Asymmetric Hydrogenation of Imines and Olefins Using Phosphine‐Oxazoline Iridium Complexes as Catalysts

Abstract

Herein we describe the synthesis of a new class of chiral phosphine-oxazolines and their application as ligands in iridium-catalyzed hydrogenations. Mechanistic aspects of olefin hydrogenation with this class of iridium catalysts are discussed and a selectivity model to help rationalize the results obtained is also presented.

Chiral Hetero‐ and Carbocyclic Compounds from the Asymmetric Hydrogenation of Cyclic Alkenes

“…The ligands in catalyst class 2 , with bicyclic backbones and P( o ‐tol) 2 P donors, are more sterically demanding than the ligands of class 1 . These catalysts derive their chirality from amino acids and have been applied to the iridium‐catalyzed asymmetric hydrogenations of imines,24 enamines,25 and vinyl boronates 26…”

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

“…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. Furthermore, achiral secondary and tertiary amine motifs are also present in many bioactive compounds, and their synthesis by reductive amination is often advantageous over N‐alkylation, which requires toxic alkylating agents and can easily lead to overaddition products.…”

Oxidation and Reduction

“…341 Highly enantioselective hydrogenation of N -protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd) 2 Mechanistic aspects of the hydrogenation of alkenes catalysed by iridium complexes with a new class of chiral phosphine-oxazoline ligands have been discussed and a selectivity model to help rationalize the results obtained has been presented. 344 The complex [IrH 2 (η 6 -C 6 H 6 ) (i-Pr 3 P)]BF 4 is an effective catalyst for the hydrogenation of N -benzylideneaniline under mild conditions by what is postulated to be an outer-sphere, ionic, bifunctional mechanism. 345 Under simple experimental conditions, the dinuclear complex [Ir 2 (μ-H)(μ-Pz) 2 H 3 (NCMe)(i-Pr 3 P) 2 ] has been transformed into derivatives such as [Ir 2 (μ-H)(μ-Pz) 2 H 2 (L)(NCMe)(i-Pr 3 P) 2 ]BF 4 [L = η 2 -H 2 , NH(Ph)CH 2 Ph] or [Ir 2 (μ-H)(μ-Pz) 2 H 2 (OSO 2 CF 3 )(NCMe)(i-Pr 3 P) 2 ], which are also very efficient catalyst precursors for the hydrogenation of N -benzylideneaniline through an ionic mechanism.…”