Kinetics and mechanisms of homogeneous catalytic reactions: Part 13. Regioselective reduction of quinoline catalysed by Rh(acac)(CO)[P(^tBu)(CH₂CH=CH₂)₂]

Abstract

The complex Rh(acac)(CO)[P(^tBu)(CH₂CH=CH₂)₂] (1) proved to be an efficient precatalyst for the regioselective hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) under mild reaction conditions (125 °C and 4 atm H₂). A kinetic study of this reaction led to the rate law:

$$ r \, = \{ K_{1} k_{2} /(1 \, + \, K_{1} {\text{H}}_{ 2} )\} [{\text{Rh}}][{\text{H}}_{ 2} ]^{2} $$

which becomes

$$ r \, = \, K_{1} k_{2} [{\text{Rh}}][{\text{H}}_{ 2} ]^{2} $$

at hydrogen pressures below 4 atm. The active catalytic species is the cationic complex {Rh(Q)₂(CO)[P(^tBu)(CH₂CH=CH₂)₂]}⁺ (2). The mechanism involves the partial hydrogenation of one coordinated Q of (2) to yield a complex containing a 1,2-dihydroquinoline (DHQ) ligand, {Rh(DHQ)(Q)(CO)[P(^tBu)(CH₂CH=CH₂)₂]}⁺ (3), followed by hydrogenation of the DHQ ligand to give THQ and a coordinatively unsaturated species {Rh(Q)(CO)[P(^tBu)(CH₂CH=CH₂)₂]}⁺ (4); this reaction is considered to be the rate-determining step. Coordination of a new Q molecule to (4) regenerates the active species (2) and restarts the catalytic cycle.