Unexpected Direct Hydride Transfer Mechanism for the Hydrogenation of Ethyl Acetate to Ethanol Catalyzed by SNS Pincer Ruthenium Complexes

Abstract

The hydrogenation of ethyl acetate to ethanol catalyzed by SNS pincer ruthenium complexes was computationally investigated by using DFT. Different from a previously proposed mechanism with fac-[(SNS)Ru(PPh )(H) ] (5') as the catalyst, an unexpected direct hydride transfer mechanism with a mer-SNS ruthenium complex as the catalyst, and two cascade catalytic cycles for hydrogenations of ethyl acetate to aldehyde and aldehyde to ethanol, is proposed base on our calculations. The new mechanism features ethanol-assisted proton transfer for H cleavage, direct hydride transfer from ruthenium to the carbonyl carbon, and C-OEt bond cleavage. Calculation results indicate that the rate-determining step in the whole catalytic reaction is the transfer of a hydride from ruthenium to the carbonyl carbon of ethyl acetate, with a total free energy barrier of only 26.9 kcal mol , which is consistent with experimental observations and significantly lower than the relative free energy of an intermediate in a previously postulated mechanism with 5' as the catalyst.

The Guerbet Reaction Network – a Ball‐in‐a‐Maze‐Game or: Why Ru‐MACHO‐BH is Poor in Coupling two Ethanol to n‐Butanol

“…As starting values for the activation barriers, the results from DFT calculations of Ru-MACHO or at least ruthenium-pincer-complex catalyzed reactions were taken from literature. [20][21][22] Those barriers were adapted to reproduce the experimental concentration profiles (ESI chapter and 6). A change of the activation enthalpies seems to be allowed, since all DFT calculations either neglect the solvent influence or do not take the actual solvent composition into account.…”

“…Those single steps cannot be resolved by IR-spectroscopy independently, because the reduced and oxidized catalyst species are not detectable under process conditions. Thus, the kinetic model of the redox steps is created from activation barriers of DFT calculations found in literature, [20][21][22] but the barrier heights are adapted to reproduce the experimental concentration profiles. The kinetic models of all subsystems are finally assembled to a microkinetic model of the Guerbet reaction network in MatLab.…”

Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

“…In contrast with the reaction mechanisms for C=O hydrogenation by bifunctional catalysts, there is no consensus on how these catalysts promote the C-N bond protonolysis of the hemiaminal intermediate, which appears to be substrate-and catalyst-dependent. In addition, it has been reported that secondary amides, alcohols and other slightly acidic molecules can be involved in the mechanisms of C-N bond protonolysis [37][38][39][40][41][42][43]. Therefore, we classified these reaction mechanisms in three different categories: (1) catalyst-assisted, (2) additive-assisted, and (3) assisted by both catalyst and additive (see Fig.…”

Selective Hydrogenation of α,β‐Unsaturated Aldehydes and Ketones by Air‐Stable Ruthenium NNS Complexes

“…The flexibility of the ligand may actually be an important factor for catalyst activity. [15] Hydrogenation reactions Catalytic hydrogenations were performed at 80 8Cw ith 30 bar H 2 in isopropanol.Abase screening was performed for the overnight hydrogenation reaction of trans-cinnamaldehyde to cinnamyl alcohol at 1mmol scale with 0.05 mol % C2 as catalyst. As shown in Ta ble 1, full conversions wereo btained with (16), Ru1ÀP1 2.2960(5), Ru1ÀCl1 2.4199(5), Ru1ÀCl2 2.4273(5);S1-Ru1-N1 84.23 (5), S1-Ru1-N2 162.42 (5), N1-Ru1-N278.27(6), N1-Ru1-P1 175.62 (5), Cl1-Ru1-Cl2 171.97(2), Cl1-Ru1-N1 84.80 (5), Cl1-Ru1-N2 83.65 (5), Cl1-Ru1-S1 93.239 (18), C1-N1-C71 15.23 (16);b )c omplex C3:Ru1ÀS1 2.3184(4), Ru1ÀN1 2.1184(14), Ru1ÀN2 2.1818(14), Ru1ÀP1 2.3161(4), Ru1ÀCl1 2.4327(4), Ru1ÀCl2 2.4207(4),; S1-Ru1-N1 84.05(4), S1-Ru1-N2 160.…”

“…As shown in Ta ble 1, full conversions wereo btained with (16), Ru1ÀP1 2.2960(5), Ru1ÀCl1 2.4199(5), Ru1ÀCl2 2.4273(5);S1-Ru1-N1 84.23 (5), S1-Ru1-N2 162.42 (5), N1-Ru1-N278.27(6), N1-Ru1-P1 175.62 (5), Cl1-Ru1-Cl2 171.97(2), Cl1-Ru1-N1 84.80 (5), Cl1-Ru1-N2 83.65 (5), Cl1-Ru1-S1 93.239 (18), C1-N1-C71 15.23 (16);b )c omplex C3:Ru1ÀS1 2.3184(4), Ru1ÀN1 2.1184(14), Ru1ÀN2 2.1818(14), Ru1ÀP1 2.3161(4), Ru1ÀCl1 2.4327(4), Ru1ÀCl2 2.4207(4),; S1-Ru1-N1 84.05(4), S1-Ru1-N2 160. 75(4), N1-Ru1-N276.94 (5), N1-Ru1-P1 172.48(4), Cl1-Ru1-Cl2 171.162 (15), Cl1-Ru1-N1 86.67(4), Cl1-Ru1-N2 94.49(4), Cl1-Ru1-S1 87.308 (15), C1-N1-C81 13.66(13); c) complex C7:Ru1ÀN1 2.215(2), Ru1ÀN2 2.069(2), Ru1ÀS1 2.3039 (7), Ru1ÀCl1 2.4445 (7), Ru1ÀCl2 2.4535 (7), Ru1ÀP1 2.2827 (7); N1-Ru1-N278.47(9), N1-Ru1-S185.11(6), N2-Ru1-S1 93.72(6), N1-Ru1-P1 177.83 (7),S1-Ru1-Cl2 172.07(3),N1-Ru1-Cl2 87.79(6), N2-Ru1-Cl1 170.59(6), Cl1-Ru1-Cl2 94.14(2), C1-N1-C8 108.8 (2),C l1-Ru1-S1 82.65 (3),N 2-Ru1-Cl2 88.32(6), P1-Ru1-Cl294.35 (2). tert-butoxide bases (entries1-4).…”