Asymmetric Reactions Catalyzed by Chiral Metal Complex. LXII. Efficient Asymmetric Hydrogenation of Imines Catalyzed by a Neutral Iridium(I) Complex of (4R,5R)-MOD-DIOP.

Asymmetric Hydrogenation of Imines

“…Inspired by the investigation of the halide effect and the high reactivity of Ir(III) complexes, Genet, Mashima, and co-workers successfully synthesized mononuclear halide-carboxylate and cationic triply halogen-bridged dinuclear Ir(III) complexes of BINAP and SYNPHOS, and found that these complexes exhibited excellent enantioselectivities in cyclic imine hydrogenations (Scheme 18) [62]. As expected on the basis of the reported tendency of halide effects for iridium catalyst systems [8,32,35,42], the iodide served as a better catalyst precursor in terms of catalytic activity among the dinuclear catalysts, though halogen atoms bound to the Ir(III) center did not clearly affect enantioselectivity. Recently, Zhang and co-workers reported the preparation of a similar triply halogen-bridged Ir(III) complex of the ferrocene-based electron-donating diphosphine ligand f-Binaphane and its application to the enantioselective hydrogenation of cyclic imines (Scheme 19) [47,48].…”

“…Many chiral diphosphine ligands that have been studied in asymmetric reactions with other transition metals such as Ru and Rh, or have been applied to the [35]; the application of DIOP* ligand and its derivatives by Zhang and co-workers obtained 85.0% ee for TMI [36]; the application of BCPM and MCCPM ligands, also by Achiwa et al, achieved 91% ee and 90% ee for TMI [37]; the application of BICP ligand by Zhang and co-workers achieved 95% ee for TMI [38]; and the application of BINAP by Achiwa et al obtained up to 86% ee for the isoquinoline-type imines (Scheme 12) [39]. The remarkable success of Xyliphos in industrial applications revealed the great potential of diphosphine ligands based on a ferrocene backbone, which could possibly enhance the air-stability and electron-donating ability of the ligands.…”

“…It should also be noted that virtually all of the other systems employed for imine hydrogenation have been derived from late transition metals. Several common types of cyclic imines such as 2,3,3-trimethylindolenine (TMI) [35], 2-phenyl-1-pyrroline [20], and tetrahydroisoquinolines [20] have been investigated primarily in the past two decades, while some new substrate classes, such as benzodiazepinones [137] and 2H-1,4-benzoxazines [138], have recently attracted attention (Scheme 45). Some representative results of the hydrogenation of these substrate categories are summarized in Tables 7, 8 Scheme 45 Non-activated cyclic imine substrate types [19,101,137,138] …”

Catalysis, kinetics and mechanisms of organo-iridium enantioselective hydrogenation-reduction

“…The introduction of p-methoxy and m′mdimethyl groups into the phenyl groups of biphosphine ligands improved the enantioselectivity of this IrIJI) complex to 81.4% ee. 29 The development of Josiphos, extremely modular and tunable chiral ferrocenyl diphosphine ligands, provided very efficient ligands for several commercial applications. 30,31 Xyliphos 53, (Scheme 16) was capable of full conversion of the substrate 49 in 4 h with 79% ee.…”

Chiral Amines from Transition‐Metal‐Mediated Hydrogenation and Transfer Hydrogenation

“…Osborn [60,61] obtained moderate enantioselectivities by screening various diphosphine ligands in the iridium-catalyzed hydrogenation of acyclic and cyclic substrates. Improvements in the stereoselectivity were achieved using iodide salts [62][63][64], protic amines [65], 1,3-indandione [64], or imides [64,66] as additives. In the past decade, several chiral iridium-diphosphine complexes have been applied to imine hydrogenation [67][68][69][70][71][72][73], and a few give >90% ee for analogues of A with substituents on the N-aryl moiety [69,71,73].…”