Carbon stable conjugated

Very often the carbon framework of the future allene is already present in the substrate and often it is propargylic in nature. For example, base-catalyzed isomeriza-tions of acetylenic hydrocarbons - with the triple bond in a non-terminal (40) or terminal position - were often used to prepare allenic hydrocarbons in the early days of allene chemistry [8]. The disadvantage of this approach consists in the thermodynamic instability of the allenes produced if not prohibited for structural reasons, the isomerizations do not stop at the allene but proceed to the more stable conjugated diene stage. In practice, complex mixtures are often formed [9] (see also Chapter 1). 

Nonconjugated dienes and polyenes undergo double-bond migration to form the most stable conjugated system in the presence of basic catalysts. Dienes in which the two double bonds are separated by only one carbon atom exhibit the highest reactivity. Double-bond shifts in dienes with more separated double bonds take place in consecutive steps under catalytic conditions that is, only the monoanion is formed.122... 

The reaction with carbon tetrahalides [213] probably involves free-radical attack e.g. by CBra) rather than electrophilic attack on the enol, but is still subject to selection of the reaction site leading to the more stable conjugated product (c/. p. 159). The reaction with methyl chloride (and a base) has been described only in a patent [214], and no mechanistic details are available. It is surprising that so unreactive an electrophile should attack the enol ether. There is no clear reason for chloromethylation favouring C(4), beyond the suggestion [21 ] that the reagent may first form a complex with the 3-acetoxy group. 

Because the oxygen atom is much more electron ative than carbon, oxygen more readily accepts a negative charge, making OH much more stable than CH3. H2O is a stronger acid than CH4 because OH is a more stable conjugate base than CH3. This is a specific example... 

When you are asked to compare two items, pay attention to how they differ ignore where they are the same. These two compounds differ only in the halogen atom that is attached to the middle carbon of the molecule. Because fluorine is more electronegative than bromine, there is greater electron withdrawal from the oxygen atom in the fluori-nated compound. The fluorinated compound, therefore, will have the more stable conjugate base, so it will be the stronger acid. 

Carbonates can be described as diester derivatives of carbonic acid, formed by condensation with hydroxyl compounds. However, carbo q lic acids do not possess the necessaiy reactivity to synthesize such products. Therefore bifunctional reactive molecules such as phosgene or carbonyldiimidazoles like N,N -carbonyldiimidazole (CDI) and disuccinimidyl carbonate (DSC) are commonly used (Scheme 1.13). These compounds readily react with nucleophiles to form stable conjugates. They react with hydro)q l-containing molecules to form the amino-reactive carbonate or carbamate intermediates. These intermediates react in a subsequent step with primaiy amines to form stable carbamate bonds (aliphatic urethanes). The reaction is usually carried out in alkaline media (pH 7-9) and in the absence of competing amino and... 

The possible formation of the nonconjugated bis-allylic 11-hydroperoxide from linoleate after hydrogen abstraction from carbon-11 has been debated for a long time in the literature. Whether or not this proposed bis-allylic 11-hydroperoxide is unstable and immediately isomerized to the more thermally stable conjugated 9- and 13-hydroperoxides is not clear. Recently, the bis-allyl 11-hydroperoxide was produced in about 5-10% relative yield by kinetically... 

A study of 2-biphenylylcyclohexanone deprotonation by lithium base in THF has established that the unconjugated lithium enolate (existing predominantly as a tetramer) is formed preferentially before converting to a monomer-dimer mixture of the more stable conjugated enolate this occurs by proton transfer from the tertiary carbon, which can then be alkylated. ... 

Allenylcyclobutanol 49 underwent asymmetric ring-opening reorganization in the presence of chiral rhodium(I) catalysts to yield cyclohex-2-enone 50 having an asymmetric quaternary carbon center at the 5-position (Scheme 3.28) [38]. Inorganic bases were used to accelerate double bond isomerization to the more stable conjugated enone. 

Deprotonation at any one of these three locations will result in a resonance-stabilized conjugate base. The most acidic proton is the one that leads to the most stable conjugate base, shown below. Notice that the negative charge is spread over two oxygen atoms (just like a doubly stabilized enolate), as well as three carbon atoms. This anion is therefore more stable than an ethoxide ion, so the equihbrium will favor formation of this resonance-stabihzed anion ... 

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