Kinetics and mechanism of the regioselective homogeneous hydrogénation of quinoline using [Rh(COD) (PPh3)2]PF6 as the catalyst precursor

Article abstract of DOI:10.1021/om00035a013

The kinetics and mechanism of the regiospecific homogeneous hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) using [Rh(COD)(PPh₃)₂]PF₆ (1) as the catalyst precursor in toluene solution under mild reaction conditions have been studied. The experimentally determined rate law is r_i = k_cat[Rh][H₂]², where k_cat = 50 ± 6 M^-2 s^-1 at 370 K; the corresponding activation parameters are ΔH? = 9 ± 1 kcal mol^-1, ΔS? = -27.0 ± 0.3 eu, and ΔG? = 19.1 ± 0.3 kcal mol^-1. Complex 1 was shown to react rapidly with Q to yield [Rh(COD)(PPh₃)(Q)]PF₆ (2) at room temperature and [Rh(COD)Q₂]PF₆ (4) in boiling toluene; complex 4 was also isolated almost quantitatively from the catalytic runs. Reaction of 1 with H₂ followed by interaction with Q at room temperature produces [Rh(PPh₃)₂Q₂]PF₆ (3). These findings indicate that 4 is the catalytically active species and are consistent with a mechanism involving a rapid and reversible hydrogenation of one coordinated Q in 4 to dihydroquinoline (DHQ), followed by a rate-determining reduction of this intermediate to yield THQ. A catalytic cycle accounting for these results is postulated.