Reactions of dienes

1
Chapter 11

• Reactions of dienes

2
Conjugated or not?

• There is a strong difference in reactivity
  between isolated (unconjugated) double bonds and
  conjugated double bonds
• When exactly are double bonds conjugated ?

3
Allenes (1,2-dienes)

• Allenes (1,2-dienes) contain two double bonds
  that are unconjugated ! There is no overlap of
  the p-orbitals of the first double bond with
  p-orbitals of the second double bond they are
  perpendicular to each other

4
Other properties of allenes

• Consider the molecule 1,3-dimethylallene. It
  exists as two different isomers. What type of
  isomers are they?
• See also the Tutoruur 23/11/00 from OC 1A.

5
How can one make allenes?

• By using a strong base (NaNH2), isomerization of
  a triple bond can be achieved
• The anion that remains is protonated in the workup

6
The reaction continues

• With the strong base present, the reaction
  continues and the terminal acetylene will be
  formed
• Why??

7
pKas revisited
Base CH3CO2 (CH3CH2)3N CH3O HO CH3CH2O (CH3)3
CO NH2
pK 4.2 10.7 15.2 15.7 15.9 19 35
Acid HI H3O HF H2S MeOH HC?CH HC?CCH3 H2CCH2
pKa 10 1.7 3.2 7.0 15.2 25 38 45

• The pK values are those of the conjugated acids
• If the pKa of the conjugated base is higher than
  the pKa of the acid deprotonation is possible

8
Conjugated or not?

• The orbitals of adjacent double bonds can give
  overlap with each other in other words, they are
  conjugated
• This is not possible in isolated double bonds
• Question draw resonance forms of 1,3-butadiene

9
MO orbitals of 1,3-butadiene

• The MO picture of butadiene can be obtained by
  combining the MOs of two ethylene fragments
• Note that the number of nodes increases with
  one
• Which of these orbitals are occupied with
  electrons?

10
The consequences of conjugation

• The s-bond between the double bonds is
  significantly shorter than a regular s-bond

11
Two possible conformations

• We know that the barrier for E/Z-isomerization in
  alkenes is very high (ca. 66 kcal/mol)
• 1,3-Dienes display an equilibrium between the
  s-cis and s-trans conformation (barrier is only
  4-5 kcal/mol)

12
Stability of dienes

• A smaller amount of energy is released when
  conjugated double bonds are hydrogenated

13
Or in other words....

• Dienes are relatively stable compared to isolated
  double bonds

14
The chemistry of vision (I)

• Extremely conjugated double bonds undergo facile
  isomerization, e.g. under the influence of an
  enzyme or light

15
The chemistry of vision (II)

• cis-Retinal binds to the protein opsin, forming
  rhodopsin
• The absorbance of light by this complex causes
  isomerization of the double bond so that the
  complex undergoes a structural change. This
  change causes a nerve impulse that is perceived
  by the brain as light

16
Reactions of 1,3-dienes

• Addition of HCl to 1,3-butadiene gives two
  products
• Question explain the outcome of the reaction
  (give the mechanism)

17
Other addition reactions

• Rationalize the outcome of the reactions (give
  the mechanism)

18
Kinetic vs thermodynamic product

• At low temperature, the least stable product is
  formed predominantly, at higher temperatures the
  most stable product is formed in excess.

19
The energy picture

• At low temperature, there is kinetic control, at
  higher temperature there is thermodynamic control
  (see also Chapter 7)

20
The explanation

• Kinetic product immediate attack of the chloride
  anion at the most electrophilic carbon
• Thermodynamic product chloride reacts at the
  least reactive position to give a more stable
  product

21
Problems

• Make problems 11.40 a-c, 11.41, 11.42

22
The allyl cation

• Only one orbital is filled (2 electrons)
• Empty orbital is stabilized by the adjacent
  occupied orbitals

23
SN1 reactions with allylic cations

• Consider the nucleophilic substitution of an
  allylic halide via an SN1 mechanism

24
Rate differences

• Relative rates of SN1 reactions in 50 EtOH at 45
  ºC

25
SN2 substitutions

• SN2 reactions are also enhanced at the allylic
  position
• This effect is less pronounced

26
Examples

• Relative rates of SN2 reaction with ethoxide in
  ethyl alcohol at 45 ºC

27
Allylic radicals

• Due to the stability of the allylic radical, only
  one of the allylic hydrogens is abstracted

28
The allylic anion

• As a result of the resonance stabilization of the
  allylic anion, the hydrogen atoms at the allylic
  position are more acidic than normal hydrogen
  atoms

29
Problems

• Make problems 11.54, 11.57.

30
The Diels-Alder reaction
diene
dienophile

• The Diels-Alder reaction was discovered in 1928
  by Prof. Otto Diels and his student Kurt Alder
• Awarded with the Nobel Prize in 1950
• Perhaps, thé reaction of the 20th century......

31
The orbital picture

• Orbitals of the diene (HOMO) and the dienophile
  (LUMO) overlap to give a cyclic product
• Question visualize the HOMO-LUMO interactions by
  drawing the signs of the different orbitals

32
The reaction is exothermic

• Netto, 2 p-bonds are exchanged for 2 s-bonds
  this results in an overall exothermic process

33
Conformation of the diene

• In order to be able to participate in a Diels
  Alder reaction, the diene has to be in the s-cis
  conformation

34
Examples

• A diene in which the double bonds are locked in
  the s-trans conformation cannot react in a
  Diels-Alder reaction

35
More about the mechamism

• Is the reaction a concerted or a stepwise
  process?
• Concerted all bonds are broken and formed
  simultaneously
• Stepwise bonds are broken and formed in
  consecutive steps

36
Concerted or stepwise?

• Energy diagrams for both processes (compare for
  example with SN1 and SN2 processes)

37
The reaction is concerted

• Stepwise processes would have led to the
  formation of other products (e.g.
  vinylcyclobutane) as well. However, such products
  have never been observed

38
Additional proof

• The fact that the geometry of the double bond is
  always retained in the product also points to a
  concerted mechanism

39
Examples

• Z-olefins lead to cis-substituted rings,
  E-olefins lead to trans-substituted rings

40
Reactivity of dienophiles

• The rate of Diels-Alder reactions is enhanced by
  using electron poor dienophiles and electron rich
  dienes
• Acetylenes react in the same manner as olefins
  leading to 1,4-dienes

41
Formation of bicyclic products

• The use of cyclic dienes leads to bridged
  bicyclic products
• Note the nomenclature the electron-withdrawing
  X-group at the inside is the endo-product, the
  other one is the exo-product
• Draw the transition states of the formation of
  both products

42
Preference for the endo-product

• In most cases, there is a strong preference for
  the endo-product (more pronounced than in these
  two examples)

43
A rationale

• A reason for this phenomenon could be the
  presence of stabilizing interactions resulting
  from additional overlap of p-orbitals on the
  dienophile with p-orbitals of the diene

44
Problems

• Make problems 11.45 (except g), 11.50, 11.51,
  11.54d