Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 11. Regioselective Hydrogenation of Quinoline Catalyzed
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Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 11. Regioselective Hydrogenation of Quinoline Catalyzed by Rhodium Systems Containing 1,2-Bis(diphenylphosphino)ethane Merlı´n Rosales • Luis Jhonatan Bastidas • Beatriz Gonza´lez • Regina Vallejo • Pablo J. Baricelli
Received: 20 November 2010 / Accepted: 31 May 2011 / Published online: 18 June 2011 Ó Springer Science+Business Media, LLC 2011
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)(j2-dppe) (j1-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. Keywords Rhodium 1,2-Bis(diphenylphosphino)ethane Quinoline Hydrogenation Kinetics M. Rosales (&) L. J. Bastidas B. Gonza´lez R. Vallejo Departamento de Quı´mica, Laboratorio de Quı´mica Inorga´nica, Facultad Experimental de Ciencias, La Universidad del Zulia (L.U.Z.), Apdo. 526, Maracaibo, Venezuela e-mail: [email protected]; [email protected] P. J. Baricelli Centro de Investigaciones Quı´micas, Facultad de Ingenierı´a, Universidad de Carabobo, Valencia, Venezuela
1 Introduction The hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) is an area of considerable interest because it is the first step of the hydrodenitrogenation (HDN) process [1–3]; some homogeneous complexes catalyze this reaction, such as RuHCl(PPh3)3 [4], RhCl(PPh3)3 [5], [Rh(g5-Cp*)(NCMe)3]2? [6], [M(COD)(PPh3)x(NCPh)y]? (M = Rh, Ir; x = 2, y = 0 [7, 8]; x = 1, y = 1 and x = 0, y = 2 [9]), [MH(CO)[NCMe)2(PPh3)2]? (M = Ru, Os) [10, 11], the systems RuCl2(NCMe)4/Tp and M2Cl2 (COE)4/Tp (Tp = tris(pyrazolyl)borate ligands) [12] and more recently the systems prepared in situ by addition of mono-, bi- and tridentate phosphine to M2Cl2(COE)4 (M = Rh, Ir; COE = cyclooctene) [13, 14]; RuH2(g2-H2)2 (PCy3)2 has been the only precatalyst reported that hydrogenates the nonheterocyclic ring of Q [15]. Recently, we reported a kinetic a
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