Kinetics and mechanisms of homogeneous catalytic reactions. Part 5. Regioselective reduction of heteroaromatic nitrogen compounds catalysed by [OsH(CO)(NCMe)2(PPh3)2]BF4

The OsH(CO)(NCMe)₂(PPh₃)₂]BF₄ complex (1) is an efficient and regioselective precatalyst for the hydrogenation of the nitrogen-containing ring of quinoline (Q), isoquinoline (iQ), 5,6- and 7,8-benzoquinoline (BQ), and acridine (A) under mild reaction conditions (125 °C and 4 atm H₂). Kinetic studies of the hydrogenation of Q and iQ to give tetrahydroquinoline (THQ) and tetrahydroisoquinoline (THiQ), respectively, lead to the rate law r = K ₁ k ₂/(1 + K ₁H₂])Os]H₂]², which becomes r = K ₁ k ₂Os]H₂]², at low hydrogen concentrations (below 1 atm H₂); the catalytically active species is of the type OsH(CO)(L)( ¹-N)(PPh₃)₂]BF₄ (2a): L = NCMe, N = Q; (2b): L = N = iQ]. The generic mechanisms involve a rapid and partial hydrogenation of the coordinated substrate (N) of complex (2) to yield the corresponding dihydroderivative (DHN) species OsH(CO)(L)( ¹-DHN)(PPh₃)₂]BF₄ (3a): L = NCMe, DHN = DHQ; (3b): L = iQ or THiQ, DHN = DHiQ], followed by the rate-determining second hydrogenation of the DHN ligand, which yield OsH(CO)(L)( ¹-THN)(PPh₃)₂]BF₄ (4a): L = NCMe, THN = THQ; (4b): L = iQ or THiQ, THN = THiQ]; substitution of the THN ligand by a new molecule of the respective substrate regenerates the active species and restarts the catalytic cycle. For the hydrogenation of acridine to give 9,10-dihidroacridine (acridane), the rate law was r = k ₁Os]H₂]; the mechanism involves the hydrogenation of the active species OsH(CO)(NCMe)( ¹-A)(PPh₃)₂]BF₄ (2c) to yield acridane and the unsaturated species OsH(CO)(NCMe)(PPh₃)₂]BF₄ as the rate-determining step.