Hydrogenation Pathway of Quinolines over Raney Nickel and Ru/C

Article abstract of DOI:10.1246/bcsj.63.3167

Quinoline, 2-methylquinoline, and 8-methylquinoline were hydrogenated over Raney Nickel (R-Ni) under 10 atm hydrogen pressure at about 200 deg C and over ruthenium on carbon (Ru/C) under 100 atm hydrogen pressure at 150 deg C.All the substrates were commonly hydrogenated into the initial products, 1,2,3,4-tetrahydroquinolines.The initial products were competitively converted over R-Ni to the final products, decahydroquinolines, directly or via 5,6,7,8-tetrahydroquinolines which were mainly formed from the initial products by isomerization.Ru/C promoted exclusively the direct hydrogenation of 1,2,3,4-tetrahydro derivatives to the final products.The hydrogenation and isomerization of 1,2,3,4-tetrahydroquinoline was completely inhibited in the competitive hydrogenation of quinoline and isoquinoline over R-Ni.Such features of these substrates are explained by the strong basicity of 1,2,3,4-tetrahydroisoquinoline.Roles of 1,2,3,4-tetrahydroisoquinoline are much moderate on Ru/C, where the ?-coordination may be important.The effects of methyl substituent and different reactivities of quinoline and isoquinoline are discussed in terms of the steric hindrance on adsorption, heats of hydrogenation, basicities, and electronic properties of the related compound, which are calculated according to the MNDO-PM3 method.