Mechanism of isomerization of 1,2,3,4-tetrahydroisoquinoline to 5,6,7,8-tetrahydroisoquinoline over raney nickel

Abstract

The isomerization of 1,2,3,4-tetrahydroisoquinoline to 5,6,7,8-tetrahydroisoquinoline proceeded over Raney nickel at ca. 200 °C under hydrogen and nitrogen pressure in a closed reactor. Based on kinetic analyses and quantum chemical calculations we propose that the isomerization proceeds through a series of consecutive steps where dehydrogenation and hydrogenation take place in an alternate manner. The isomerization is initiated by dehydrogenation at the C-1 and N positions of 1,2,3,4-tetrahydroisoquinoline to form 3,4-dihydroisoquinoline which, in turn, is rehydrogenated into 3,4,6,7-tetrahydroisoquinoline and 3,4,5,6,7,8-hexahydroisoquinoline. The hexahydroisoquinoline is finally dehydrogenated into the isomerized product. No extra hydrogen appears to be required for the isomerization. Under a nitrogen stream in an open reactor, the selective formation of isoquinoline occurred.