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2-(Aminomethyl)pyridine−Phosphine Ruthenium(II) Complexes:  Novel Highly Active Transfer Hydrogenation Catalysts

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posted on 28.03.2005, 00:00 by Walter Baratta, Eberhardt Herdtweck, Katia Siega, Micaela Toniutti, Pierluigi Rigo
The complexes trans,cis-RuCl2(PPh3)2(ampy) (1) and trans-RuCl2[Ph2P(CH2)4PPh2](ampy) (2) have been prepared in high yield by reaction of RuCl2(PPh3)3 and RuCl2(PPh3)[Ph2P(CH2)4PPh2] with 2-(aminomethyl)pyridine (ampy) at room temperature by PPh3 displacement. Heating compound 1 in refluxing toluene leads to the isomer cis,cis-RuCl2(PPh3)2(ampy) (3), which has been proven to be a good precursor for the preparation of the complexes cis-RuCl2(PP)(ampy) [PP = (S,S)-Chiraphos, 4; Ph2P(CH2)3PPh2, 5; (S,S)-Skewphos, 6; Ph2P(CH2)4PPh2, 7; (R,R)-Diop, 8] by displacement of two PPh3 with the appropriate diphosphine. The derivatives cis-RuCl2(PP)(ampy) [PP = (R,S)-Josiphos, 9; (R,S)-tBu-Josiphos, 10] have been synthesized from RuCl2(PPh3)3 and PP followed by addition of ampy. The chiral complexes 4, 6, 8, 9, and 10 are formed stereoselectively, as inferred by NMR data in solution. For the derivatives 7 and 9 the molecular structures have been determined by X-ray measurements. The monohydride complex trans,cis-RuHCl(PPh3)2(ampy) (11) has been prepared from RuHCl(PPh3)3 and ampy in heptane by PPh3 substitution. Compound 11 reacts with sodium isopropoxide in toluene, affording the dihydride derivative cis,trans-Ru(H)2(PPh3)2(ampy) (12) via the alkoxide route. The intermediate species cis,cis-Ru(H)2(PPh3)2(ampy) (A) has been also characterized by NMR in solution. All these complexes have been found to be highly efficient transfer hydrogenation catalysts. With the complexes cis-RuCl2(PP)(ampy) a large number of ketones (dialkyl, diaryl, and alkyl-aryl) can be quantitatively reduced to alcohols in 2-propanol and in the presence of NaOH (ketone/Ru/NaOH = 2000/1/40) with remarkably high TOF values (up to 400 000 h-1 at 50% conversion). The derivatives containing chiral diphosphines afforded rapid (TOF > 105 h-1) and enantioselective (ee up to 94%) reduction of methyl-aryl ketones using low loading of catalysts (0.05−0.01 mol %). In the absence of base the dihydride compound 12 catalyzes the transfer hydrogenation of acetophenone.