Efficient Transfer Hydrogenation of Ketones Catalyzed by the Bis(isocyanide)−Ruthenium(II) Complexes trans,cis,cis-[RuX₂(CNR)₂(dppf)] (X = Cl, Br; dppf = 1,1‘-Bis(diphenylphosphino)ferrocene):  Isolation of Active Mono- and Dihydride Intermediates

Abstract

The bis(isocyanide)−ruthenium(II) complexes trans,cis,cis-[RuX2(CNR)2(dppf)] (X = Cl, R = CH2Ph (2a), Cy (2b), tBu (2c), 2,6-C6H3Me2 (2d), (S)-(−)-C(H)MePh (2e); X = Br, R = CH2Ph (3a), Cy (3b), tBu (3c), 2,6-C6H3Me2 (3d), (S)-(−)-C(H)MePh (3e)) have been prepared by reaction of the bis(allyl)−ruthenium(II) derivative [Ru(η3-2-C3H4Me)2(dppf)] (1) with the appropriate isocyanide ligand, in CH2Cl2 at room temperature and in the presence of the corresponding hydrogen halide HX. The structure of the compound trans,cis,cis-[RuCl2(CN-2,6-C6H3Me2)2(dppf)] (2d) has been confirmed by X-ray crystallography. The catalytic activity of complexes 2 a−e and 3a−e in the transfer hydrogenation of acetophenone by propan-2-ol has been studied, and the most active complex, trans,cis,cis-[RuCl2(CNCH2Ph)2(dppf)] (2a), has been also tested as a catalyst in the transfer hydrogenation of a large variety of ketones. The hydride derivatives cis,cis-[RuHCl(CN-2,6-C6H3Me2)2(dppf)] (4) and cis,cis,cis-[RuH2(CN-2,6-C6H3Me2)2(dppf)] (5) have been isolated and characterized. Although both hydride complexes catalyze the transfer hydrogenation of acetophenone in the absence of base, the reactions proceed ca. 5 times faster with 5 than with 4, pointing out that the real active species are dihydride−ruthenium complexes.

Homogeneous Hydrogenation of Aldehydes, Ketones, Imines and Carboxylic Acid Derivatives: Chemoselectivity and Catalytic Activity

“…Highly efficient transfer hydrogenation of ketones can be achieved by the use of the transfer hydrogenation catalyst trans,cis,cis-[RuX 2 (CNR) 2 (dppf)] (X = Cl or Br; R = CH 2 Ph, cy, t-Bu, 2,6-C 6 H 3 Me 2 ) [59]. These are the first examples of isocyanide-ruthenium species being used for the transfer hydrogenation of ketones.…”

Dicarbonylruthenium(II) complexes of diphosphine ligands and their catalytic activity

“…Despite the large number of Ru(II) catalysts reported for this particular transformations [27][28][29][30], the use of ruthenium(II) carbonyl species (which are generally considered as sluggish catalysts for hydrogenation reaction [31]) are quite scanty [29]. However, aldehydes are difficult to reduce by catalysts commonly used for transfer hydrogenation, and controlling the chemoselectivity of the reaction presents a further challenge [32,33].…”

Ruthenium cationic species for transfer hydrogenation of aldehydes: Synthesis and catalytic properties of [(PPh3)2Ru(CH3CN)3Cl]+[A]− {A=BPh4 or ClO4} and structure of [(PPh3)2Ru(CH3CN)3Cl]+[BPh4]−

“…However, Cadierno et al [26] have recently isolated cis,cis-[RuHCl-(CN-2,6-C 6 H 3 Me 2 ) 2 (dppf)] and cis,cis,cis-[RuH 2 (CN-2, 6-C 6 H 3 Me 2 ) 2 (dppf)] [dppf = 1,1 0 -Bis(diphenylphosphino)-ferrocene] and found that, both the monohydrido and dihydridoruthenium compounds are active in the reduction of acetophenone by isopropanol, however, the dihydrido species was found to be more active. Recently, it has been reported that, cationic complexes of the type [CpRu(P-N)(CH 3 CN)] + , where (P-N) is bidentate aminophosphene ligands, are excellent pre-catalysts for transfer hydrogenation for ketones and [30].…”

“…Transfer hydrogenation of carbonyl compounds using 2-propanol as reducing agent as well as solvent has attracted a lot of attention over last few years due to the comparatively benign nature of the reagents [23][24][25][26][27][28]. A number of transition metal catalysts have been used for this transformation.…”

“…A number of transition metal catalysts have been used for this transformation. The main focus has been on ruthenium(II) complexes [23][24][25][26][27][28][29]. However, in most of the cases, the reactions are effective only in the cases of ketones.…”