Classes of Dienes

Allylic carbocations and aUylic radicals are conjugated systems involved as reactive intermediates in chemical reactions. The third type of conjugated system that we will examine, conjugated dienes, consists of stable molecules. 

A hydrocarbon that contains two double bonds is caUed an alkadiene, and the relationship between the double bonds may be described as isolated, conjugated, or cumulated. Isolated diene units are those in which two carbon-carbon double bond units are separated from each other by one or more -hybridized carbon atoms. 1,4-Pentadiene and 1,5-cyclooctadiene have isolated double bonds  

Conjugated dienes are those in which two carbon-carbon double bond units are directly connected to each other by a single bond. 1,3-Pentadiene and 1,3-cyclooctadiene contain conjugated double bonds  

Cumulated dienes are those in which one carbon atom is common to two carbon-carbon donble bonds. The simplest cnmnlated diene is 1,2-propadiene, also called allene, and componnds of this class are generally referred to as allenes. 

PROBLEM 10.6 Many naturally occurring substances contain several carbon-carbon double bonds some isolated, some conjugated, and some cumulated. Identify the types of carbon-carbon double bonds found in each of the following substances  

If necessary, review the suggested sections to prepare for this chapter  

MO Theory and Pi Bonds (Sections 1.8, 1.9) Enthalpy, Entropy and Gibbs Free Energy (Sections 6.1-6.3) 

PLUS Visit www.wileyplus.com to check your understanding and for valuable practice. 

The relative spatial arrangement of the p orbitals in cumulated, conjugated, and isolated dienes. 

Dienes are compounds that possess two C C bonds. Depending on the proximity of the it bonds, they are classified as cumulated, conjugated, or isolated. 

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Another important class of dienes which react readily with ADC compounds are the vinyl arenes. However, the reaction of styrene with ADC compounds does not give simple tetrahydrocinnolines. Styrene and DEAZD give a 1 2 adduct formed by the initial Diels-Alder adduct reacting rapidly in an ene reaction with more DEAZD (Eq. 18).181 Substituted styrenes react... 

The preparation of this class of dienes has received considerable attention during the last few years. Scheme 127 depicts the current approaches, such as 1,4-eliminations of o-xylene derivatives via reduction (544) - (M2), thermal dehydrohalogenation ( 5) (542), and fluoride-induced desilylation... 

Two classes of diene have been copolymerized successfully with ethylene/propene, in both of which one double bond is deactivated such that the monomer behaves as a mono-olefin. These are unconjugated diolefins, such as cis and trans 1,4-hexadiene or 3,7-dimethyl 1,6-octa-diene, and compounds containing the 2-norbornene structure... 

A third class of dienes, of increasing interest to organic chemists, contain cumulated double bonds these compounds are known as allenes ... 

Fig. 4-30 Energies of the frontier orbitals of 1 -substituted dienes. The energies are typical values for each class of diene...

The regiochemical relationships summarized in Scheme 10.3 can be understood by considering the atomic coefficients of the frontier orbitals. Eigure 10.5 gives the approximate energies and orbital coefficients for the various classes of dienes and dienophiles. 1-ERG substituents (X ) raise the HOMO level and increase the coefficient... 

A new class of dienes (22) has been found to undergo cycloaddition with nitriles to give amino-pyridines (23) (Scheme 11). 4-Dimethylaminobuta-l,3-diene-l,l-dicarbonitriles (24), prepared as shown in Scheme 12, have been cyclized, by reaction with ammonia at 150 °C, to the corresponding 2-amino-3-cyano-pyridines (25). ... 

Thus, the comprehensive investigations above demonstrate that a variety of multicyclic compounds with quaternary stereogenic centres can be obtained from this class of dien-ynes, depending on the length of the tethering chain as well as on the substituents of the unsaturated functions. The efficiency of cationic Tol-BEM-AP/Rhodium complexes as catalysts for many reactions of this class has been assessed. 

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