Organic Chemistry

1
Organic Chemistry

• William H. Brown Christopher S. Foote

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Conjugated Dienes
Chapter 23
3
Conjugated Dienes

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

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Conjugated Dienes

• .

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Conjugated Dienes

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

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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

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Conjugated Systems

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

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1,2- and 1,4-Addition

• Addition of 1 mole of HBr to butadiene at -78C
  gives a mixture of two constitutional isomers
• we account for these products by the following
  two-step mechanism

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1,2- and 1,4-Addition

• the key intermediate is a resonance-stabilized
  allylic carbocation

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1,2- and 1,4-Addition

• Addition of 1 mole of Br2 to butadiene at -15C
  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

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Additional Exptl Info

• for addition of HBr at -78C and Br2 at -15C,
  the 1,2-addition products predominate at higher
  temperatures (40 to 60C), 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

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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

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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

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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

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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

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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

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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

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Diels-Alder Reaction

• the conformation of the diene must be s-cis

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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

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Diels-Alder Reaction

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

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Diels-Alder Reaction
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Diels-Alder Reaction

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

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Diels-Alder Reaction

• exo and endo are relative to the double bond
  derived from the diene

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Diels-Alder Reaction

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

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Diels-Alder Reaction

• the configuration of the dienophile is retained

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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

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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

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Aromatic Transition States

• decarboxylation of ?-keto acids and
  ?-dicarboxylic acids (Section 17.9)
• Cope elimination of amine N-oxides (Section 22.11)

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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

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Claisen Rearrangement

• Claisen rearrangement a thermal rearrangement of
  allyl phenyl ethers to o-allyl phenols

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Claisen Rearrangement
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Claisen Rearrangement

• Example 23.7 Predict the product of this Claisen
  rearrangement

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Cope Rearrangement

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

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Cope Rearrangement

• Example 23.8 Predict the product of these Cope
  rearrangements

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Prob 23.21

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

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Prob 23.22

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

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Prob 23.24

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

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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.

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Prob 23.26

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

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Prob 23.27

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

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Prob 23.28

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

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Prob 23.29

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

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Prob 23.30

• Propose a structural formula for compound A.

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Prob 23.31

• Predict the product of each Diels-Alder reaction.

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Prob 23.32

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

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Prob 23.33

• Provide a mechanism for each step in this
  sequence.

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Prob 23.34

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

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Prob 23.35

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

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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.

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Prob 23.37

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

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Prob 23.38

• Propose a mechanism for this Claisen
  rearrangement.

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Prob 23.39

• Provide a mechanism for each reaction

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Prob 23.40

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

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Prob 23.41

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

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Prob 23.42

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

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Prob 23.43

• 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, Cl2CS, as the
  source of CS group.

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Conjugated Dienes
End Chapter 23