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Chapter 10: Conjugation in Alkadienes and Allylic Systems



Summary | Conjugation | Resonance | Allylic Systems | Dienes | Self Assessment | Quiz |


Dienes

Chapter 10: Conjugation in Alkadienes and Allylic Systems

Dienes
Conjugated
The double bond units occur consecutively giving a continuous p system since the adjacent "p" orbitals can all overlap with each other.
The result is that conjugated dienes reactivity differs to that of simple alkenes.
The extra bonding interaction between the adjacent p systems  makes the conjugated dienes the most stable type of diene.
a conjugated diene pi system
Isolated
The double bond units occur separately.  The p systems are isolated from each other by sp3 hybridized centers.
The result is that isolated dienes have reactivity that is characteristic of simple alkenes.
an isolated diene pi system
Cumulated
The double bond units share a common sp hybridized C atom. The result is that cumulated dienes have reactivity more like simple alkynes.
a cumulated diene pi system

Preparation of Conjugated Dienes

eliminations to prepare conjugated dienes
 
What would the products of the following sequences be ?
1-butene reacted with N-bromosuccinimide (NBS) then treated with KOH / heat 1-butene via allylic bromination to 3-bromo-1-butene then eliminate to 1,3-butadiene
cyclohexene reacted with Br2 / CH2Cl2 then with KOH / heat electrophilic addition to 1,2-dibromocyclohexane then double elimination to 1,3-cyclohexadiene


Kinetic and Thermodynamic Control

The potential outcome of a reaction is usually influenced by two factors:

  1. the realtive stability of the products  (i.e. thermodynamic factors)
  2. the rate of product formation  (i.e. kinetic factors)
The following simple reaction coordinate diagram provides a basis for the key issues about kinetic and thermodynamic control:
 
Consider the case where a starting material, SM, can react to give two different products, P1 and P2 via different pathways (represented by green and blue  lines). 

Reaction 1 (green) generates P1.
This will be the faster reaction since it has a more stable transition state, TS1, and therefore a lower activation barrier. So P1 is the kinetic product

Reaction 2 (blue) generates P2.
P2 is the more stable product since it is at lower energy than P1. So P2 is the thermodynamic product.

 

 

Reaction coordinate diagram showing kinetic and thermodynamic products
Now consider what happens as we alter the reaction temperature and so the average energy of the molecules changes.

1. At low tempearture, the reaction preferentially proceeds along the green path to P1 and stops since they lack sufficient energy to reverse to SM, i.e.  it is irreversible, so the product ratio of the reaction is dictated by the rates of formation of P1 and P2, k1: k2.

2. At some slightly higher temperature, reaction 1 will become reversible while reaction 2 remains irreversible. So although P1 may form initially, over time it will revert to SM and react to give the more stable P2.

3. At high temperature, both reaction 1 and 2 are reversible and the product ratio of the reaction is dictated by the equilibrium constants for P1 and P2, K1 : K2.

Summary :

At low temperature, the reaction is under kinetic control (rate, irreversible conditions) and the major product is that from fastest reaction.

At high temperature, the reaction is under thermodynamic control (equilibrium, reversible conditions) and the major product is the more stable system

Reactions of Dienes

In general terms, dienes undergo electrophilic addition reactions in a similar fashion to alkenes (review)

general mechanism for electrophilic addition

However, in a little more detail:

Our attention here will focus on conjugated dienes.
 
  • The p bonds are a region of high electron density (red) so dienes are typically nucleophiles.
  • Dienes react with electrophiles (e.g. H+, X+)
  • Dienes can undergo addition reactions in which one or both of the p bonds are converted to new stronger s bonds.
  • Overall reaction :  Electrophilic addition
electrostatic potential of 1,3-butadiene
 
The reactions that will be considered here are:

 

Addition of Hydrogen Halides to Dienes

Conjugated dienes undergo addition reactions in a similar manner to simple alkenes, but two modes of addition are possible.
These differ based on the relative positions of H and X in the products:

addition of HX to dienes

Direct H-X adds "directly" across the ends of a C=C
Conjugate H-X adds across the ends of the conjugated system
The numbers 1,2- and 1,4- denote the relative positions of H and X in the products
The distribution of the products depends on the reaction conditions as shown by the example below:

addition of HBr to butadiene

At low temperature, the reaction is under kinetic control (rate, irreversible conditions) and the major product is the from fastest reaction, that of the bromide with the secondary cation.

At room temperature, the reaction is under thermodynamic control (equilibrium, reversible conditions) and the major product is the more stable system (note the more highly substituted alkene). This is supported by the fact that heating pure samples of either 3-bromo-1-butene (direct addition product) or 1-bromo-2-butene (conjugate addition product) gives the same 15 : 85 ratio of 3-bromo-1-butene to 1-bromo-2-butene.

Addition of Halogens to Dienes

Like the addition of hydrogen halides to conjugated dienes, halogens add to dienes via direct and conjugate addition pathways:

addition of X2 to dienes

The major products are usually the more stable, conjugate addition products with the more stable E configuration of C=C.

Diels-Alder Reaction (Nobel Prize in 1950)

simplest Diels-Alder reaction, butadiene and ethene to cyclohexene
Diels-Alder reaction of butadiene and ethyne to 1,4-cyclohexadiene
The concerted mechanism of the Diels-Alder reaction
Dienes
Common dienes
Dienophiles
common dienophiles

Stereoselectivity:

cis-dienophile gives cis-substituents in the product. stereoselective aspects of the Diels-Alder reaction
trans-dienophile gives trans-substituents in the product.
If both substituents on the diene are Z, then both end up on the same face of the product
If substituents on the diene are E and Z, then they end up on opposite  faces of the product

Cyclic dienes can give stereoisomeric products depending on whether the dienophile lies under or away from the diene in the transition state.  The endo product is usually the major product (due to kinetic control)
 

endo and exo products from cyclic dienes
formation of endo product
Diene and dienophile aligned directly over each other gives the endo product
(dienophile under or in = endo)
formation of exo product
Diene and dienophile staggered with respect to each other gives the exo product
(dienophile exposed or out  = exo)
 

Compare the relative position of the dienophile fragment in the following CHIME images

 

Diels-Alder Reaction (Nobel Prize in 1950)

Students often find it difficult to "spot" the diene and dienophile components in the Diels-Alder products, particularly when the products are bicyclic.  Try using the following CHIME images to help you identify the starting materials.
 

Diene
Dienophile
Product
Highlight
diene C atoms 
dienophile C atoms
Reset 
Highlight
diene C atoms 
dienophile C atoms
Reset
Highlight
diene C atoms 
dienophile C atoms
Reset
Highlight
diene C atoms
dienophile C atoms
Reset

 

 



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