Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 10. Regioselective Hydrogenation of Quinoline Catalyzed by the Systems M2Cl2(COE)4/2 Triphos [M = Rh, Ir; COE = Cyclooctene; Triphos = 1,1,1-Tris(diphenylphosphinomethyl)Ethane

Abstract

The systems prepared in situ by addition of two equivalents of 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) to M2Cl2(COE)4 (M = Rh, Ir; COE = cyclooctene) showed to be efficient and regioselective precatalysts for the hydrogenation of quinoline. For both systems, kinetic studies lead to the rate laws r = {K 1 k 2/(1 + K 1[H2])}[M][H2]2; it was proposed that the catalytically active species are the cationic unsaturated complexes [M(Q)(triphos)]+. The general mechanism involves a rapid and partial hydrogenation of these species to generate complexes of the type [M(H)2(Q)(triphos)]+ (isolated and characterized for M = Ir), which transfer the hydride ligands to the coordinated Q to yield species containing a 1,2-dihydroquinoline (DHQ) ligand, followed by a second oxidative addition of H2, considered as the rate-determining step of the cycle; hydrogen transfer toward the DHQ ligand yield THQ, regenerates the active species and restarts the catalytic cycle. The systems prepared in situ by addition of two equivalents of 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) to M2Cl2(COE)4 (M = Rh, Ir; COE = cyclooctene) showed to be efficient and regioselective precatalysts for the hydrogenation of quinoline. For both systems, kinetic studies lead to the rate laws r = {K 1 k 2/(1 + K 1[H2])}[M][H2]2; it was proposed that the catalytically active species are the cationic unsaturated complexes [M(Q)(triphos)]+. The general mechanism involves a rapid and partial hydrogenation of these species to generate complexes of the type [M(H)2(Q)(triphos)]+ (isolated and characterized for M = Ir), which transfer the hydride ligands to the coordinated Q to yield species containing a 1,2-dihydroquinoline (DHQ) ligand, followed by a second oxidative addition of H2, considered as the rate-determining step of the cycle; hydrogen transfer toward the DHQ ligand yield THQ, regenerates the active species and restarts the catalytic cycle.