Synthetical Applications of Activated Metal Catalysts. XVII. The Hydrogenolysis of Aromatic Ethers by Raney Nickel

Abstract

W7 Raney nickel reacts with diphenyl ether at atmospheric pressure to give benzene and cyclohexanol. Traces of phenol are also formed. Less cyclohexanol and more phenol are produced when this reaction is carried out either at room temperature or with a degassed Raney nickel catalyst. W7 Raney nickel has also been reacted with 4,4'-dimethyl diphenyl ether (to give toluene and trans-4-methyl-cyclohexanol), with 4,4'-dicarboxydiphenyl ether (to give benzoic acid and p-hydroxy-benzoic acid), with anisole (to give benzene and methanol), with dibenzofuran (to give trans-2-phenylcyolohexanol), and with cyclohexyl phenyl ether (to give benzene and cyclohexanol). Dicyclohexyl ether is not affected by W7 Raney nickel. These results are discussed in terms of a mechanism which involves the following steps (i) adsorption of the ether on the catalyst via the oxygen atom; (ii) fission of one of the carbon-oxygen bonds to give the more-stable hydrocarbon anion and phenol (or alcohol for an alkyl aryl ether) ; and (iii) hydrogenation of the phenol to give a cyclohexanol. The effect of variation of the structure of the ether on each of these steps is discussed. In agreement with this mechanism, an excess of W7 Raney nickel reduces at atmospheric pressure phenol and p-cresol (as the anions) to give cyclohexanol and trans-4-methylcyclohexanol respectively, while p-hydroxybenzoic acid and benzoic acid are largely recovered. cis-4-Methylcyclohexanol is not epimerized by W7 Raney nickel.