Pentadiene

In organic chemistry, pentadiene is any hydrocarbon with an open chain of five carbons, connected by two single bonds and two double bonds. All those compounds have the same molecular formula C₅H₈. The inventory of pentadienes include:

• 1,2-pentadiene, or ethyl allene, H₂C=C=CH−CH₂−CH₃.^[1] It and 2,3-pentadiene are the least common isomers of pentadiene.
• 1,3-pentadiene, H₂C=CH−CH=CH−CH₃ with two isomers:^[2][3]
  • cis-1,3-pentadiene.
  • trans-1,3-pentadiene, also known as piperylene.
• 1,4-pentadiene, H₂C=CH−CH₂−CH=CH₂.^[4]
• 2,3-pentadiene, H₃C−CH=C=CH−CH₃, with two enantiomers (R and S).^[5] It and 1,2-pentadiene are the least common isomers of pentadiene.

Well known derivatives containing pentadiene groups include hexadienes, cyclopentadiene, and especially three fatty acids linoleic acid, α-linolenic acid, and arachidonic acid as well as their triglyceride esters (fats).

Preparation and basic reactions

1,4-Pentadiene can be prepared from 1,5-pentadiol via the diacetate.^[6]

1,3-Pentadiene, like 1,3-butadiene, undergoes a variety of cycloaddition reactions. For example, it forms a sulfolene upon treatment with sulfur dioxide.^[7]

Pentadienyl

Pentadienyl refers to the organic radical, anion, or cation with the formula [CH₂(CH)₃CH₂]^z, where z = 0, −1, +1, respectively.

Biochemistry

Main article: Lipid peroxidation

Methylene-interrupted polyenes are 1,4-pentadiene groups found in polyunsaturated fatty acids linoleic acid, α-linolenic acid, and arachidonic acid. These pentadiene derivatives are susceptible to lipid peroxidation, far moreso than monounsaturated or saturated fatty acids. The basis for this reactivity is the weakness of doubly allylic C-H bonds, leading to pentadienyl radicals. A range of reactions with oxygen occur. Products include fatty acid hydroperoxides, epoxy-hydroxy polyunsaturated fatty acids, jasmonates, divinylether fatty acids, and leaf aldehydes. Some of these derivatives are signallng molecules, some are used in plant defense (antifeedants), some are precursors to other metabolites that are used by the plant.^[8]

Cyclooxygenases ("COX") are enzymes that generate prostanoids, including thromboxane and prostaglandins such as prostacyclin. Aspirin and ibuprofen exert their effects through inhibition of COX.

Mechanism of COX activation and catalysis. The tyrosyl radical can abstracts the 13-pro(S) hydrogen of arachidonic acid to generate a pentadienyl radical, initiating the COX cycle.

Drying and rancidification

Simplified chemical reactions associated with the formation of a hydroperoxide from a typical polyunsaturated fatty acid. In the second step, the hydroperoxide combines with another unsaturated side chain to generate a crosslink.^[9]

Fats containing 1,4-pentadiene groups are drying oils, i.e. film-forming liquids suitable as paints. One practical consequence is that polyunsaturated fatty acids have poor shelf life owing to their tendency toward autoxidation, leading, in the case of edibles, to rancidification. Metals accelerate the degradation.

Organometallic chemistry

In organometallic chemistry, the pentadienyl anion is a ligand, the acyclic analogue of the more-common cyclopentadienyl anion. The pentadienyl anion is generated by deprotonation of pentadiene. A number of complexes are known, including bis(pentadienyl) iron, Fe(C₅H₇)₂, the "open" analog of ferrocene. Only few pentadienyl complexes feature simple C₅H₇ ligands. More common is the dimethyl derivative 2,4-Me₂C₅H₅. Additionally, many pentadienyl ligands are cyclic, being derived from the addition of hydride to η⁶-arene complexes or hydride abstraction from cyclohexadiene complexes.^[10][11]

The first pentadienyl complex to be reported was derived from protonolysis of a complex of pentadienol:^[12]

Fe(C₅H₇OH)(CO)₃ + H⁺ → [Fe(C₅H₇)(CO)₃]⁺ + H₂O

Treatment of this cation with sodium borohydride gives the pentadiene complex:

[Fe(C₅H₇)(CO)₃]⁺ + H⁻ → Fe(C₅H₈)(CO)₃

Further reading

• Juergen Herzler , Jeffrey A. Manion, and Wing Tsang (2001): "1,2‐Pentadiene decomposition". International Journal of Chemical Kinetics, volume 33, issue 11, pages 755-767. doi:10.1002/kin.1072