Cooperative Catalysis with Iron and a Chiral Brønsted Acid for Asymmetric Reductive Amination of Ketones

Abstract

The direct asymmetric reductive amination (DARA) of ketones with anilines is described by combining a chiral Brønsted acid (TRIP) and the non-chiral Knçlker iron complex as the catalyst system. In situ-formed imines are reduced with molecular hydrogen to give chiral amines in high yields (90%) and enantioselectivities of up to 99% ee.Enantiomerically pure chiral amines are of significant commercial value in the fine chemical and pharmaceutical industries in view of their applications as resolving agents, chiral auxiliaries/chiral bases and catalysts for asymmetric synthesis. [1] Moreover, chiral amines often possess pronounced biological activity in their own right, and hence they constitute privileged building blocks for pharmaceuticals and agrochemicals ( Figure 1). With respect to their synthesis, the asymmetric reduction of ketimines is one of the most direct and efficient methods and impressive results have been achieved using organocatalysts [2] and transition metal catalysts [3] in the last decade. However, the major drawback of these methods is that the somewhat unstable ketimines have to be synthesized, isolated and purified. Obviously, the direct asymmetric reductive amination (DARA) of ketones with amines represents a more convenient, simple and practical method for the synthesis of chiral amines. Here, the in situ-formed ketimine is subsequently reduced.An important breakthrough in enantioselective reductive amination was presented by Blaser [4] for the synthesis of (S)-Metolachlor. Since then, a number of studies on this area have been published based on Rh, [5] Ir, [6] Pd, [7] Ru [8] or others. [9] Notably, List and coworkers presented the first example for the direct asymmetric reductive amination using an organocatalytic approach. [2b,10] In a two-step, one-pot synthesis chiral amines are available in the presence of a chiral phosphorus acid (TRIP) as catalyst using stoichiometric amounts of Hantzsch esters. The corresponding imine, generated in situ from acetophenone and paraanisidine, is protonated by the chiral Brønsted acid and reduced by transfer hydrogenation with Hantzsch ester to afford the corresponding amine in 92% yield Figure 1. Selected aniline-based pharmaceuticals.

Comparative Study of the Catalytic Amination of Benzylic C–H Bonds Promoted by Ru(TPP)(py)2 and Ru(TPP)(CO)

“…Analytical data of 3a, [19] 3b, [19] 3c, [59] 3d, [19] 3e, [19] 3f, [19] 3g, [33] 3h [33] and 3i [60] were in accord with those reported in the literature.…”

Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes

“…More recently, many new catalytic applications of cyclopentadienone 1 and related complexes have been reported [32][33][34][35][36][37][38][39][40][41][42][43][44][45].…”

Identification of Novel Bacterial Members of the Imine Reductase Enzyme Family that Perform Reductive Amination

“…One of the most powerful ways to construct these architectures is through the reductive amination of a carbonyl compound with an amine, because it can enable the coupling of two smaller fragments directly with concomitant formation of a stereogenic center. A number of advances have recently been made in the field of catalytic asymmetric reductive amination processes, although there are still considerable challenges such as the chemoselectivity of the catalyst for imine functionality, the disfavored equilibrium of imine formation in water, and the enantioselectivity of the reaction . Alternatively, preformation of the imine followed by asymmetric transition‐metal‐catalyzed hydrogenation is an option, although it is a two‐step process and brings with it additional safety considerations around the use of hydrogen.…”