Mechanistic insights into asymmetric transfer hydrogenation of pyruvic acid catalysed by chiral osmium complexes with formic acid assisted proton transfer

Intramolecular Proton-Coupled Hydride Transfers with Relatively Low Activation Barriers

We report that bifunctional molecules containing hydroxyl and carbonyl functional groups can undergo an effective transfer hydrogenation via an intramolecular proton-coupled hydride transfer (PCHT) mechanism. In this reaction mechanism, a hydride transfer between two carbon atoms is coupled with a proton transfer between two oxygen atoms via a cyclic bond rearrangement transition structure. The coupled transfer of the two hydrogens as H^δ+ and H^δ- is supported by atomic polar tensor charges. The activation energy for the PCHT reaction is strongly dependent on the length of the alkyl chain between the hydroxyl and carbonyl functional groups but relatively weakly dependent on the functional groups attached to the hydroxyl and carbonyl carbons. We investigate the PCHT reaction mechanism using the Gaussian-4 thermochemical protocol and obtain high activation energy barriers (ΔH^‡₂₉₈) of 210.5-228.3 kJ mol^-1 for chain lengths of one carbon atom and 160.2-163.9 kJ mol^-1 for chain lengths of two carbon atoms. However, for longer chain lengths containing 3-4 carbon atoms, we obtain ΔH^‡₂₉₈ values as low as 101.9 kJ mol^-1. Importantly, the hydride transfer between two carbon atoms proceeds without the need for a catalyst or hydride transfer activating agent. These results indicate that the intramolecular PCHT reaction provides an effective avenue for uncatalyzed, metal-free hydride transfers at ambient temperatures.

Density functional theory investigation of Ru(II) and Os(II) asymmetric transfer hydrogenation catalysts

Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(II) (Noyori-type) and Os(II) sulfonyldiamine 16-electron active catalysts [Ru/Os(η⁶-p-cymene)(TsDPEN-H₂)], where TsDPEN is N-tosyl-1,2-diphenylethylenediamine containing S,S or R,R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH₂, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N,N-chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p-cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p-cymene coordination from η⁶ to η². Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH₂ followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M-H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(II) catalyst is more stable than the Ru(II) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d⁶ and 5d⁶ second-row and third-row transition metal congeners in group 8 of the periodic table.

Arene-Osmium(II) Complexes in Homogeneous Catalysis

Although the application of arene-osmium(II) complexes in homogeneous catalysis has been much less studied than that of their ruthenium analogues, different works have shown that, in some instances, a comparable or even superior effectiveness can be achieved with this particular class of compounds. This review article focuses on the catalytic applications of arene-osmium(II) complexes. Among others, transfer hydrogenation, hydrogenation, oxidation, and nitrile hydration reactions, as well as different C-C bond forming processes, are comprehensively discussed.

Computational Prediction of Chiral Iron Complexes for Asymmetric Transfer Hydrogenation of Pyruvic Acid to Lactic Acid

Density functional theory calculations reveal a formic acid-assisted proton transfer mechanism for asymmetric transfer hydrogenation of pyruvic acid catalyzed by a chiral Fe complex, FeH[(R,R)-BESNCH(Ph)CH(Ph)NH2](η6-p-cymene), with formic acid as the hydrogen provider. The rate-determining step is the hydride transfer from formate anion to Fe for the formation and dissociation of CO2 with a total free energy barrier of 28.0 kcal mol-1. A series of new bifunctional iron complexes with η6-p-cymene replaced by different arene and sulfonyl groups were built and computationally screened as potential catalysts. Among the proposed complexes, we found 1g with η6-p-cymene replaced by 4-isopropyl biphenyl had the lowest free energy barrier of 26.2 kcal mol-1 and excellent chiral selectivity of 98.5% ee.