Formation phenols

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

The initial rates of phenol formation at different substrate concentrations at pH 7.0 and 25.0 °C are as follows... 

Phenol formation is favoured in less coordinating and/or less polar solvents however, for clean reactions affording the Cr(CO)3-coordinated benzannulation products, ethereal solvents are the solvents of choice. 

The influence of the copper catalyst ratio on the hydrolysis of arylhalide has been investigated on the chlorobenzene. The yield of the phenolate formation in these reaction conditions is depending on the initial molar ratio of the cuprous oxide to the starting chlorobenzene (Fig. 15). 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

Products from the photolysis of the cyclopropyl ketone (44) are dependent on the pH of the solvent.(42) In aqueous dioxane only the 2,3-diphenyl-phenol (45) is formed along with the photoacid (46). Bond cleavage at c takes place via both the singlet and triplet states, whereas bond cleavage at a with phenol formation takes place in the triplet state ... 

The importance of phenol formation by the proposed pathway was probed by irradiating l,3-diphenoxy-2-methyl-2-propanol (5) under the same conditions. Compared to 3, the rate of phenol formation was approximately 2 times slower. Since the 11-transfer step in Scheme II is not available to 5, the results provide support for the scheme as an important, but not sole, pathway for phenol formation. Irradiation of and 5 with an air purge resulted in faster rates of phenol formation (ca. 5-fold) relative to N2. These findings parallel the accelerated fluorescence intensity loss from polymer 1 films in air as compared to the results in vacuo (see Table I). 

FIGURE 4.79 Substrates, chlorobenzene and warfarin, used to test whether phenol formation involves a stepwise mechanism. 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

Solvent polarity has a strong influence on the yield of rearranged products as indicated in Table 1 for the PFR of phenyl benzoate (14) in several solvents [31]. Polar solvents favor the rearrangement, whereas nonpolar solvents favor phenol formation. An experiment carried out with methanol-ether mixtures... 

Notably, NH3 is indispensable for the catalytic phenol synthesis. In the absence of NH3, neither benzene combustion nor phenol formation occurred on the Re-CVD/HZSM-5 catalyst (Table 10.6). Other amine compounds such as pyridine and isopropylamine did not promote the catalytic reaction at aU, which indicates that the role of NH3 in the catalysis is not due to its basic function. Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N2O as an oxidant [90, 91], but N2O did not act as an active oxidant on the Re-CVD/ HZSM-5 catalyst Furthermore, no positive effects were observed by the addition of both N2O and H2O. Notably, the NH3-pretreated Re-CVD/HZSM-5 catalyst selectively converted benzene into phenol with O2 in the absence of NH3, as discussed below. 

A parallel was drawn between stable ion and AMI studies of methylphenanthrenes and solvolytic studies of K-region and non-K-region phenanthrene oxides. The carbocation formed by opening of the 1,2-epoxide closely resembled the 2-methylphenanthrene cation (and 7H ), and the regiochemistry of phenol formation (1-phenanthrol) could be understood. Similarly, phenanthrenium cations derived from the 3-methyl and dimethylated compounds served as models for carbo-cations formed by solvolysis of phenanthrene-3,4-epoxide (formation of 4-phenanthrol following hydride shift). 

SCHEME 159. Selenium-catalyzed Baeyer-Villiger oxidation of aromatic aldehydes and phenol formation via hydrolysis of the intermediary esters... 

According to the reaction medium the results in the third column of the Table V should be approached in a different way. A rigid polyethylene film suppresses phenol formation almost entirely by keeping the radicals in the solvent cage. Furthermore, all substituents decrease the quantum yield relative to hydrogen. This effect cannot be explained solely by a Hammett-type treatment of electronic effects. The effect of the substituent size is particularly pronounced in the case of the p-isooctyl substituted ester whose quantum yield is reduced drastically when compared to that of the p-methyl substituted... 

Ortho- and para-rearrangement and phenol formation on uv-irradiation of aryl esters are accompanied in several cases by decarboxylation,37,60,62,64,80,81 represented for 3,5-di-t-butylphenyl benzoate by the equation 118 -> 119-122. It was shown that this reaction cannot be sensitized,64 but the dramatic differences in product distribution could be observed by changing of the solvent.60,84 The results in Table VI indicate that in polar solvents the decarboxylation process is minimized while the formation of the photo-Fries rearrangement 119 is enhanced. The reverse appears to be true when nonpolar ethereal solvents are used. A considerable amount of biaryls are formed, and hence this reaction may prove useful for the preparation of biaryls and alkylary Is. 

The reversed dependence of this concerted decarboxylation reaction, as well as the concerted ortho-rearrangement on the polarity of solvents (cf. Tables II and VI), and the competition of decarboxylation to phenol formation in nonpolar solvents, are striking. Lack of information on the influence of substituents, viscosity, sensitization, or possible quenching in direct comparison with the formation of rearranged products and phenol do not yet... 

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