Ruthenium(II) Terdentate CNN Complexes: Superlative Catalysts for the Hydrogen‐Transfer Reduction of Ketones by Reversible Insertion of a Carbonyl Group into the RuH Bond

Abstract

Dedicated to Professor Fausto Calderazzo on the occasion of his 75th birthdayPincer ECE (E = N, P) transition-metal complexes, in which the terdentate ligands contain two stable five-membered cyclometalated rings, have reached a high level of sophistication and appear extremely attractive for both catalytic and stoichiometric reactions.[1] The great interest in pincer ligands arises from the high level of control over the reactivity and the stereochemistry that they impose around the metal center as a result of their electronic properties and geometric restrictions. These factors afford highly selective transformations and lead to some unique species of relevance for the investigation of elementary processes.[2] In spite of the great attention afforded to ruthenium for its versatility in catalysis, [3] no examples of terdentate ruthenium CNN catalysts have been reported thus far. It is worth noting that CNN complexes are expected to have significantly different reactivity compared to the NCN analogues, mainly because of the different geometrical disposition of the s-donor carbon atom.Recently, we have shown that 2-(aminomethyl)pyridine (ampy) shows a high ligand acceleration effect in transfer hydrogenation [4] catalyzed by ruthenium(ii) complexes with phosphane ligands. Thus, complete reduction of many ketones in 2-propanol is quickly achieved with the cyclometalated complex [RuCl(CO)(CP)(ampy)] (CP = (2-CH 2 -6-MeC 6 H 3 )PPh 2 ), with turnover frequencies (TOF values) up to 6.3 10 4 h À1 , whereas the derivatives cis-[RuCl 2 (PP)-(ampy)] (PP = diphosphane) lead to TOF values up to 4.0 10 5 h À1 and ee values up to 94 % by using chiral diphosphanes.[5] Since it is well known that 2-phenylpyridine readily gives access to orthometalated CN ruthenium complexes, [6] we decided to investigate the coordination chemistry of the related 6-(4'-methylphenyl)-2-pyridylmethylamine with the aim of obtaining terdentate CNN complexes. We report herein on novel complexes of formula [RuX(CNN)(dppb)] (X = Cl, H; dppb = Ph 2 P(CH 2 ) 4 PPh 2 ), which are remarkably active catalysts for transfer hydrogenation that afford TOF values up to 2.5 10 6 h À1 with very low loading of catalysts (0.005-0.001 mol %) compared to the most active systems reported. [2f, 7] Evidence is provided that the reduction of the ketone proceeds through the formation of a Ru IIalkoxide complex by insertion of the carbonyl group of the substrate into the RuÀH bond of a ruthenium(ii) hydride formed as an intermediate from the chloride complex 1.Treatment of [RuCl 2 (PPh 3 )(dppb)] with an equimolar amount of 6-(4'-methylphenyl)-2-pyridylmethylamine in 2-propanol at reflux in the presence of NEt 3 affords the thermally stable orthometalated ruthenium(ii) complex 1 [8] in high yield [Eq. (1)].The signals for the diastereotopic NCH 2 protons in the 1 H NMR spectrum are at d = 4.12 and 3.72 ppm with 2 J(H,H) = 15.5 Hz. The doublet at d = 52.5 ppm with a 3 J(C,P) = 2.7 Hz in the 13 C NMR spectrum corresponds to the CH 2 N group which is shifted downfield relati...

Highly Diastereoselective Formation of Ruthenium Complexes for Efficient Catalytic Asymmetric Transfer Hydrogenation

“…It is likely that the mechanism of the transfer hydrogenation involves the [RuHX(Josiphos)RPyme] (X = H, OR') species, with b-hydrogen elimination and ketone insertion reactions, [14] in accordance with our study on cis-[RuCl 2 (PP)Pyme].…”

Ethanol as Hydrogen Donor: Highly Efficient Transfer Hydrogenations with Rhodium(I) Amides

“…However, the efficiency with ethanol is excellent ( Table 1, entries 1-3). [1,2] 2-Propanol can be used as hydrogen donor but is less efficient and requires more dilute conditions to obtain comparable conversions (Table 1, entries 11, 12).…”

“…[1] Impressive activities (turnover frequencies TOF > 1 10 6 h À1 ) [2] and selectivites have been reached. Ruthenium(II) arene complexes and rhodium(III) cyclopentadienyl complexes in combination with 2-propanol or formic acid/ triethylamine mixtures as hydrogen donors are among the most popular catalytic systems.…”

Osmium(II) CNN Pincer Complexes as Efficient Catalysts for Both Asymmetric Transfer and H2 Hydrogenation of Ketones

“…[5] Since the CNN pincer complexes [RuCl(CNN)P 2 ] [6] obtained from 1-(6-arylpyridin-2-yl)methanamines (HCNN) are among the most active catalysts for the TH of carbonyl compounds, we decided to investigate the chemistry of osmium complexes with CNN ligands and report herein the isolation of the first CNN pincer osmium derivatives [OsCl(CNN)P 2 ]. These complexes are extremely active and productive catalysts for both the TH and HY of ketones (turnover frequency (TOF) of up to 1.3 10 6 h À1 ).…”