Kinetics and stereochemistry in the catalytic hydrogenation of acridine
Hydrogenation of acridine (1) using a commercial Pd–Al2O3 catalyst was kinetically and stereochemically studied under variable conditions. A consecutive pathway, (1)→ 9,10-dihydroacridine (2)→ 1,2,3,4,4a,9,9a,10-octahydroacridine (5), which competes with hydrogenative isomerization of (2) to 1,2,3,4,5,6,7,8-octahydroacridine (4)via 1,2,3,4-tetrahydroacridine (3) or (5) was suggested from the observed product distributions over the time course of the reactions and their kinetic simulation. Thus, selectivity for the hydrogenated products is found to be controlled by either kinetics or thermodynamics according to the reaction conditions. Thermodynamic stabilities of the products are discussed based on quantum chemical (MNDO) and molecular mechanics (MM2) calculations to rationalize the reaction scheme. The stereoisomers of (5) and perhydroacridine (6) were identified and quantified by detail analyses using g.c.–i.r. and 13C n.m.r. The stereoselectivity is governed by the preference in adsorption of the intermediates on the catalyst surface at a lower reaction temperature (150 °C); however, a higher temperature (250 °C) produces the thermodynamically stable products through equilibrium control.