The mechanism of the regioselective reduction of quinoline catalyzed by [RuH(CO)(NCMe)2(PR3)2]BF4: a theoretical investigation1Dedicated to Professor Yves G. Smeyers Guillemin on the occasion of his 65th birthday.1

The semi-empirical SCF-CNDO/2 method was used to study the regioselective hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) catalyzed by [RuH(CO)(NCMe)2(PH3)2]BF4 (1b). We found that the active species of the catalytic cycle is the isomer of the complex [RuH(CO)(η1(N)–Q)(NCMe)(PH3)2]BF4 (3b) where the hydride is not only cis to the Q ligand, but is also close to the alpha C atom of Q (C(2)). The results showed that hydride transfer to C(2) is favored, giving a monohydroquinolinide (MHQ) complex [Ru(CO)(η1(N)–MHQ)(NCMe)–(PH3)2]BF4 (4b), which is coordinatively unsaturated. The interaction of 4b with hydrogen produced the hydrido-dihydroquinoline species [RuH(CO)(η1(N)–DHQ)(NCMe)(PH3)2]BF4 (5b). A change in coordination from η1(N) to η1(Cα,Cβ) led to the olefin-like complex [RuH(CO)(η2–DHQ)(NCMe)(PH3)2]BF4 (6b). These results are in good agreement with the available experimental information and allowed us to propose a detailed mechanism for the reduction of Q catalyzed by [RuH(CO)(NCMe)2(PPh3)2]BF4.