Cyclometalated Dicarbonyl Ruthenium Catalysts for Transfer Hydrogenation and Hydrogenation of Carbonyl Compounds

Abstract

The dicarbonyl complex RuCl 2 (L) 2 (CO) 2 (1) was easily prepared by reaction of ruthenium chloride hydrate with formic acid and L (L = (2,6-Me 2 C 6 H 3 )PPh 2 ) in ethanol at reflux, via the [RuCl 2 (CO) 2 ] n intermediate. Alternatively, 1 was obtained from [RuCl 2 (CO) 3 ] 2 and L by CO elimination. Reaction of 1 with NEt 3 in toluene at reflux afforded the cyclometalated derivative RuCl{(2-CH 2 -6-MeC 6 H 3 )PPh 2 }-(L)(CO) 2 (2). A simple one-pot synthesis of 2 was achieved by treatment of RuCl 3 hydrate with formic acid, L, and NEt 3 . The cyclometalated dicarbonyl complexes [Ru{(2-CH 2 -6-MeC 6 H 3 )PPh 2 }(NN)(CO) 2 ]Cl (NN = ethylenediamine, 3; 2-(aminomethyl)pyridine, 4; (R,R)-1,2-diphenylethane-1,2diamine, 5) were isolated by reaction of 2 with the corresponding dinitrogen ligand in methanol at reflux. Complexes 1−4 catalyze the transfer hydrogenation (TH) of acetophenone in 2-propanol at reflux (S/C = 1000 and TOF up to 30 000 h −1 )with alkali base (1−5 mol %), whereas 5 leads to (S)-1-phenylethanol with 68% ee. The derivatives 1−5 catalyze the hydrogenation (HY) of several ketones (H 2 , 30 bar) at 70 °C in MeOH and EtOH with KOtBu (2 mol %) (S/C and TOF up to 25 000 and 14 000 h −1 ). Addition of NN ligands to 1 and 2 in situ increases both the TH and HY activity, with ampy displaying the better performance. Heating of the cationic complex 3 in solid state and in solution leads to decarbonylation, affording the neutral monocarbonyl compound RuCl{(2-CH 2 -6-MeC 6 H 3 )PPh 2 }(en)(CO) (6) which was found active in the ketone HY.

Transfer Hydrogenation from 2-propanol to Acetophenone Catalyzed by [RuCl2(η6-arene)P] (P = monophosphine) and [Rh(PP)2]X (PP = diphosphine, X = Cl−, BF4−) Complexes

“…At present, many highly efficient Ru-phosphorus systems for the TH reduction of ketones and aldehydes in the presence of 2-propanol and other H-donors (i.e., HCOOH/NEt 3 mixtures, HCOONa, HCOONH 4 ) have been reported. Within these catalytic systems, stand out the precursors developed by Noyori et al containing the BINAP motif [39][40][41] as well as Ru complexes bearing cyclometallated phosphines [42,43], or other ligands as fluoroacetate [44], bi-and tridentate nitrogen ligands in combination with mono- [45] or diphosphines, developed by Baratta et al [46][47][48][49]. The association of chiral phosphorus ligands with (chiral) primary or secondary amine functionalities (bifunctional catalysts) often results in elevated enantioselectivities [50].…”

Hydrogen Transfer Reactions of Carbonyls, Alkynes, and Alkenes with Noble Metals in the Presence of Alcohols/Ethers and Amines as Hydrogen Donors

“…Ruthenium complexes with CO ligand(s) were reported as active catalysts in the transfer hydrogenation reaction of ketones [74,75]. Cyclometalated dicarbonyl complexes such as 34, 35, and 35a-35c (Scheme 16) catalyzed the transfer hydrogenation of acetophenone in 2-propanol as a hydrogen source in the presence of alkali base (1-5 mol %) at reflux conditions (TOF up to 30000 hr −1 ) [74].…”

“…Ruthenium complexes with CO ligand(s) were reported as active catalysts in the transfer hydrogenation reaction of ketones [74,75]. Cyclometalated dicarbonyl complexes such as 34, 35, and 35a-35c (Scheme 16) catalyzed the transfer hydrogenation of acetophenone in 2-propanol as a hydrogen source in the presence of alkali base (1-5 mol %) at reflux conditions (TOF up to 30,000 h −1 ) [74]. Mechanistic considerations showed that the [Ru-H] complex most probably forms from the corresponding [Ru-O i Pr] complex via β-hydrogen elimination (inner-sphere mechanism), while complexes having NH 2 functionality in the [Ru-H] complex form from a 16-electron Ru-amide [74,76] or from a Ru-amine/alkoxide species (outer-sphere mechanism) [74,77,78].…”

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

“…Baratta et al recently extended the library of ruthenacyclic complexes bearing the anionic (κ 2 - C , P )-[2-CH 2 -6-MeC 6 H 3 PPh 2 ] − ligand by synthesizing the series of dicarbonyl derivatives 3 – 5 depicted in Scheme 2 [ 13 ]. The amine free complex 3 (0.1 mol.%) displayed poor activity in the TH of acetophenone (0.1 M) in the presence of NaO i Pr (2 mol%) as a base in 2-propanol at reflux, affording only 48% conversion into 1-phenylethanol after 8 h reaction.…”

“…This result, together with the fact that both 1 and 2 are twice as active as 5 and 4 , respectively, suggested a possible thermal dissociation of one CO ligand as the initial step of the reaction mechanism. This thermal CO displacement in the presence of 2-propanol and an alkali base would lead to the catalytically active Ru monohydride species that would then reduce acetophenone with the help of “ a hydrogen bonding network promoted by the NH 2 function ” [ 13 ].…”