A study of the aromatic cyclodehydration of 2-(2-naphthylmethyl)- 2'-chloro-5'-methylbenzophenone

Abstract

In a recent study of cyclodehydrogenation reactions of potential carcinogenic or carcinolytic hydrocarbons Zajac pointed out that ring closure of 12-(3-methylphenyl)- benz[a]anthracene might take place at either of the two ortho positions or the phenyl ring. These positions are not equivalent with respect to the methyl group and ring closure might yield either 2-methyldibenzo[a,1]pyrene or 4-methyldibenzo[a,1]pyrene or both.

The objectives of the present investigation are to study possibilities and methods to prepare 2-(2-naphthylmethyl)- 2'-chloro-5'-methylbenzophenone in a sufficient yield so that a study of the cyclization reaction under a variety of conditions can be made in order to prepare 2-methyldibenzo[a,l]pyrene.

During the course of the investigation an improved synthetic route to prepare 2-bromonaphthalene was secured.

Three methods utilizing Grignard reagents were approached in order to prepare 2-(2-naphthyl~ethyl)-2'halobenzophenones necessary for the study to obtain 2-(2-naphthylmethyl)-2'-chloro-5'-methylbenzophenone.

The only successful preparative method for the above ketones was found to be the reaction between a Grignard reagent and an acid halide. This procedure was applied to the preparation of the following new ketones: 2-(2-naphthylmethyl)-2'-fluorobenzophenone, 2-(2-naphthylmethyl)- 2'-chlorobenzophenone, and 2-(2-naphthylmethyl)- 2'-chloro-5'-methylbenzophenone.

Investigation of the sealed tube method revealed that the above ketones could be cyclized at 180° with a mixture of hydrobromic and acetic acids in seven hours. Under these conditions the new compounds 12-(2-fluorophenyl) benz[a]anthracene, 12-(2-chlorophenyl)benz[a]- anthracene, and 12-(2-chloro-5-methylphenyl)-7,12-dihydrobenzo[ a]anthracene were obtained. Further study showed that 12-(2-chloro-5-methylphenyl)-7,12-dihydro-benz[a]anthracene could be obtained in a much better yield when the above procedure was altered by using a mixture of hydriodic and acetic acids.

In order to obtain 2-methyldibenzo[a,1]pyrene 12-(2-chloro-5-methylphenyl)-7,12-dihydrobenzo[a]anthracene was reacted with potassium hydroxide and quinoline. Although the results were not definite the qualitative tests and the ultraviolet and infrared spectra suggest an intermediate dihydro compound.

The ultraviolet and infrared spectra of six new compounds were recorded.