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

Abstract

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.

Ligand Design for Catalytic Asymmetric Reduction

“…Precatalysts C1-C3 use the ''NH effect'' described above. [111][112][113] A turnover frequency, defined as moles of product per mol of catalyst per hour, of 30,000 h À1 is achieved by using of C2 and an alkaline base in 2-propanol. 112 A Rh complex C3 is an isolobal to the corresponding arene-Ru complex (see Figure 1.23).…”

“…[111][112][113] A turnover frequency, defined as moles of product per mol of catalyst per hour, of 30,000 h À1 is achieved by using of C2 and an alkaline base in 2-propanol. 112 A Rh complex C3 is an isolobal to the corresponding arene-Ru complex (see Figure 1.23). 113 The Ru complexes C4 114 and C5 115 without NH group in ligand catalyze the reaction by different mechanisms.…”

Development of a Stepwise Reductive Deoxygenation Process by Ru‐Catalysed Homogeneous Ketone Reduction and Pd‐Catalysed Hydrogenolysis in the Presence of Cu Salts

“…[13] Baratta's catalyst 17 [14] provided up to 71 % conversion to 2 at S/C 1000:1 in iPrOH/tBuOK (entry 4) and 36 % conversion at S/C 2,000:1 (entry 5). Catalyst 18 [15] provided increased activity with full conversion at S/C 2000:1 in MeOH with 5 % tBuOK (60 8C and 27 bar H 2 ).…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…These have been demonstrated to be powerful catalysts for the enantioselective reduction of ketones and imines. Recently, Baratta and co-authors [11] found that 2-(aminomethyl) pyridine (ampy) accelerated the transfer hydrogenation (TH) of ketones [12][13][14] in Ru(II) and Os(II) complex catalysts. Very recently, Morris group [15,16] reported that chiral NNPP ligands and their iron(II) complexes were effective in the ATH of ketones under mild conditions, representing a rare case of using inexpensive iron as the active metal.…”