P-hydroxybenzophenone

The reaction of phenol benzoylation catalyzed by a H-P zeolite (Si/Al ratio 75) has been studied, with the aim of determining the reaction scheme. The only primary product was phenylbenzoate, which then reacted consecutively to yield o- and p-hydroxybenzophenone and benzoylphenylbenzoate isomers, via both intermolecular and intramolecular mechanisms. 

Figure 2 plots the effect of phenol conversion on the distribution of products. The following compounds were obtained (i) phenylbenzoate, (ii) 0- and p-hydroxybenzophenone, and (iii) 0- and p-benzoylphenylbenzoate. The first product was obtained with 100% selectivity only at moderate conversion, for low reaction times. In fact, when the conversion of phenol increased due to longer reaction times. 

The data clearly indicate that the only primary product was phenylbenzoate, while all the other compounds formed by consecutive reactions upon the ester. Therefore, the scheme of reaction is that one summarized in Figure 3. The formation of the ester as the only primary product represents one important difference with respect to the Friedel-Crafts benzoylation of phenol with benzoylchloride or benzyltrichloride, catalyzed by AICI3. In the latter case, in fact, the product of para-C-acylation (p-hydroxybenzophenone) is the main product of reaction this is due to the fact that AICI3 coordinates with the 0 atom of the hydroxy group in phenol, and makes it less available for the ester formation, due to both electronic and steric reasons. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

In our case, all the compounds obtained by transformation of the intermediate, phenylbenzoate, were primary products. This indicates that the following parallel reactions occurred on phenylbenzoate (1) the intramolecular Fries transposition generated o-hydroxybenzophenone, (ii) phenylbenzoate acted as a benzoylating agent on phenol, to yield p-hydroxybenzophenone (with also possible formation of the ortho isomer) and phenol and (iii) phenylbenzoate acylated a second molecule of phenylbenzoate to generate benzoylphenylbenzoates, with the co-production of phenol. 

An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes. Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of m-, and p-hydroxybenzophenones. In non-hydrogen bond... 

The reaction rate was shown to be related to the accessibility of the acid sites-H-MFI zeolite was practically inactive and Nafion was the most active catalyst. With all the catalysts, not only the desired p-hydroxybenzophenone (/ -HBP) product but also j3-benzoxybenzophenone (p-BXBP) and o-hydroxybenzophenone (o-HBP) were formed. The para products result from benzoylation of phenol (formed by hydrolysis of PB) or of PB and o-HBP from the intramolecular rearrangement of PB. The selectivity to p-HBP can, therefore, be significantly improved by adding phenol to the reactant mixture [4]. Unfortunately, thermodynamic limitations arising from the low polarity of the medium employed (even with very polar solvents such as sulfolane) seems to preclude the use of a solid acid for the production of diphenol monomers from diphenyl benzoate [4]. 

Acylation of phenol with PhCCl3 or benzoyl chloride is effectively catalyzed by HZSM-5 (reaction 6.6). The product p-hydroxybenzophenone is used in polymers and as an intermediate in the production of tamoxifen, an anti-breast-cancer drug. High shape selectivity of the catalyst is responsible for the high para-selectivity. 

Also obtained by reduction of p-hydroxybenzophenone 2,4-dinitrophenylhydra-zone with stannous chloride dihydrate in the presence of concentrated hydrochloric acid in boiling acetic acid for 1 h (93%) [52]. 

Preparation by passing chlorine into a solution of p-hydroxybenzophenone and sodium acetate in acetic acid during some hours [140]. 

Obtained (poor yield) by reaction of benzoyl peroxide with p-hydroxybenzophenone [144] in refluxing chloroform for 16 h (9%) [278],... 

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