Kinetics and mechanisms of homogeneous catalytic reactions. Part 16. Regioselective hydrogenation of quinoline catalyzed by dichlorotris(triphenylphosphine)ruthenium(II)

Abstract

The complex RuCl2(PPh3)3 (1) showed to be an efficient and regioselective precatalyst for the hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) at P = 5.0–6.5 at m and T = 140−167 °C. This reaction showed to be first order on catalyst concentration and fractional order on dissolved hydrogen concentration (1.84). However, different to other Ru, Os, Rh and Ir precatalysts, the reaction showed an inverse fractional order on Q concentration (-0.44). The rate law may be written as: r = {KoK1k2/(Ko + [Q]+ KoK1[H2])}[Ru][H2]2. Coordination chemistry and theoretical DFT studies allowed us to propose a detailed catalytic cycle for this reaction. Under the hydrogenation conditions, the ruthenium precursor 1 is totally transformed to RuCl2(κN-Q)2(PPh3)2 (3) and subsequently to RuHCl(κN-Q)2(PPh3)2 (4) by heterolytic addition of H2. Complex 4 reversibly dissociates a Q ligand to generate RuHCl(κN-Q)(PPh3)2, which was proposed as the catalytically active species (CAS) entering to the catalytic cycle. The reversible addition of hydrogen yields a species containing a κN-1,2-dihydroquinoline (κN-DHQ) ligand (possibly through the coordination of hydrogen, migration of hydride to the N atom of Q ligand and posterior DHQ reductive elimination); this species suffers a change of coordination mode from κN- to an olefin-like η2-DHQ. Posteriorly, the addition of a second molecule of H2, considered the rate-determining step, generates a THQ species (probably through migration of hydride to C3 of DHQ ligand followed by the heterolytic addition of hydrogen). The cycle is completed by the substitution of the THQ ligand by a new Q molecule, therefore regenerating the CAS.