Diene

Dienes can be divided into three classes, depending on the relative location of the double bonds: • 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. , is the simplest cumulated diene. B: Isoprene, also known as 2-methyl-1,3-butadiene, the precursor to natural rubber. C: 1,3-Butadiene, a precursor to synthetic polymers. D: 1,5-Cyclooctadiene, an unconjugated diene (notice that each double bond is two carbons away from the other). E: Norbornadiene, a strained bicyclic and unconjugated diene. F: Dicyclopentadiene. According to the Gold Book definition, a "diene" could include one or more heteroatoms which replace unsaturated carbon atoms, giving structures that could more specifically be called heterodienes. 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 thermal cracking of butanes. In a similarly non-selective process, dicyclopentadiene is obtained from coal tars. 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. In the laboratory, more directed and delicate processes are employed, such as dehydrohalogenations and condensations. Conjugated dienes can be formed by the addition of two good leaving groups across a double bond, followed by base workup. Dienes can also be produced from the semihydrogenation of diynes. Pd catalysts tend to degrade homoconjugate dienes, but Ni works well. Myriad name reactions have been developed, including the Whiting reaction. Families of nonconjugated dienes are derived from the oligomerization and dimerization of conjugated dienes. For example, 1,5-cyclooctadiene and 4-vinylcyclohexene are produced by dimerization of 1,3-butadiene. Diene-containing fatty acids are biosynthesized from acetyl CoA. == 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. Cycloaddition 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: 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. Other diene complexes include (butadiene)iron tricarbonyl, cyclobutadieneiron tricarbonyl, and cyclooctadiene rhodium chloride dimer. ==References==