Chapter 16

Dienes, Conjugated Pi Systems, and Pericyclic Reactions

Introduction Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like…

Introduction A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering…

Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of…

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.…

Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the…

The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s…

Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide. Conjugated dienes…

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…

Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum…

UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions.…

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through…

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are…

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the…

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state.…

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between…

Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical…

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene…

The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred…

The Diels–Alder reaction is thermally reversible, meaning that the reaction reverts to the starting diene and dienophile under suitable…

The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic…

Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products. Dienophiles with…

The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that…

In a Diels–Alder reaction, the diene is usually an electron-rich system and acts as a nucleophile, whereas the dienophile is electron-deficient…

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…

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…

The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a…

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition…

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules…

Direct cross-coupling between two alkenes via vinylic C-H bond activation represents an efficient strategy for the synthesis of butadienes with high…

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial…