Kinetics and mechanisms of homogeneous catalytic reactions. Part 3. Regioselective, catalysed [RuH(CO)(NCMe)2(PPh3)2]BF4 reduction of quinoline

Abstract

A kinetic study of the regioselective homogeneous hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) was carried out using the cationic complex [RuH(CO)(NCMe)2(PPh3)2]BF4 (1) as the precatalyst. The experimentally determined rate law wasr = {k2K1/(1+K1[H2])}[Ru0][H2]2, which becomesr = {k2K1[Ru0]−[H2]2 at low hydrogen concentrations (k2K1 = 28.5M−2 s−1 at 398 K). The corresponding activation parameters were found to be ΔH≠ = 42 + 6 kJ mol−1, ΔS≠ = − 115 ± 2JK−1mol−1 and ΔG≠ = 92 ± 8 kJ mol−1. Complex(1) was found to react with Q in CHCl3 under reflux to yield [RuH(CO)(NCMe)(N-Q)(PPh3)2]BF4(2) which was also isolated from the hydrogenation runs. These experimental findings, together with the results ofab initio self-consistent-field molecular orbital calculations on the free organic molecules involved, are consistent with a mechanism involving a rapid and reversible partial hydrogenation of(2) to yield the corresponding dihydroquinoline (DHQ) species [RuH(CO)(NCMe)(DHQ)(PPh3)2]BF4(4), followed by a rate-determining second hydrogenation of DHQ to yield [RuH(CO)(NCMe)(THQ)(PPh3)2]BF4(3).