Catalytic activity of dihydride ruthenium complexes in the hydrogenation of nitrogen containing heterocycles |
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| Authors: | Piero Frediani Valentina Pistolesi Marco Frediani Luca Rosi |
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| Institution: | Department of Organic Chemistry, University of Florence, via della Lastruccia, 13 - 50019 Sesto Fiorentino (Florence), Italy |
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| Abstract: | The catalytic activity of the dihydride ruthenium complexes, RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2 and RuH2(PPh3)4, in the hydrogenation of nitrogen containing heterocycles has been tested by analyzing the influence of reaction parameters such as temperature, hydrogen pressure, catalyst concentration, on the rate and regioselectivity of the reaction.RuH2(PPh3)4 shows a better catalytic activity with an 86.7% conversion of quinoline after 24 h at 100 °C under a hydrogen pressure of 25 bar, while RuH2(CO)2(PPh3)2 and RuH2(CO)2(PnBu3)2 in the same conditions give a conversion of 37.1% and 35.6%, respectively. These results are confirmed by the reaction rate of the hydrogenation of quinoline, since the Kc in the presence of RuH2(PPh3)4 (1.46 × 10−5 s−1) is higher than others (6.37 × 10−6 s−1 for RuH2(CO)2(PPh3)2 and 6.36 × 10−6 s−1 for RuH2(CO)2(PnBu3)2).Noteworthy is the selectivity of these catalytic systems in the hydrogenation of quinoline: in all tests the three catalysts lead to 1,2,3,4-tetrahydroquinoline as the major product, furthermore this compound is the only formed in the presence of RuH2(CO)2(PPh3)2. The selectivity is affected by the presence of an acid (CH3COOH) or a base (NnBu3) in the reaction media.The complex RuH2(PPh3)4 is catalytically active, even if in a minor extent, in the hydrogenation of isoquinoline, pyridine and 2-methylpyridine.The basicity of the substrate and steric hindrance around the nitrogen atom show a great influence on the conversion.The results obtained suggest that the catalytic system activates a heterocyclic ring through the coordination of the heteroatom to the metal centre of the complexes. |
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| Keywords: | Homogeneous catalysis Ruthenium hydride Hydrogenation Quinoline Pyridine |
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