Phenoxides, hindered

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... 

Similar reactions have been used for substitution by alkoxide and phenoxide nucleophiles. Hindered binaphthyl ligands have proven useful in substitutions by alcohols.173... 

The sterically hindered dianionic bidentate phenoxide ligand (205) gives several tetrahedral iron(II) complexes, e.g., [Fe (205)(THF)2], [Fe (205)(py)2], [Fe (205)(bipy)], and [Fe (205)(2,6-xylylNC)2]. The first of these is prepared from FeCl2 and (205)H2 in tetrahydrofuran the others are prepared from the dimer [Fe2(205)2]. The 2,6-xylylNC complex is low-spin, the others high-spin. There is also a five-coordinate iron(III) complex, red-black [Fe(205)(bipy)Cl], whose structure is intermediate between trigonal bipyramidal and square pyramidal." ... 

The intensification of studies of the derivatives of sterically hindered ligands such as OCR3 [where R = Ph, Bu (tritox)],OCHBu2 (ditox), OSiR3, 2,6-or 2,4,6- substituted phenoxides RnC6HJ.nO (where R= Me, Pr, Bu, Ph) is... 

The phenoxides of Nb(II) and alkoxides of Ta(III), as well as the corresponding derivatives of vanadium should be stabilized by the sterically hindered ligands. The alkoxide chlorides of niobium (Eft) and (TV) are diamagnet-... 

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. 

Epoxides are sensitive to nucleophilic ring-opening reactions. Phenoxide ion attacks the less hindered carbon to yield l-phenoxy-2-propanol. 

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. 

But is the same cyclopropanone an intermediate in the Favorskii reaction If the bromoketone is treated with methoxide in methanol, it gives the Favorskii product but, if it is treated with a much more hindered base, such as the potassium phenoxide shown, it gives the same cyclopropanone with the same stereochemistry. 

An interesting example of the use of a hindered magnesium phenoxide has been reported, though without experimental details [20],... 

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 carboxylic acid group in the starting material will also be converted to a salt in carbonate solution. Although phenoxide is a better nucleophile than carboxylate ion, this phenoxide may not react as rapidly as the carboxylate with benzyl bromide because it is much more sterically hindered (by the ortho methyl and nitro groups). 8, 2 reactions are slowed considerably by steric hindrance. 

The resonances in the butyl ether region occur in three distinct bands. Chemical shift data for the a carbon atom resonances in about 20 ethers indicate that the resonances centered about 872.9 may result from hindered aryl ethers, for example, butyl 2,6-dimethylphenyl ether, butyl benzyl ethers, or butyl n-alkyl ethers, for example, dibutyl ether. The resonances in this region could arise from tetrahydrofuran residues in the coal product. However, the results obtained in this laboratory and in Larsen s laboratory are much more compatible with interpretations that exclude the involvement of tetrahydrofuran and focus on the reactions of the labeled butylation reagent with 2,6-disubstituted phenoxides, benzylic oxides, and primary alkoxides liberated in the formation of the coal polyanion. The most intense resonance centered at... 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.)... 

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