Aromatic ring enols

The enol form is phenol and the stabilization gamed by forming an aromatic ring is more than enough to overcome the normal preference for the keto form... 

The formation of a stable imino-enole tautomer 358 is due to the conjugation of the C=C and C=N double bonds with the aromatic ring and hydroxyl group. The enaminoketone tautomer 357 is present in a negligible amount. 

As in the case of benzothiazoles and benzimidazoles, the excited-state proton transfer in 2-(2 -hydroxyphenyl)benzoxazole was studied both experimentally and computationally. The results closely resemble the observations for the other species The cw-enol form is preferred in the Sq ground state and the cw-keto form in the 5i excited state. Moreover, the proton transfer appears to be due to vibrational relaxation rather than thermal activation, suggesting that the aromatic ring has an impact on the transfer reaction of these systems [95JPC12456, 99JST255]. 

Alkylation (Sections 8.8, 16.3, 18.2. 22.7) Introduction of an alkyl group onto a molecule. For example, aromatic rings can be alkylated to yield arenes, and enolate anions can be alkylated to yield a-substituted carbonyl compounds. 

As shown above, the electronic properties have a serious effect on the rate of the reaction. It means that the aromatic ring should occupy the same plane with that of the estimated intermediate enol moiety. Then, it is supposed that the conformation of the substrate is already restricted when it binds to the active site of the enzyme. The evidence that supports this estimation is the inactiveness of a-methyl-o-cWorophenyl and a-naphthylmalonic acids. This is a marked difference with the fact that a-methyl-p-Cl-phenyl and methyl-(3-naphthylmalonic acids are... 

Scheme 2.11 shows some examples of Robinson annulation reactions. Entries 1 and 2 show annulation reactions of relatively acidic dicarbonyl compounds. Entry 3 is an example of use of 4-(trimethylammonio)-2-butanone as a precursor of methyl vinyl ketone. This compound generates methyl vinyl ketone in situ by (3-eliminalion. The original conditions developed for the Robinson annulation reaction are such that the ketone enolate composition is under thermodynamic control. This usually results in the formation of product from the more stable enolate, as in Entry 3. The C(l) enolate is preferred because of the conjugation with the aromatic ring. For monosubstituted cyclohexanones, the cyclization usually occurs at the more-substituted position in hydroxylic solvents. The alternative regiochemistry can be achieved by using an enamine. Entry 4 is an example. As discussed in Section 1.9, the less-substituted enamine is favored, so addition occurs at the less-substituted position. 

Tejedor and coworkers have utilized a combination of two domino processes for a microwave-promoted synthesis of tetrasubstituted pyrroles [344]. The protocol combines two coupled domino processes the triethylamine-catalyzed synthesis of enol-protected propargylic alcohols and their sequential transformation into pyrroles through a spontaneous rearrangement from 1,3-oxazolidines (Scheme 6.183). Overall, these two linked and coupled domino processes build up two carbon-carbon bonds, two carbon-nitrogen bonds, and an aromatic ring in a regioselective and efficient manner. The tetrasubstituted pyrroles could be directly synthesized from the enol-protected propargylic alcohols and the primary amines by microwave irradia-... 

Considering the electrophilic nature of the reaction mechanism, suitable coupling components for the synthesis of azo pigments should carry a nucleophilic center on the aromatic ring system (naphthols) or should be enolizable compounds with reactive methylene functions. Naphthols enter the reaction as naphtholates compounds with reactive methylene groups participate as enolates. 

A further attempt has been made to develop a predictive model for chirality transfer achieved through alkylation reactions of ester enolates which feature chiral auxiliaries. " Hippurate esters (30) derived from (lI , 25 )-trani-2-(p-substituted phenyl)cyclohexanols were found, on reaction with benzyl bromide, to give (31) with predominantly the S configuration at the alkylation centre but with no correlation between the degree of stereoselectivity (20-98%) and the electron density on the aromatic ring. 

The racemization process involves removal of the a-hydrogen to form the enolate anion, which is favoured by both the enolate anion resonance plus additional conjugation with the aromatic ring. Since the a-protons in esters are not especially acidic, the additional conjugation is an important contributor to enolate anion formation. The proton may then be restored from either side of the planar system, giving a racemic product. 

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