Enantioselective Hydrogenation of Imines Using a Diverse Library of Ruthenium Dichloride(diphosphine)(diamine) Precatalysts

Abstract

A range of aromatic and cyclic imines were subjected to asymmetric hydrogenation with catalysts derived from complexes of the type RuCl 2 (diphosphine)(diamine). Good to high enantioselectivities were observed. For each imine, a library of chiral complexes based on different diphosphine and diamine combinations was screened. A different combination of diphosphine and diamine was required each time to obtain the optimum enantioselectivity.

Highlights of Transition Metal‐Catalyzed Asymmetric Hydrogenation of Imines

“…The best enantiose-lectivity reported (92 %) was achieved with the combination of (R,R)-EtDuPHOS 49 and (R,R)-DACH 50 (DACH = 1,2-diaminocyclohexane; Figure 4). [39] N-tosylimines (6 a-p) are hydrogenated with Ru-binap catalyst (binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) with enantioselectivities up to 84 %. [40] Despite these results, the asymmetric hydrogenation of imines with Ru catalysts and molecular hydrogen as a source of protons has received less attention.…”

Highly Enantioselective Imine Hydrogenation Catalyzed by Ruthenium Phosphane–Phosphite Diamine Complexes

“…As a first approach, similar reaction conditions to those reported by Cobley et al were used (i.e., substrate/catalyst = 100, iPrOH as solvent, 20 bar H 2 , [4 a]/A C H T U N G T R E N N U N G [tBuOK] = 1, 60 8C). [13] Under these conditions, compounds 1 a and 1 b gave moderate conversions, and the diphenyl catalyst provided a significantly better enantioselectivity (Table 1, Entries 1 and 2). Moreover, precatalysts with a smaller phosphite fragment (1 c; Table 1, Entry 3) and an ethane backbone (1 d; Table 1, Entry 4) were also examined.…”

“…[5] Moreover, Cobley and co-workers, by following a systematic optimization of both the diamine and diphosphane ligands, found a highly enantioselective catalyst, based on Et-DuPHOS and DACH, for the hydrogenation of N-(1-phenylethylidene)aniline. [13] Very recently, Ohkuma et al described a highly efficient catalyst based on Xyl-skewphos and DPEN ligands for the hydrogenation of a wide range of imines. [14] A remarkable feature of […”

Iridium‐Catalyzed Asymmetric Hydrogenation of N‐Alkyl α‐Aryl Furan‐Containing Imines: an Efficient Route to Unnatural N‐Alkyl Arylalanines and Related Derivatives.

“…[15] We then moved on to investigating pressure hydrogenation conditions. Noyori-type Ru bisphosphine/diamine catalysts [16] have been used previously for hydrogenations of N-aryl/benzyl imines derived from acetophenone; however conversions with our substrate were very low both at 30 8C and 508C. Ir-based systems were then Scheme 1.…”