Simple heterogeneous Ru/CeO 2 catalysts as well as Ru/ZrO 2 catalysts were found to be quite effective for the selective direct synthesis of indole via intramolecular dehydrogenative N-heterocyclization of 2-(2aminophenyl)ethanol, while catalysts supported on SiO 2 , Al 2 O 3 , TiO 2 , and MgO were less effective. Ru/CeO 2 catalysts that were calcined at a relatively low temperature, 200 o C, showed excellent activity and gave indole in a yield over 99% by the reaction at 140 o C for 24 h (Ru catalyst 2.5 mol%). Spectroscopic studies of the Ru/CeO 2 catalysts indicated the formation of Ru IV =O species on the surface, which is considered to be transformed to the catalytically-active species at the initial stage of the reaction. Hot filtration tests and an ICP-AES analysis indicated that these Ru/CeO 2 catalysts act heterogeneously and that the leaching of ruthenium species into the solution is negligible. These catalysts could be recycled without a significant loss of activity, which suggests that the present oxide-supported catalysts are promising alternatives to conventional homogeneous catalysts. 65 based on the spectroscopic characterization of the catalysts.
Experimental Section2.1. Material Tris(acetylacetonato)ruthenium(III) (Aldrich), tris(acetylacetonato)iron(III) (Dojin), tris(acetylacetonato)rhodium(III) 70 (Aldrich), tris(acetylacetonato)iridium(III) (Wako), palladium(II) acetate (Nacalai Tesque), aqueous ammonia solution (28%, Nacalai Tesque), potassium hydroxide (Nacalai Tesque), tetrahydrofuran (dehydrated, stabilizer-free, Wako), 2-(2-H 2 consumption (a.u.) Temperature ( o C) Ru/CeO 2 -as-imp. Ru/CeO 2 -150 Ru/CeO 2 -200 Ru/CeO 2 -300 Ru/CeO 2 -400 40 60 80 100 120 140
A solid ceria-supported ruthenium catalyst (Ru/CeO 2 ) as well as a zirconia-supported ruthenium catalyst (Ru/ZrO 2 ) showed high activities toward the intermolecular coupling of alkynes with acrylates to give the corresponding 1,3-dienes in high yields. The reaction proceeded in the presence of sodium formate and both internal or terminal alkynes and norbornene could be applied for the catalytic systems. The solid ruthenium catalyst could be recycled for several times without significant loss of activity. The FT-IR and X-ray absorption fine structure (XAFS) studies of ceria-supported ruthenium catalysts showed that a distorted ruthenium(IV)-oxo species on ceria was transformed during the initial stage of the reaction into a low-valent ruthenium species that was effective for the coupling reactions.
Ceria‐supported ruthenium (Ru/CeO2) was found to be an effective catalyst for linear‐selective dimerization of styrenes. The dimerization of styrene in the presence of Ru/CeO2 together with ethanol and formalin in mesitylene at 150 °C gave (E)‐1,4‐diaryl‐1‐butene in 63 % yield with high linear selectivity (95 %). The presence of ethanol and/or formaldehyde is indispensable. Whereas a zirconia‐supported ruthenium showed moderate activity, catalysts supported on other metal oxides were not effective at all. The hot filtration tests indicated that the Ru/CeO2 catalyst acts heterogeneously. The leaching of ruthenium species into the solution was negligible according to analysis by using inductively coupled plasma atomic emission spectroscopy. The regioselectivity can be switched through the choice of a suitable solvent: the reactions in N,N‐dimethylacetamide or N‐methyl‐2‐pyrrolidone gave the corresponding branched dimers in good yields with complete regioselectivity.
on Ceria as a Key Precursor of Active Species. -It is found that treatment of distorted Ru(IV)-oxo species on ceria or zirconia with HCOONa leads to low-valent Ru compounds which efficiently promote the coupling of acrylates with some alkynes as well as norbornene. The catalyst can be easily removed and reused several times without significant loss of activity. -(MIURA, H.; SHIMURA, S.; HOSOKAWA, S.; YAMAZOE, S.; WADA*, K.; INOUE, M.; Adv. Synth. Catal. 353 (2011) 14-15, 2837-2843, http://dx.doi.org/10.1002/adsc.201100415 ; Dep. Energy Hydrocarbon Chem., Grad. Sch. Eng., Kyoto Univ., Nishikyo, Kyoto 615, Japan; Eng.) -Jannicke 13-057
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