Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 11. Regioselective Hydrogenation of Quinoline Catalyzed by Rhodium Systems Containing 1,2-Bis(diphenylphosphino)ethane

Abstract

The system prepared in situ by addition of two equivalents of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh2Cl2(COE)4 (COE = cyclooctene) showed to be an efficient and regioselective precatalyst for the hydrogenation of quinoline (Q). This reaction showed to be independent of the Q concentration and of fractional order on H2 and catalyst concentrations (1.5 and 0.6, respectively). The fractional order on catalyst concentration indicates that several catalytic species with different activities are present in the reaction medium; however, the cationic species [Rh (dppe)2]+ was the only phosphorous-containing compound detected by 31P{1H} NMR. For the acac salt of this cationic bis(dppe) complex, a kinetic study led to the rate law r = {K1k2/(1 + K1[H2])}[M][H2]2; [M(Q)(κ2-dppe)(κ1-dppe)]+ was proposed as the catalytically active species (CAS) of the cycle. The general mechanism involves a reversible oxidative addition of H2 to generate a dihydrido complex, which transfers 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 yields THQ, regenerates the CAS and restarts the catalytic cycle. The system prepared in situ by addition of two equivalents of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh2Cl2(COE)4 (COE = cyclooctene) showed to be an efficient and regioselective precatalyst for the hydrogenation of quinoline (Q). This reaction proceeds through several catalytic species with different activities; the cationic species [Rh (dppe)2]+ was the only phosphorous compound detected by 31P{1H} NMR. For the acac salt of this cationic bis(dppe) complex, a kinetic study lead to the rate law r = {K1k2/(1 + K1[H2])}[M][H2]2; [M(Q)(κ2-dppe)(κ1-dppe)]+ was proposed as the catalytically active species (CAS) of the cycle. The general mechanism involves a reversible oxidative addition of H2 to generate a dihydrido complex, 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 CAS and restarts the catalytic cycle.