Organic Chemistry

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

1,2- 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

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

1,2- and 1,4-Addition • Addition of 1 mole of Br2 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 Br2 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 FeCl3 or ZnCl2 • 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

1,2- and 1,4-Addition • We interpret these results using the concepts of kinetic and thermodynamic control of reactions • Kinetic control: the distribution of products is determined by their relative rates of formation • in addition of HBr and Br2 to a conjugated diene, 1,2-addition occurs faster than 1,4-addition

1,2- and 1,4-Addition • Thermodynamic control: the distribution of products is determined by their relative stabilities • in addition of HBr and Br2 to a butadiene, the 1,4-addition product is more stable than the 1,2-addition product

1,2- 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: 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: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: 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: 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, 14.... 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: a thermal rearrangement of allyl phenyl ethers to o-allyl phenols

Claisen Rearrangement

Claisen Rearrangement Example 23.7 Predict the product of this Claisen rearrangement

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

Cope Rearrangement Example 23.8 Predict the product of these Cope rearrangements

Prob 23.21 Draw the structural formula for the Diels-Alder adduct of cyclopentadiene with each of the following.

Prob 23.22 Propose structural formulas for A and B, and specify the configuration of B.

Prob 23.24 Draw a Lewis structure for butadiene sulfone, and show by curved arrows the path of this reverse Diels-Alder reaction.

Prob 23.25 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 23.26 Draw a structural formula for the Diels-Alder adduct.

Prob 23.27 Propose a structural formula for the product of this intramolecular Diels-Alder reaction.

Prob 23.28 Draw a structural formula for the product of this Diels-Alder reaction, and show the stereochemistry of the adduct.

Prob 23.29 Propose a synthesis for the starting diene from cyclopentanone and acetylene. Rationalize the stereochemistry of the target dicarboxylic acid.

Prob 23.30 Propose a structural formula for compound A.

Prob 23.31 Predict the product of each Diels-Alder reaction.

Prob 23.32 Show how (1) and (2) react to give (3).

Prob 23.33 Provide a mechanism for each step in this sequence.

Prob 23.34 Propose a structural formula for A, and a mechanism for formation of the bicyclic product of this sequence.

Prob 23.35 Propose a mechanism for the following reaction, which is known alternatively as an allylic rearrangement or a conjugate addition.

Prob 23.36 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 23.37 Show how to synthesize the tricyclic diene on the left from 2-bromopropane, cyclopentadiene, and 2-cyclohexenone.

Organic Chemistry