The invention relates to a process for the preparation of meta-substituted halogenophenols by selective dehalogenation of more highly halogenated phenols.
The 3-halogenophenols and 3,5-dihalogenophenols obtainable in accordance with the process of the invention can only be synthesised in a highly labor-intensive and cost-intensive manner if the processes of preparation hitherto known are used. For example, the preparation of 3,5-dichlorophenol hitherto customary is carried out via the chlorination of p-nitroaniline to 1-amino-2,6-dichloro-4-nitrobenzene. The amino group is then diazotized and replaced by hydrogen, by reduction. The resulting 3,5-dichloro-nitrobenzene is in turn reduced to 3,5-dichloroaniline, which is then diazotized, and the product is boiled to give 3,5-dichlorophenol (Journal of the Chemical Society, London, 1927, 2217; Ber. dtsch. chem. Ges. 8, 143; Japanese Patent Application No. 4,862,729-Q). Processes which start from the halogenation of a nitrobenzene require catalytic reduction of the 3-halogeno-1-nitrobenzene, thus obtained, to the corresponding aniline, and subsequent diazotisation and boiling to give the 3-halogenophenol (compare Beilsteins Handbook of Organic Chemistry, 4th edition, volume VI, page 185).
The catalyst hydrogenation of halogenated phenols is also known. However, the hydrogenation does not take place selectively and leads either exclusively to phenols or to mixtures of different chlorophenols (Houben-Weyl, Methods of Organic Chemistry, 4th edition, volume V/IV, page 772). Thus, for example, the hydrogenation of pentachlorophenol with Raney nickel as the catalyst leads to complete dehalogenation (Bull. Soc. Chim. Fr. 1963, 2442) and the hydrogenation of polychlorophenols in the gas phase, over an activated alumina catalyst which has been treated with copper-(I) chloride, leads to mixtures of phenols of various chlorine contents (German Published Specification Mo. 1,109,701).