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

Abstract

The conventional homogeneous catalysts for the enantioselective hydrogenation or transfer hydrogenation of ketones are based on platinum metals and, in particular, ruthenium. This method provides valuable enantiopure alcohols for the fine chemical industries. This tutorial review summarizes recent successes in replacing expensive and toxic ruthenium in these catalysts with "greener" iron substitutes including my lab's recent progress in this area using iron complexes containing readily-prepared tetradentate ligands. It will enlighten chemists interested in homogeneous catalysis and asymmetric synthesis.

Asymmetric catalysis using iron complexes – ‘Ruthenium Lite’?

“…21 The mechanism of the reduction reaction is not yet fully understood, however Morris has speculated that the 80 imine group in the ligands may be reduced, in-situ, to give the saturated complexes, which act as the active catalyst precursors. 21,22 Evidence for this came from the observation that complexes 4 and 6 give very similar conversions of ketones to hydrogenation products under the same conditions. Should this be the case, then the mechanism may resemble that commonly associated with the closely-related ruthenium 5 complexes (Figure 2), 16 in which hydrogen is transferred to substrate through a concerted, 6-membered ring mechanism, the well-defined nature of which contributes to the high level of enantiocontrol in the reduction.…”

“…In recent years, significant breakthroughs have been made in 10 the development and applications of homogeneous iron-based catalysts to asymmetric transformations. [1][2][3][4][5][6][7][8][9][10][11] Several excellent reviews have been published which describe the key findings and many of the non asymmetric precedents for the catalysts in this review. Here the focus will be on recent developments 15 in asymmetric reactions, although some non-asymmetric reactions will be discussed where they serve to place new findings into context.…”

Enantioselective Synthesis of Alcohols and Amines by Iridium‐Catalyzed Hydrogenation, Transfer Hydrogenation, and Related Processes

“…[3] Much effort has been devoted to the design and synthesis of ligands and the catalytic efficiency has been evaluated, in particular, with Ru, Rh, Ir, and recently also Fe complexes. [98] In 1995, Noyori, Ikariya, and co-workers reported the first example of what has become the most notable type of catalysts used in the ATH of ketones: Ruthenium complexes with monotosylated diamines, such as [ Figure 1, left) were able to reduce simple ketones into chiral alcohols with exceptional efficiency. [99] This result triggered deep mechanistic investigations into what has become known as "metal-ligand bifunctional catalysis".…”

“…In response, numerous related [IrA C H T U N G T R E N N U N G (Cp*)(L) n ] complexes have been reported and various ethylenediamine ligands have been combined with other Ir I and Ir III sources. For instance, Lemaire used a catalyst formed in situ from TsDPEN (98) and [IrClA C H T U N G T R E N N U N G (cod)] 2 in 2-PrOH/tBuOK to reduce acetophenone with an ee of 92 % (87 % conv.) after one day at room temperature (Scheme 52).…”

Exploring the decomposition pathways of iron asymmetric transfer hydrogenation catalysts

“…1,2 In addition, iron is the most abundant transition metal found in earth's crust; thus catalysts based on iron are significantly more sustainable than those reliant on trace metals. [3][4][5][6] A well-known use of iron in industry is as a heterogeneous catalyst for the production of ammonia via the Haber-Bosch process. 7 Unfortunately, the progress of homogeneous iron catalysis has not had the same type of success, unlike the platinum group metals.…”