Understanding the Role of Water in Aqueous Ruthenium‐Catalyzed Transfer Hydrogenation of Ketones.

Abstract

We report an accurate computational study of the role of water in transfer hydrogenation of formaldehyde with a ruthenium-based catalyst using a water-specific model. Our results suggest that the reaction mechanism in aqueous solution is significantly different from that in the gas phase or in methanol solution. Previous theoretical studies have shown a concerted hydride and proton transfer in the gas phase (M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000, 122, 1466-1478;J.-W. Handgraaf, J. N. H. Reek, E. J. Meijer, Organometallics 2003, 22, 3150-3157; D. A. Alonso, P. Brandt, S. J. M. Nordin, P. G. Andersson, J. Am. Chem. Soc. 1999, 121, 9580-9588; D. G. I. Petra, J. N. H. Reek, J.-W. Handgraaf, E. J. Meijer, P. Dierkes, P. C. J. Kamer, J. Brussee, H. E. Schoemaker, P. W. N. M. van Leeuwen, Chem. Eur. J. 2000, 6, 2818-2829), whereas a delayed, solvent-mediated proton transfer has been observed in methanol solution (J.-W. Handgraaf, E. J. Meijer, J. Am. Chem. Soc. 2007, 129, 3099-3103). In aqueous solution, a concerted transition state is observed, as in the previous studies. However, only the hydride is transferred at that point, whereas the proton is transferred later by a water molecule instead of the catalyst.

Extending the range of pentasubstituted cyclopentadienyl compounds: The synthesis of a series of tetramethyl(alkyl or aryl)cyclopentadienes (Cp ∗ R ), their iridium complexes and their catalytic activity for asymmetric transfer hydrogenation

“…This leads to the conclusion that a concerted pathway is not the mechanism of reduction, but a step-wise pathway. The reduction of acetophenone by the fully N-methylated [(g 5 -C 5 Me 4 iPr)Ir(N,N-dimethyl-glycinate)Cl] gives experimental credence to the theoretical mechanisms presented by Ikariya [47] and Meijer, and are in agreement with results previously presented by Xiao and co-workers [9,47,48].…”

“…Since conversion still takes place with a methylated ligand, the hydrogen source must be solvent itself, not an amine proton. A step-wise mechanism, which has been presented by several groups prior to this study, accounts for the reduction when no amine protons are present [9,47,48]. A summary of the reduction of acetophenone is displayed in Table 6.…”

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…As an example we mention the role of assisting water molecules on a recently proposed reaction cycle for olefin elimination during methanol conversion in H-SAPO-34. [74] That protic molecules affect proton transfer and possibly also reaction mechanisms, can be well understood by the application of dynamical methods, as was already proven for chemical reactions in aqueous solution [75] and zeolites [42,44] . Whether methanol and water forms ion pairs upon adsorption in H-SAPO-34 is a point of discussion in literature and seems to depend on the applied conditions.…”

Computational Studies Explain the Importance of Two Different Substituents on the Chelating Bis(amido) Ligand for Transfer Hydrogenation by Bifunctional Cp*Rh(III) Catalysts

“…2, 3 Considerable effort has been devoted to understand these mechanisms and experimental and computational studies have concurred to show the prevalence of the outer sphere mechanism. 2, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] In the latter case, the proton is transferred from the hydrogen donor to a ligand that has an electron rich atom such as nitrogen or oxygen and the hydride is transferred to the metal. The catalyst is considered as bifunctional since it involves the metal and the ligand in the transformation between its unsaturated 16 electron (16e) and saturated 18e metal hydride forms.…”