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Ruthenium Catalyst


Light-mediated catalysis enables unique reaction pathways that were previously unattainable under thermal control and forms various unconventional bonds in synthetic organic chemistry. Among them, the most common mechanisms by which photocatalysts are capable of converting light to chemical energy with simultaneous selective molecular activation, including energy transfer, organometallic excitation, light-induced atomic transfer, and light oxidation catalysis. Ruthenium complexes with polypyridyl ligands are widely used in the field of organic synthesis as visible light photocatalysts[1]. The classical ruthenium catalyst Ru(bpy)32+ can participate in photoredox reactions under extremely mild conditions, thereby generating free addition, deleterious side reactions hardly occur. The photocatalytic efficiency can be improved by structural modification of such complexes.

Chemical structures of common Ruthenium photocatalystsFig. 1 Chemical structures of common Ruthenium photocatalysts[1].


Ruthenium catalysts are widely used in organic reactions such as reduction of electron-deficient alkenes, reduction of nitrogen functional groups and oxidation of benzyl halides.

  • Reduction of electron-deficient alkenes

Ru(bpy)32+-mediated reduction of electron-deficient alkenes was described in 1981. 1-benzyl-1,4-dihydronicotinamide (BNAH) with the redox-active 1,4-dihydropyridine unit was used as the terminal reducing agent which formed a catalyst system with catalytic quantities of Ru(bpy)32+. Dimethyl maleate as substrate undergoes free radical cleavage under visible light irradiation, and the catalytic system composed of Ru(bpy)32+ catalyzed the reduction of electron-deficient alkenes to get the saturated product dimethyl succinate[2].

Photoredox reduction of electron-deficient olefinsFig. 2 Photoredox reduction of electron-deficient olefins[2].

  • Reduction of nitrogen functional groups

Ruthenium catalysts has been used for the reduction of many nitrogen-containing functional groups. For example, Ru(bpy)32+ or the related photocatalyst Ru(bpy)2(MeCN)2(PF6)2 could reduce nitrobenzenes to anilines. And the catalytic system consisting of Ru(bpy)32+, Hü nig's base and Hantzsch ester are capable of reducing azides to amines. Furthermore, Ru(bpz)32+, a tris(bipyrazyl) analogue of Ru(bpy)32+, as a powerful photoredox catalyst, can be suited for oxidatively induced photoredox transformations. N-phenyl-N-benzoylhydrazine can be reduced to N-phenylbenzamide under the catalysis of photoredox catalyst Ru(bpz)32+. In addition, N-methyl-N-phenylhydrazine can also be reduced to N-methylaniline via Ru(bpz)32+ catalysis[3].

Reduction of nitrogen functional groupsFig. 3 Reduction of nitrogen functional groups[3].

  • Oxidation of benzyl halides

Pyridine derivatives as photocatalysts are able to participate in photoredox oxidation, activating α-haloester substrates to obtain α-ketoesters at room temperature. In this reaction, molecular oxygen serves as a terminal oxidant as well as the source for introducing oxygen atoms into the product. In the presence of Ru(bpy)32+ as catalyst and 4-methoxypyridine as cocatalyst, ethyl α-bromophenylacetate can be converted to ethyl benzoylformate in good yield under air. Overall, photocatalysts and organic catalysts work together to efficiently oxidize α-aryl halogen derivatives to α-aryl carbonyl compounds, which is a novel radical reaction catalyzed by pyridine derivatives[4].

Aerobic oxidation of benzylic halidesFig. 4 Aerobic oxidation of benzylic halides[4]

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  • Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis. Chem Rev. 2013, 113: 5322
  • Pac, C.; et al. Tris(2,2'-bipyridine)ruthenium(2+)-mediated photoreduction of olefins with 1-benzyl-1,4-dihydronicotinamide: a mechanistic probe for electron-transfer reactions of NAD(P)H-model compounds. J Am Chem Soc. 1981, 103: 6495.
  • Zhu, M.; Zheng, N. Photoinduced cleavage of N-N bonds by visible light. Synthesis 2011, 14: 2223.
  • Su, Y.; Zhang, L.; Jiao, N. Utilization of natural sunlight and air in the aerobic oxidation of benzyl halides. Org. Lett. 2011, 13: 2168.

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