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Chapter 16
Dienes, Conjugated Pi Systems, and Pericyclic Reactions

Structure of Conjugated Dienes
Introduction Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like…
Stability of Conjugated Dienes
Introduction A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering…
π Molecular Orbitals of 1,3-Butadiene
Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of…
π Molecular Orbitals of the Allyl Cation and Anion
An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond.…
π Molecular Orbitals of the Allyl Radical
Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the…
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s…
Electrophilic 1,2- and 1,4-Addition of X<sub>2</sub> to 1,3-Butadiene
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide. Conjugated dienes…
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the…
UV&ndash;Vis Spectroscopy of Conjugated Systems
Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum…
UV&ndash;Vis Spectroscopy: Woodward&ndash;Fieser Rules
UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions.…
Pericyclic Reactions: Introduction
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through…
Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are…
Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the…
Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state.…
Cycloaddition Reactions: Overview
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between…
Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical…
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene…
[4+2] Cycloaddition of Conjugated Dienes: Diels&ndash;Alder Reaction
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred…
Diels&ndash;Alder vs Retro-Diels&ndash;Alder Reaction: Thermodynamic Factors
The Diels–Alder reaction is thermally reversible, meaning that the reaction reverts to the starting diene and dienophile under suitable…
Diels&ndash;Alder Reaction Forming Cyclic Products: Stereochemistry
The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic…
Diels&ndash;Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry
Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products. Dienophiles with…
Diels&ndash;Alder Reaction: Characteristics of Dienes
The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that…
Diels&ndash;Alder Reaction: Characteristics of Dienophiles
In a Diels–Alder reaction, the diene is usually an electron-rich system and acts as a nucleophile, whereas the dienophile is electron-deficient…
Thermal Sigmatropic Reactions: Overview
Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are…
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement
The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves…
[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement
The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a…
Woodward&ndash;Hoffmann Selection Rules and Microscopic Reversibility
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition…
Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes
Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules…
Facile Preparation of (2<em>Z</em>,4<em>E</em>)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high…
The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry
A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial…

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