A study of aromatic cyclodehydration, I. Ortho effects II. The catalytic effect of metal oxides
Spangler, Martin Ord Lee
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In the light of previous investigations of the cyclodehydration of o-benzylbenzophenones and analogous compounds to polycyclic aromatic hydrocarbons a study of 2'-substituted-2-benzylbenzophenones was undertaken. The previous work had been concerned with the rates of cyclization or 3'-substituted and 4'-substituted ketones in which no ortho effects were operating. The results of these investigations could be adequately interpreted according to the electronic theory of the English school. In the 2'-series ortho effects are superimposed upon the usual electronic effects. When the 2'-substituents were halogens a decrease in rate was observed with an increase in the size of the halogen atom. This indicates that one aspect of these ortho effects is steric. These reactions were carried out in a sealed tube in hydrobromic and acetic acids at 150°C rather than at 117.5°C which was the temperature used previously. This increase was necessary to get a reaction with all the ketones concerned. Even under these conditions the 2'-hydroxy-2- benzylbenzophenone cyclized very poorly. This was attributed to a second ortho effect due to intramolecular hydrogen-bonding between the hydroxy group and the keto group. Infrared spectroscopy demonstrated that this type of hydrogen-bonding is present. The 3'-hydroxy-2-benzylbenzophenone apparently cyclizes very readily since the 3'-methoxy compound is cyclized to 9-(3-hydroxyphenyl)anthracene quite readily. Thus, the two reactions in the 3'-series, ether cleavage and the cyclization reaction both proceed more rapidly than the cyclization of the 21-hydroxyketone. The 9-(2- methoxyphenyl)anthracene can be prepared in good yield by the use of phenyl acid phosphate or polyphosphoric acid as a cyclizing agent. This compound can be easily converted to 9-(2-hydroxyphenyl)anthracene by heating under reflux with a mixture of hydrobromic and acetic acids. The halogen substituted ketones are very viscous oils. In order to have solid derivatives for identification purposes these ketones and the methyl derivative were oxidized to the corresponding o-dibenzoylbenzenes which are all solids. Chromium trioxide in acetic acid was usually found to be the best reagent for this purpose. Two of the 9-phenylanthracenes were oxidized to the corresponding 9-phenyl-9-hydroxy-10-anthrones using sodium dichromate in acetic acid. This reaction is carried out very readily and in excellent yield. It may prove to be valuable in the identification of other aromatic polycyclic hydrocarbons. The reduction of 2'-fluoro-2-benzylbenzophenone to 2'-fluoro-2-benzylbenzhydrol was carried out using sodium borohydride in pyridine. The infrared spectrum showed no evidence of intramolecular hydrogen-bonding between the fluorine atom and the hydroxy group. A study of the use of solid surfaces in aromatic cyclodehydration was made using 4'-chloro-2-benzylbenzophenone as a model compound. Experiments with a large number of solids brought to light two factors which seem to contribute to the ability of a surface to catalyze the reaction. One is the acidity of the surface and the other is the ability of a surface to bind the ketone irreversibly. Surface acidity was measured in terms of the H₀ function by the use of Hammett indicators adsorbed on the surface. The color of the adsorbed indicators was used to obtain the H₀ values. The activity of each surface was measured in terms of the percent yield of the product. The solid of highest activity and highest acidity was a silica-alumina cracking catalyst containing 13% alumina. This catalyst gave a 94% yield with 4'-chloro-2-benzylbenzophenone compared to only about 25% on the commonly used Fisher activated alumina. 2'-Methyl-2-benzylbenzophenone was cyclized in 60% yield. This ketone gave a yield of only 20% when cyclized in the hydrobromic-acetic acid mixture at 180° for 9 hr. 2’,6’-Dimethyl-2-benzylbenzophenone has resisted all attempts to cyclize it including an attempt with the most active solid surface available, the silica-alumina cracking catalyst.
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