Enantioselective Imine Reduction Catalyzed by Phosphenium Ions

Abstract

The first use of phosphenium cations in asymmetric catalysis is reported. A diazaphosphenium triflate, prepared in two or three steps on a multigram scale from commercially available materials, catalyzes the hydroboration or hydrosilylation of cyclic imines with enantiomeric ratios of up to 97:3. Catalyst loadings are as low as 0.2 mol %. Twenty-two aryl/heteroaryl pyrrolidines and piperidines were prepared using this method. Imines containing functional groups such as thiophenes or pyridyl rings that can challenge transition-metal catalysts were reduced employing these systems.

Zinc‐Catalyzed Asymmetric Hydrosilylation of Cyclic Imines: Synthesis of Chiral 2‐Aryl‐Substituted Pyrrolidines as Pharmaceutical Building Blocks

“…Application of ruthenium(II)‐ and iridium(II)‐oxazolinyl‐ferrocenylphosphine‐complexes provided optically pure 2‐substituted pyrrolidine derivatives with good enantioselectivities (up to 88% ee ), whereas replacing substrates to six‐membered imines was completely ineffective [9a] . Recently, Speed and co‐workers presented an alternative method to this approach by asymmetric hydroboration using chiral phosphenium cations [4a] . Although authors tested several hydrosilanes, the highest yields and enantioselectivites (72–94% ee ) of the corresponding chiral pyrrolidines and piperidines were obtained in case of the use of HB(pin) as a reducing agent [4a] …”

A diversity of recently reported methodology for asymmetric imine reduction

“…62). 76 Wang et al have enantioselectively hydrogenated 2-substituted quinoline-type compounds using newly developed chiral…”

Diazaphospholene and Diazaarsolene Derived Homogeneous Catalysis

“…This gives the desired reduced imine and regenerates the catalyst. The proposed catalytic cycle is shown in Scheme …”