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Dienes and Polyenes

Dienes can be isolated, conjugated or cumulated (allenes). In conjugated dienes the p orbitals of the four carbons overlap and the system reacts as a whole: it gives 1,4-additions, the Diels-Alder cycloaddition and pericyclic reactions.

YouTube video: dienes and polyenes

YouTube video: cumulated dienes (allenes)

Structural Aspects

Types of dienes: isolated (CH₂=CHCH₂CH=CH₂), conjugated (CH₂=CHCH=CH₂) and cumulated, or allene (CH₂=C=CH₂)

Nomenclature

Molecular model of 1,3-butadiene

Physical and Bond Properties

Reactivity

Energy diagram of conjugate addition: the allylic carbocation gives the 1,2-product faster (kinetic control) and the more stable 1,4-product (thermodynamic control)

Conjugated Electrophilic Addition

Animation of the Diels-Alder reaction

Diels-Alder Cycloaddition

Cyclization of (E,Z,E)-2,4,6-octatriene: thermally it gives cis-5,6-dimethyl-1,3-cyclohexadiene and photochemically the trans isomer

Pericyclic Reactions

Enzymatic cyclization of squalene (C30 triterpene) to the cholesterol precursor

Polymerization