Iridium(I) versus Ruthenium(II). A Computational Study of the Transition Metal Catalyzed Transfer Hydrogenation of Ketones

Abstract

We present a density functional theory based computational study comparing simplified models for the ruthenium(II)-and iridium(I)-catalyzed transfer hydrogenation of ketones. For the ruthenium compound our results confirm earlier findings that the hydrogenation involves a ruthenium hydride and occurs via a concerted hydrogen transfer mechanism with no direct ruthenium-ketone binding along the reaction path. In contrast, for the iridium compound our calculations suggest that the reaction proceeds via direct hydrogen transfer between simultaneously coordinated ketone and alcohol. We find that for both metal complexes the formation of a very stable metal-alkoxide complex plays an important role. For the ruthenium-catalyzed reaction it constitutes a resting state that does not take an active part in the transfer hydrogenation, while for the iridium-catalyzed reaction it is an important intermediate along the reaction path.

Asymmetric hydrosilylation, transfer hydrogenation and hydrogenation of ketones catalyzed by iridium complexes

“…This mechanism is also lowenergy for the model complex RhH(C 2 H 4 ) 2 (MeHNCHRCHRNMeH) (R = H, Me, Ph) [100]. The only computational study specifically addressing an Ir catalyst that has appeared to date, to the best of our knowledge, examines the model systems IrH(COD)(NH 2 CH 2 CH 2 X) (X = OH, SCH 3 ) [101]. In this study, the three cycles A, B and D of Scheme 58 are compared, the critical transition states being represented in Scheme 69.…”

Transfer Hydrogenation Including the Meerwein‐Ponndorf‐Verley Reduction

“…A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48][49][50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re- leasing a ketone (14) (b).…”

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.…”