Hydrogenation of 2-(2′-Hydroxy-5′-methylphenyl)benzotriazole on Raney Nickel in the Liquid Phase

Liquid-phase hydrogenation of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole on a nickel catalyst proceeds through successive stages to form 4,5,6,7-tetrahydro-2-(2′-hydroxy-5′-methylphenyl)benzotriazole and 1,3,4,5,6,7,8,9-octahydro-2-(2′-hydroxy-5′-methylphenyl)benzotriazole. A mixture of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole and its tetrahydro derivative in a molar ratio of 3 : 2 exhibits stronger photostabilizing action than the individual components. Therefore, clarification of the reasons and the nature of the effect of the composition and the nature of the solvent on the rate and selectivity of hydrogenation of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole has theoretical and practical significance. Organic solvents, a 1 M aqueous solution of sodium hydroxide, and 2-propanol–water mixtures with additions of a base or an acid were used as the liquid phase. The highest initial hydrogenation rates were recorded in alcohol and water–alcohol media. The use of dimethylformamide, toluene, and mixed solvents with an increased content of sodium hydroxide or acetic acid led to a noticeable decrease in the hydrogenation rate. The maximum selectivity toward the tetrahydro derivative of benzotriazole was observed in an alkaline solution with the addition of alcohol and in 2-propanol with the addition of acetic acid. The dependence of the volume of absorbed hydrogen, the yield of 4,5,6,7-tetrahydro-2-(2′-hydroxy-5′-methylphenyl)benzotriazole, and the rate and duration of hydrogenation on hydrogen pressure, temperature, and catalyst amount is also considered. The experimental data indicate that to obtain 4,5,6,7-tetrahydro-2-(2′-hydroxy-5′-methylphenyl)benzotriazole with a yield of at least 95%, hydrogenation of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole on Raney nickel should be carried out at a hydrogen pressure of 0.5–1 MPa and a temperature of 348–358 K, at a catalyst : reactant weight ratio not exceeding 1 : 8 in a 1 M aqueous solution of sodium hydroxide with the addition of 10 vol % 2-propanol, provided that an additional amount of base is introduced sufficient to convert the initial hydroxy-substituted benzotriazole into the phenolate.