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

The complexes trans,cis -RuCl 2 (PPh 3 ) 2 (ampy) ( 1 ) and trans -RuCl 2 [Ph 2 P(CH 2 ) 4 PPh 2 ](ampy) ( 2 ) have been prepared in high yield by reaction of RuCl 2 (PPh 3 ) 3 and RuCl 2 (PPh 3 )[Ph 2 P(CH 2 ) 4 PPh 2 ] with 2-(aminomethyl)pyridine (ampy) at room temperature by PPh 3 displacement. Heating compound 1 in refluxing toluene leads to the isomer cis,cis -RuCl 2 (PPh 3 ) 2 (ampy) ( 3 ), which has been proven to be a good precursor for the preparation of the complexes cis -RuCl 2 (PP)(ampy) [PP = ( S, S )-Chiraphos, 4; Ph 2 P(CH 2 ) 3 PPh 2, 5; ( S, S )-Skewphos, 6; Ph 2 P(CH 2 ) 4 PPh 2, 7; ( R, R )-Diop, 8 ] by displacement of two PPh 3 with the appropriate diphosphine. The derivatives cis -RuCl 2 (PP)(ampy) [PP = ( R, S )-Josiphos, 9; ( R, S )- t Bu-Josiphos, 10 ] have been synthesized from RuCl 2 (PPh 3 ) 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(PPh 3 ) 2 (ampy) ( 11 ) has been prepared from RuHCl(PPh 3 ) 3 and ampy in heptane by PPh 3 substitution. Compound 11 reacts with sodium isopropoxide in toluene, affording the dihydride derivative cis, trans -Ru(H) 2 (PPh 3 ) 2 (ampy) ( 12 ) via the alkoxide route. The intermediate species cis, cis -Ru(H) 2 (PPh 3 ) 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 -RuCl 2 (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 > 10 5 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.