addition  Is it a general rule that where two or more products are formed from a common intermediate, that the thermodynamically less stable product is formed at a greater rate?  No whether the thermodynamically more or less stable product is formed at a greater rate from a common intermediate depends very much on the particular reaction and reaction conditions

Diels-Alder Reaction  Diels-Alder reaction:  Diels-Alder reaction: a cycloaddition reaction of a conjugated diene and certain types of double and triple bonds dienophile: diene-loving Diels-Alder adduct: the product of a Diels-Alder reaction

Diels-Alder Reaction alkynes also function as dienophiles cycloaddition reaction:cycloaddition reaction: a reaction in which two reactants add together in a single step to form a cyclic product

Diels-Alder Reaction we write a Diels-Alder reaction in the following way the special value of a D-A reaction is that it (1) forms six-membered rings (2) forms two new C-C bonds at the same time (3) is stereospecific and regioselective

Diels-Alder Reaction the conformation of the diene must be s-cis

Diels-Alder Reaction (2Z,4Z)-2,4-hexadiene is unreactive in Diels-Alder reactions because nonbonded interactions prevent it from assuming the planar s-cis conformation

Diels-Alder Reaction reaction is facilitated by a combination of electron- withdrawing substituents on one reactant and electron-releasing substituents on the other

Diels-Alder Reaction

Diels-Alder Reaction the Diels-Alder reaction can be used to form bicyclic systems

Diels-Alder Reaction exo and endo are relative to the double bond derived from the diene

Diels-Alder Reaction for a Diels-Alder reaction under kinetic control, endo orientation of the dienophile is favored

Diels-Alder Reaction the configuration of the dienophile is retained

Diels-Alder Reaction  Mechanism no evidence for the participation of either radical of ionic intermediates chemists propose that the Diels-Alder reaction is a pericyclic reaction  Pericyclic reaction  Pericyclic reaction: a reaction that takes place in a single step, without intermediates, and involves a cyclic redistribution of bonding electrons

Aromatic Transition States  Hückel criteria for aromaticity:  Hückel criteria for aromaticity: the presence of (4n + 2) pi electrons in a ring that is planar and fully conjugated  Just as aromaticity imparts a special stability to certain types of molecules and ions, the presence of (4n + 2) electrons in a cyclic transition state imparts a special stability to certain types of transition states reactions involving 2, 6, 10, electrons in a cyclic transition state have especially low activation energies and take place particularly readily

Aromatic Transition States decarboxylation of  -keto acids and  -dicarboxylic acids (Section 17.9) Cope elimination of amine N-oxides (Section 22.11)

Aromatic Transition States the Diels-Alder reaction  We now look at examples of two more reactions that proceed by aromatic transition states Claisen rearrangement Cope rearrangement

Claisen Rearrangement  Claisen rearrangement:  Claisen rearrangement: a thermal rearrangement of allyl phenyl ethers to o-allyl phenols

Claisen Rearrangement

Claisen Rearrangement Example 23.7 Example 23.7 Predict the product of this Claisen rearrangement

Cope Rearrangement  Cope rearrangement:  Cope rearrangement: a thermal isomerization of 1,5-dienes

Cope Rearrangement Example 23.8 Example 23.8 Predict the product of these Cope rearrangements

Prob Draw the structural formula for the Diels-Alder adduct of cyclopentadiene with each of the following.

Prob Propose structural formulas for A and B, and specify the configuration of B.

Prob Draw a Lewis structure for butadiene sulfone, and show by curved arrows the path of this reverse Diels-Alder reaction.

Prob This triene undergoes an intramolecular Diels-Alder reaction to give the bicycloalkene on the right. Show how the triene is coiled to give this product, and show by curved arrows how the product is formed.

Prob Draw a structural formula for the Diels-Alder adduct.

Prob Propose a structural formula for the product of this intramolecular Diels-Alder reaction.

Prob Draw a structural formula for the product of this Diels- Alder reaction, and show the stereochemistry of the adduct.

Prob Propose a synthesis for the starting diene from cyclopentanone and acetylene. Rationalize the stereochemistry of the target dicarboxylic acid.

Prob Propose a structural formula for compound A.

Prob Predict the product of each Diels-Alder reaction.

Prob Show how (1) and (2) react to give (3).

Prob Provide a mechanism for each step in this sequence.

Prob Propose a structural formula for A, and a mechanism for formation of the bicyclic product of this sequence.

Prob Propose a mechanism for the following reaction, which is known alternatively as an allylic rearrangement or a conjugate addition.

Prob All attempts to prepare cyclopentadienone give only a Diels-Alder adduct. Cycloheptatrienone, however, has been prepared by several methods and is a stable compound. Draw a structural formula for the Diels-Alder adduct, and account for the differences in stability of the two ketones.

Prob Show how to synthesize the tricyclic diene on the left from 2-bromopropane, cyclopentadiene, and 2- cyclohexenone.

Prob Propose a mechanism for this Claisen rearrangement.

Prob Provide a mechanism for each reaction

Prob Propose a mechanism for this example of a Carroll reaction.

Prob Use curved arrows to show the flow of electrons in each photoisomerization.

Prob Draw a structural formula for the product of this reaction, and show the stereochemistry of the product.

Prob Propose a mechanism for the formation of A, and show how A can be converted to tolciclate. Use 3-methyl-N- methylaniline as the source of the amine nitrogen, and thiophosgene, Cl 2 C=S, as the source of C=S group.

Conjugated Dienes End Chapter 23