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.

Graphical 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 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.