• The simplest types of polyenes are those in which there are two double bonds, "dienes".
• The relative arrangement of the double bonds dictates the characteristic reactions of the systems.
• There are three possible scenarios, which are described below (from most to least stable)

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.
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.
Cumulated	The double bond units share a common sp hybridized C atom. The result is that cumulated dienes have reactivity more like simple alkynes.

Preparation of Conjugated Dienes

• Dienes can be prepared by elimination reactions (review) of unsaturated alcohols and alkyl halides.
• The outcome of eliminations typically favors the more stable product,
• Since conjugated dienes are more stable than isolated dienes, the formation of the conjugated diene is usually favored over the isolated diene unless the structure prevents the formation of the conjugated system.

 

What would the products of the following sequences be ?
1-butene reacted with N-bromosuccinimide (NBS) then treated with KOH / heat
cyclohexene reacted with Br2 / CH2Cl2 then with KOH / heat

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)

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.

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)

However, in a little more detail:

• Conjugated dienes undergo addition but the proximity of the conjugated C=C influences the reactions
• Isolated dienes react just like alkenes (chapter 6)
• Cumulated dienes react more like alkynes (after all, both have sp hybridised C atoms)

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

• addition of hydrogen halides
• addition of halogens
• Diels-Alder reaction

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:

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

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:

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)

• The Diels-Alder reaction is a conjugate addition reaction of a conjugated diene to an alkene (the dienophile) to produce a cyclohexene.
• The simplest example is the reaction of 1,3-butadiene with ethene to form cyclohexene:

• The analogous reaction of 1,3-butadiene with ethyne to form 1,4-cyclohexadiene is also known:

• Since the reaction forms a cyclic product, via a cyclic transition state, it can also be described as a "cycloaddition".
• The reaction is a concerted process:

• Due to the high degree of regio- and stereoselectivity (due to the concerted mechanism), the Diels-Alder reaction is a very powerful reaction and is widely used in synthetic organic chemistry.
• The reaction usually thermodynamically favorable due to the conversion of 2 p-bonds into 2 new stronger s-bonds.
• The two reactions shown above require harsh reaction conditions, but the normal Diels-Alder reaction is favored by electron withdrawing groups on the electrophilic dienophile and by electron donating groups on the nucleophilic diene.
• Some common examples of the components are shown below:

Dienes
Dienophiles

Stereoselectivity:

• The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.
• Addition is syn on both components (bonds form from same species at the same time)
• This is shown by the examples below:

cis-dienophile gives cis-substituents in the product.
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)
 

Diene and dienophile aligned directly over each other gives the endo product (dienophile under or in = endo)	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.
 

Copyright ©2000 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.