Quinoline transfer hydrogenation by a rhodium bipyridine catalyst

• Published in: Inorganica Chimica Acta Vol. 359; no. 9; pp. 2650 - 2657
• Main Authors: Frediani, Piero, Rosi, Luca, Cetarini, Lorenza, Frediani, Marco
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2006
• Subjects: Alkene; Ketone; Pyridine; Quinoline; Rhodium; Transfer hydrogenation; Quinoline; Transfer hydrogenation; Pyridine; Alkene; Rhodium; Ketone
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2005.10.044

The pre-activated rhodium(III) complex cis-[Rh(bipy) 2Cl 2]Cl is catalytically active in the transfer hydrogenation of organic substrates containing a C C, C O or C N group giving the corresponding saturated compounds. High conversion is usually obtained depending on the temperature, catalyst or substrate concentration. A catalytic cycle is suggested. The catalytic activity of the rhodium complex cis-[Rh(bipy) 2Cl 2]Cl · 2H 2O in the transfer hydrogenation of different unsaturated substrates is reported. This complex, if pre-activated, is very active in the transfer hydrogenation of ketones (i.e., cyclohexanone is reduced with a 38.1% conversion at 283 K and 100% at 313 K) while in the case of hex-1-ene, a 36.8% conversion was reached at 293 K. A cyclic olefin (cyclohexene) was also reduced with a lower, but still significant, conversion. It is interesting to note the catalytic activity of this complex in the transfer hydrogenation of a C N double bond belonging to imides or nitrogen-containing heterocycles. For instance, N-benzylidenaniline was hydrogenated to N-benzylaniline at 303 K with a conversion of 27.3%. Increasing the temperature to 353 K, the conversion rised to 91.8%. A nitrogen containing heterocycle, quinoline, was also reduced by transfer hydrogenation at 353 K with a 11.7% conversion giving 1,2,3,4-tetrahydroquinoline (selectivity of 96.6%). The conversion rised up to 54.2% with a still high selectivity (84.5%) when the temperature was 383 K. Almost the same activity was shown in the reduction of pyridine to pyperidine (conversion, 51.1% at 383 K), while 2-methylpyridine was hydrogenated with a 24.7% conversion.