Diene
In
Classes
Dienes can be divided into three classes, depending on the relative location of the double bonds:[1]
- Cumulated dienes have the double bonds sharing a common atom. The result is more specifically called an allene.
- Conjugated dienes have conjugated double bonds separated by one single bond. Conjugated dienes are more stable than other dienes because of resonance.
- Unconjugated dienes have the double bonds separated by two or more single bonds. They are usually less stable than isomeric conjugated dienes. This can also be known as an isolated diene.
According to the
Compounds that contain more than two double bonds are called polyenes. Polyenes and dienes share many properties.
Synthesis of dienes
On an industrial scale, butadiene is prepared by
In the laboratory, more directed and more delicate processes are employed such as
Diene-containing
α,ω-Dienes have the formula (CH2)n(CH=CH2)2. They are prepared industrially by ethenolysis of cyclic dienes. For example, 1,5-hexadiene and 1,9-decadiene, useful crosslinking agents and synthetic intermediates, are produced from 1,5-cyclooctadiene and cyclooctene, respectively. The catalyst is derived from Re2O7 on alumina.[2]
Reactivity and uses
Polymerization
The most heavily practiced reaction of alkenes, dienes included, is polymerization. 1,3-Butadiene is a precursor to rubber used in tires, and isoprene is the precursor to natural rubber. Chloroprene is related but it is a synthetic monomer.
Cycloadditions
An important reaction for conjugated dienes is the Diels–Alder reaction. Many specialized dienes have been developed to exploit this reactivity for the synthesis of natural products (e.g., Danishefsky's diene).
Other addition reactions
Conjugated dienes add reagents such as bromine and hydrogen by both 1,2-addition and 1,4-addition pathways. Addition of polar reagents can generate complex architectures:[3]
Metathesis reactions
Nonconjugated dienes are substrates for
Acidity
The position adjacent to a double bond is
As ligands
Dienes are widely used chelating ligands in organometallic chemistry. In some cases they serve as placeholder ligands, being removed during a catalytic cycle. For example, the cyclooctadiene ("cod") ligands in bis(cyclooctadiene)nickel(0) are labile. In some cases, dienes are spectator ligands, remaining coordinated throughout a catalytic cycle and influencing the product distributions. Chiral dienes have also been described.[5] Other diene complexes include (butadiene)iron tricarbonyl, cyclobutadieneiron tricarbonyl, and cyclooctadiene rhodium chloride dimer.
References
- ^
- ISBN 0-471-23896-1.
- PMID 17962753.
- PMID 13129348.
- PMID 35972018.