Phenoxides, hindered phenols

Mononuclear octahedral complexes have been obtained for carbonyl-bound acetic acid, and five-coordinate complexes of sterically hindered phenoxides of the type [V(py)3(C6H3Ph2-2,6)2]. A more hindered phenol gave a four-coordinate complex, [V (C6H3Pr2-2,6)4 Li(thf) 2] in which two pairs of cfr-phenolates were linked by a single Li(thf)+ unit. A binuclear complex of a substituted acetylacetone has the stmcture shown in(l). 

The one-electron oxidation of tyrosine by a copper(II) center (step B of Figure 7) is modeled by two systems (Figure 9). Treatment of (1) with sodium 2,6-di(tertbutyl)phenolate yields copper(I) products, which can be trapped with an isocyanide to give (50) 3,3, 5,5 -tetr<2 w(tertbutyl)-4,4 -dibenzoquinone is formed in this reaction in its maximum theoretical yield of 25%. Similar reactions using less sterically hindered phenolates instead yield the stable copper(II) phenoxide compounds (4)-(7). Similarly, (51) and (52) decompose rapidly at room temperature to give... 

The acetylation (with AC2O and HC104) and Vilsmeier formylation of sterically hindered phenols have been investigated. Substituted o-hydroxy-benzophenones have been prepared in 18-68% yields by treatment of the HMPT complexes of bromomagnesium phenoxides with aromatic aldehydes. " Phenolic O- vs. C-benzoylation has been studied, with particular reference to 3,4-disubstituted phenols. Previous work on the use of trifluoroacetic anhydride to promote aromatic acylation has been extended to the preparation of symmetrical and unsymmetrical benzophenones via reaction between the methyl and benzyl ethers of orcinol and the same ethers of phloroglucinolcarboxylic acid. " Other phenolic acylations include some chalcone syntheses and the acetylation and benzoylation of 2-hydroxy-4-methoxyacetophenone (peonol). ... 

The reaction of 2,4,6-tribromopyridine with phenoxide ion illustrates, in our opinion, the effect of hydrogen bonding as discussed in Section II, B, 3. Reaction (150°, 24 hr) in water gave approximately equal amounts (18% yields) of 2- and 4-monosubstitution, but in phenol under the same conditions only the 2-phenoxy derivative (in high yield plus a small amount of the 2,6-diphenoxy compound) was formed. In water, reaction at the adjacent 2- and 6-position is hindered by the hydrogen bonding (cf. 61) of the solvent to the azine-nitrogen, compared to reaction at the 4-position. On the other hand, in... 

The available rate data for the substitution reactions of phenol, diphenyl ether, and anisole are summarized in Table 5. The elucidation of the reactivity of phenol is hindered by its partial conversion in basic media into the more reactive phenoxide anion. Because of the high reaction velocity of phenol and the even greater reactivity of phenoxide ion the relative rates are difficult to evaluate. Study of the bromination of substituted phenols (Bell and Spencer, 1959 Bell and Rawlinson, 1961) by electrochemical techniques suitable for fast reactions indicates the significance of both reaction paths even under acidic conditions. 

Therefore, the radical coupling occurs easily to give a hydroperoxyben-zoquinone intermediate, which is converted to benzoquinone, completing the catalytic cycle. When the phenol is hindered, the inner-sphere coupling between the phenoxide and hydroperoxide does not proceed effectively and homo lysis of the Cu(II)—OPh bond occurs. The resultant phenoxy radicals couple together to give the diphenoquinone. 

The first step is the attack of a nucleophile on an epoxide. It s an Sn2 reaction, because it goes with inversion of configuration, and we need a phenol as the nucleophile. To make the phenol more reactive, we probably want to deprotonate it to make the phenoxide, and NaOH will do this. Why does this end of the epoxide react Well, it is next to a phenyl ring, and benzylic Sn2 reactions are faster than reactions at normal secondary carbons. Next the end hydroxyl group is made into a leaving group (a mesylate ), for which we need methanesulfonyl chloride (mesyl chloride) and triethylamine. The primary hydroxyl group must react faster than the secondary one because it is less hindered. 

Phenoxide ions oxidise at lower potentials than either the corresponding phenols or phenoxy radicals. In consequence phenoxy radicals formed by the oxidation of hindered phenoxides are relatively stable and are only transformed to the carbonium ion at more anodic potentials. Phenoxy radicals from less hindered phenoxides undergo dimerisa-tion. ... 

Yamamoto and coworkers followed a promising approach, developing a rich chemistry with aluminium phenoxides prepared from sterically hindered 2,6-substituted phenols. The reaction of hullqr phenols with trimethyl aluminium led to methylaluminium bis(2,6-di-tert-bulyl-4-methylphenoxide) (MAD) and aluminium tris(2,6-diphenylphenoxide) (ATPH), as shown in Scheme 18.8, which are as expected monomeric in organic solvents. 

Transposition of substituents takes place from aromatic compounds based on ortho effects from hydroxyphenyl ketones via assumed nucleophilic attack on the carbonyl carbon atom of the phenoxide site to give a tight tetravalent intermediate that promptly decomposes through benzyne neutral release and formation of a carboxylate anion (Scheme 17.19a). This reaction is hindered from meta- and para-substituted phenols. Alternatively, radical alkane loss is also observed that can be rationalized by considering the formation of an ion-neutral complex (Scheme 17.19b) comprised of quinone-like and alkylide groups. The relatively low ionization energy allows the generation of odd-electron quinone-like species and the elimination of the alkane radical (Scheme 17.19b). 

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