KINETICS AND MECHANISM OF THE REGIOSELECTIVE HOMOGENEOUS HYDROGENATION OF QUINOLINE USING [RH(COD)(PPH3)2]PF6 AS THE CATALYST PRECURSOR

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][H-2]2, where k(cat) = 50 +/- 6 M-2 s-1 at 370 K; the corresponding activation parameters are DELTAH(double dagger) = 9 +/- 1 kcal mol-1, DELTAS(double dagger) = -27.0 +/- 0.3 eu, and DELTAG(double dagger) = 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.