Ortho-Hydroxylated products, formation

Photocatalytic degradation of 4-chlorophenol in TiOz aqueous suspensions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated product, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields hydroxy hydroquinone, which can readily be oxidized and mineralized to carbon dioxide. Complete dechlorination and mineralization of 4-chlorophenol can be achieved. In contrast, direct photolysis of 4-chlorophenol produces hydroquinone and p-benzoquinorie as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiO2, is generated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring. 

Pedersen and coworkers investigated the El mass spectra of several 2-hydroxyphenyl alkyl sulfones (39) and sulfoxides (Section II.B). The methyl derivative seemed to fragment only via sulfinate ester formation giving the primary product ions m/z 157 and 109 (equation 14). Obviously hydrogen bonding between the ortho hydroxyl and the sulfone sulfur makes the loss of CH3SO2 difficult in contrast to the situation in methyl phenyl sulfone. The sulfinate ester rearrangement is not important when R>Et in 39. 

Phenol is photooxidized via its hydroxylated compounds into CO2 and H2O. Para-dihydroxybenzene (p-DHB), ortho-dihydroxybenzene (o-DHB) and 1,2,3-trihydroxybenzene (1,2,3-THB) are identified as primary and secondary hydroxylation products (Al-Ekabi Seipone, 1988). 1,2,4-trihydroxybenzene (1,2,4-THB) and 1,4-benzoquinone (1,4-SQ) are also documented as intermediate species. These species appear to be present prior to the total phenol mineralization (Trillaseta/., 1996 Winterbottom et al., 1997). Tseng and Huang (1990) reported two intermediate species,p-DHB and 1,4-BQ, during the photodegradation of 5.10 mole/L of phenol. Several other species are also suspected as possible inteimediates with their formation following the aromatic ring break up muconic acid, maleic acid, oxalic acid, formic acid, and acetic acid. Researchers have however, experienced difficulty until now to directly detect these species. 

Finn also showed the formation of 2H-chromenes under the same reaction conditions, using alkenylboronic acids and morpholine in dioxane at 90 °C. A more convenient route to the 2H-chromenes was then developed using a catalytic amount of dibenzylamine in the presence of alkenylboronic adds and salicylaldehyde (42, Scheme 7.11) [30]. Chromenes 43 were reported to arise from the initial Petasis borono-Mannich adducts 44 via an add promoted intramolecular S 2 attack of the ortho-hydroxyl group onto the protonated allylic amine of intermediate 45. A more likely mechanism involves elimination from 45 to intermediate 46, followed by 6n-electrocychzation to the product The reaction is tolerant of various functional groups and substitution patterns on the salicylaldehyde, and could also be promoted using a polymer-supported base, such as Merrifield resin-supported dibenzylamine (40-50 mol%) [30]. 

Diazophenols, ie, o-hydroxyaryldiazonium salts, couple to 1-naphthol in weaMy basic solution primarily in the para position, but as the hydroxyl ion concentration is increased, formation of the ortho isomer is favored and is frequentiy the sole product. Pyridine and pyridine derivatives, urea, and acetate, etc, used as buffers can also catalyze azo coupling reactions (28). l-amino-2-naphthol-4-sulfonic acid [116-63-2] (1,2,4-acid) and 1-naphthol yield the important Eriochrome Black A [3564-14-5] (18a, R = H) (Cl Mordant Black 3 Cl 14640) which is reportedly (20) a mixture of ortho and para isomers. 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

Scheme 4.2) does not produce diazepine derivatives, but 2-(2-oxyphenyl)benzimi-dazole 6 was found to be the only product isolated. However, there are known data on the interaction between o-PDA and orr/zooxybenzalacetone 7 (the structure of which is similar to that of ortho-oxychalcone) resulting in the formation of benzodiazepine 8 [9]. The reaction involves an intramolecular cyclization in which the hydroxyl group and the azomethine bond participate ... 

The microbial hydroxylation of aromatic substrates with activating substituents appears to follow the normal rules of electrophilic substitution in that ortho and para products predominate (776,181), and the formation of 177 (major) and 178 (minor) in both the microbial and mammalian metabolism of acronycine has been attributed (777) to the directing influence of the nitrogen atom in a process involving a monooxygenase enzyme. 

V.D). When the electron density in the ring is high (as in polyalkyl phenols) and the ortho- and/or para position (with respect to the OH group) is vacant, the formation of ortho- or para-benzoquinone also occurs. Indeed, in the hydroxylation of phenol to catechol and hydroquinone, one of the major side products (and the main cause of the tar formation) is the formation of benzo-quinones and products derived from them. The benzoquinones of polyalkyl-benzenes are starting materials for many products in the photographic and fine chemicals industries. Trukhan et al. 234) reported the oxidation of 2,3,-... 

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