Acylation of 1,1-dichloroethylene by the arylalkanoic acid chlorides: synthesis of 4-chloro-2-naphthols

Abstract

The acylation reactions of 1,1-dichloroethylene by the arylalkanoic acid chlorides were studied and a convenient approach to the synthesis of 4-chloro-2-naphthols was demonstrated. As starting materials for naphthols’ preparation, the available arylacetic acids, which are readily obtained by hydrolysis of the corresponding arylacetonitriles, were selected. It was found that the acylation reaction of 1,1-dichloroethylene by the arylacetic acid chlorides with acceptor substituents was completed by the formation of classical products of 4,4,4-trichloro-1-arylbutan-2-ones. The last ones had a tendency to readily cleave HCl and to convert into the corresponding 4,4-dichlorobut-3-ene-2-one derivatives. It should be noted that in the case of non-compliance with the reaction temperature conditions, the 4,4-dichlorobut-3-ene-2-one derivatives were found to be side products of the reaction. In contrast to the acceptor substituents, the corresponding 4-chloro-2-naphthols in high yields were formed in the reaction of unsubstituted phenylacetic or tolylacetic acid chlorides with 1,1-dichloroethylene. Obviously, in the case of donor substituents, the formed carbocation easily attacked the aromatic ring with the formation of an intermediate which was aromatized by the elimination of HCl. The reaction was exothermic. Minor resinous impurities were removed by recrystallization of naphthols from hexane. The lengthening of the carbon chain did not lead to the formation of the desired cyclic β -chloroketones. In particular, in the case of 3-phenylpropanoyl chloride, the main reaction product was 2,3-dihydroinden-1-one, which was a consequence of the intramolecular cyclization of the acylium ion. The product of 1,1-dichloroethylene acylation of 1,1-dichloro-5-phenylpent-1-en-3-one was found in a small amount. Attempts to use phenoxyacetic acid chlorides and 4-(4-methoxyphenoxy)butanoic acid chlorides as reagents led to the formation of the resinous products. In summary, a convenient way of synthesis of 4-chloro-2-naphthol derivatives from arylacetic acids was demonstrated and the general perspectives and limitations of the use of arylalkanoic acid chlorides in Friedel–Crafts acylation of 1,1-dichloroethylene for intramolecular cyclization to obtain cyclic β -chloroketones was discussed. Key words: arylacetic acids, 1,1-dichloroethylene, β -naphthol, intramolecular cyclization, Friedel–Crafts reaction.