Chemoselective Transfer Hydrogenation of Aldehydes with HCOONH₄ Catalyzed by RuCl(CNN^Ph)(PP) Pincer Complexes

Abstract

Aldehydes were chemoselectivelyr educed to primary alcohols by using HCOONH 4 as the hydrogen donor throught ransfer hydrogenation catalyzed by benzo [h]quinoline pincer complexes RuCl(CNN Ph )(PP) at substrate to catalystm olar ratios of 2000 to 20 000. This practical reaction performed with aldehydes of commercial-grade purity in aw ater/tolueneb iphasic system afforded alcohols without the formation of condensation or amination side products.The search of well-designed and productive catalysts for the selectiveh ydrogenation [1] and transfer hydrogenation (TH) [2] of aldehydes to primary alcohols is af undamentalr eactiono f broad application in industry. [3] This catalytic route promoted by transition metals results in al ower environmental impact and an easier workup with respect to the classical approach entailing NaBH 4 ,L iAlH 4 ,b oranes, and Al alkoxides as reducing agents. [4] Among the transition metalsu sed in catalytic homogeneousr eductions,r uthenium,w hichi sc heaper than rhodium and iridium,h as played ac rucial role, and this has led to an umber of efficient catalysts for aldehyde reduction by using H 2 under pressure [5] or 2-propanol. [5e, 6] Whereas the TH with 2propanol is an equilibrium reactiont hat, on scale, requires the removalo fa cetone to drivet he reaction to completion, the use of formic acid derivatives as hydrogen donors has the advantageo fg enerating CO 2 ,w hich is released from the reaction solution,a nd this drives the reaction and minimizesr eversibility problems. [7] Alkali formates HCOOM (M = Na, K) have been employed in the TH of aldehydes with RuCl 2 (PPh 3 ) 3 , [8] [RuCl 2 (mtppms) 2 ] 2 (mtppms = sodium 3-diphenylphosphinobenzenesulfonate), [9] CpRuCl(PPh 3 )(PN) (Cp = h 5 -cyclopentadienyl;P N = diphenyl-2-pyridylphosphine), [10] RuCl 2 (PTA) 4 (PTA = 1,3,5-triaza-7-phosphaadamantane), [11] and RuCl 2 (PO) 2 [PO = (2-methoxyethyl)diphenylphosphine] [12] catalysts at substrate to catalystm olar ratios (S/C) 1000, whereas the HCOOH-NEt 3 system has been used with Ru-arenec omplexes, but at al ow S/C = 50-200. [13] Ammonium formate is ac heap and readily accessible reducing agent widely employed in organic transformations. In addition to the Leuckart-Wallach reductive amination of carbonyl compounds, [14] metal-catalyzed versions of this reactionw ere reported by Kitamura [15] and Ta lwar [16] with Cp*R h III (Cp* = h 5pentamethylcyclopentadienyl) and Ir III complexes, respectively, whereas Kadyrov described the asymmetric version with RuCl 2 [(R)-Tol-BINAP][(R)-Tol-BINAP = (R)-(+)-2,2'-bis(di-4-tolylphosphino)-1,1'-binaphthyl]. [17] HCOONH 4 has also been widely used in the Pd/C-catalyzed conversion of carbonyl compounds into alkanes, [18] nitro derivatives into amines [19] and in the hydrodehalogenation of aromatic chlorides. [20] For the employment of HCOONH 4 in the TH of carbonyl compounds, Grainger [21] recently described the use of Wills' tethered Ru complexesi nt he reduction of ketones,w hereas Nie [22] observed the concomitant TH and reductive aminati...

Transfer Hydrogenation of Unsaturated Substrates by Half-sandwich Ruthenium Catalysts using Ammonium Formate as Reducing Reagent

“…Further optimisation of reaction conditions allowed us to reduce the catalyst load to 0.5 mol %. As expected, a significant improvement in efficiency over the reduction of nitriles and imines was observed, although this catalytic system is still less reactive than the classical Noyori diamine Ru catalyst (operates at the 0.5 % catalyst load but at 28° C) and falls short of the benchmark Barrata's catalyst for the transfer hydrogenation of aldehydes with ammonium formate (operates at the substrate : catalyst ratio of 5000:1) …”

“…At this point, the Leuckart‐Wallach reductive amination products were not observed in this catalytic system, which is in agreement with the recent report from the Barrata group …”

Preparation of monocarbonyl ruthenium complexes bearing bidentate nitrogen and phosphine ligands and their catalytic activity in carbonyl compound reduction

“…It is worth pointing out that aldehydes are substrates that are not easily reduced on account of side reactions, which may lead to catalyst deactivation. 67,68 A higher activity has been observed with the ampy dppb 18 for g and h with 98 and 99% conversion in 12 and 1 h (entries 5-6). 18 h 1 99 a The conversions has been determined by GC analysis.…”

Homogeneous Catalyzed Valorization of Furanics: A Sustainable Bridge to Fuels and Chemicals

“…The chemoselective reduction of FAL to the corresponding alcohol was achieved by Baratta and coworkers using benzo[h]quinoline RuCl 2 (CNN Ph )(PP) pincer complex (0.01 mol%) and HCOONH 4 as a hydrogen donor. The heteroaromatic aldehyde was selectively reduced to FA with a conversion of 98% at 90 • C in toluene/water after 20 h [134].…”