Double bond conjugation

The electrophilicity of C = C double bonds conjugated with electron withdrawing groupings leads to a -synthons. This is an important example of the vinyiogous principle ... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Both enols have their carbon-carbon double bonds conjugated to a carbonyl group and can form an intramolecular hydrogen bond They are of comparable stability... 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

Compounds having a cyclic double bond conjugated to an aromatic ring (e.g. indene) undergo oxofluorination, with FCIO3 acting as a 2-centre electrophile ... 

Molecules with alternating single and double bonds ( conjugated polyenes ) often exhibit unusual physical and chemical properties. Chemists have postulated the involvement of zwitterionic resonance structures to account for these properties. 

The isoindole-isoindolenine equilibrium has been studied quantitatively only in the case of certain 1-arylisoindoles. Although two structurally different isoindolenines are possible, only that with the carbon-nitrogen double bond conjugated with both aromatic rings was observed. Investigation of the isoindole-isoindolenine ratios for three compounds by NMR and ultraviolet spectroscopy indicated a... 

Diolefms are hydrocarbon compounds that have two double bonds. Conjugated diolefins have two double bonds separated by one single bond. Due to conjugation, these compounds are more stable than mono-olefms and diolefms with isolated double bonds. Conjugated diolefins also have different reactivities than monoolefins. The most important industrial diolefmic hydrocarbons are butadiene and isoprene. 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

For phenylethylene, in which a double bond conjugated with a benzene ring permits struc-... 

Later, Patterson and Hibbert (S3) stated that ultraviolet absorption of lignin corresponded with the absorptions of hydroxy derivatives of 4-(4,hydroxy-3,methoxyphenyl)-l-propanone and l-(4,hydroxy-3, 5,dimethoxyphenyl)-4-propanone. These authors, therefore, assumed the presence in lignin of a carbonyl group, or of an ethylenic double bond conjugated with an aromatic nucleus. They ascribed the 280 mp band to m-position freedom in the benzene ring and the 310 mp baud to a masked carbonyl group in conjugation with a benzene nucleus. 

The addition of a carbanion to a double bond conjugated with a carbonyl, nitrile, or sulfonyl group can give the usual simple addition product or it can give an intermediate carbanion which continues the reaction as a chain polymerization. 

The isomerization of the double bonds conjugated with the ester moiety to enol ethers can be carried out using RuCl(H)(CO)(PPh3)3 as catalyst (Scheme 42).69... 

The electrocatalytic reduction of dienes, like that of monoenes, is difficult when the double bond is not activated. Polyolefins with isolated double bonds cannot be reduced on mercury cathode, while double bonds conjugated to another -system can be reduced. 

A water molecule trapped during the crystallization is axially coordinated (05) and induces the zinc ion to move out of the plane of the four nitrogen atoms by 0.31 A. It can be seen that the saturation of the carbon responsible for the break of the double bond conjugation is localized on the C5 carbon atom. 

In addition to the described reduction of double bonds conjugated to a carbonyl group, sodium hydrogen telluride and phenyltellurol reduce double (and triple) bonds conjugated to aromatic systems. " ... 

Double bonds conjugated with aromatic rings and with carbonyl, carboxyl, nitrile and other functions are readily reduced by catalytic hydrogenation and by metals. These reductions are discussed in the appropriate sections aromatics, unsaturated aldehydes and ketones, unsaturated acids, their derivatives, etc. 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

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