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

The complex Rh(acac)(CO)[P( t Bu)(CH 2 CH=CH 2 ) 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 H 2 ). A kinetic study of this reaction led to the rate law: r = { K 1 k 2 / ( 1 + K 1 H 2 ) } [ Rh ] [ H 2 ] 2 which becomes r = K 1 k 2 [ Rh ] [ H 2 ] 2 at hydrogen pressures below 4 atm. The active catalytic species is the cationic complex {Rh(Q) 2 (CO)[P( t Bu)(CH 2 CH=CH 2 ) 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( t Bu)(CH 2 CH=CH 2 ) 2 ]} + ( 3 ), followed by hydrogenation of the DHQ ligand to give THQ and a coordinatively unsaturated species {Rh(Q)(CO)[P( t Bu)(CH 2 CH=CH 2 ) 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.