Enantioselective Hydrogenation of Activated Aryl Imines Catalyzed by an Iron(II) P-NH-P′ Complex

Abstract

Reports of the use of first row transition metal complexes for the catalytic AH of imine are limited. In 2011 Beller and coworkers reported that a range of N-aryl substituted imines were hydrogenated to amines with ee ranging from 88 to 98% using Knölker's iron complex FeH(CO)2(Cp'OH) (5 mol%) in combination with the homochiral phosphoric acid (S)-TRIP (1 mol%) at 50 bar H2 and 65 °C. 23 In 2014 our group reported that an iron complex [FeBr(CO)2(L 1)]BF4 with the asymmetric ligand (S,S)-PPh2CHPhCHMeNHCH2CH2P i Pr2 (L 1) when activated with LiAlH4/iso-amylalcohol was active in the presence of 10 mol% KO t Bu for the AH of the N-diphenylphosphinoyl imine derived from 1-propiophenone to the amine (90% ee) at 20 atm H2 and 50 °C. 24 Bai et al have described the use of homochiral cyclopentadienone)iron complex for the AH of activated imines but the enantioselectivity was poor (ee of the amine up to 40%). 25 Otherwise there have been several reports of the ATH of activated imines by catalysts based on iron, 26-29 cobalt 30,31 and nickel. 32

Bifunctional homogeneous catalysts based on first row transition metals in asymmetric hydrogenation

“…According to calculations, the hydride transfer and the splitting of dihydrogen had similar energy barriers and could define the rate limiting step and the turn-over frequency depending on the ketone substrate. Finally, the calculated energies of the assumed pro-S and pro-R transition states were consistent with the observed enantioselectivity Mechanism of the AH of ketones catalysed by iron(II) complex 40a based on a chiral PNHP ligand by Morris et al In a recent contribution, Morris et al reported the application of an unsymmetrical iron PNHP′ pre-catalyst (41a) for the AH of imines activated with N-diphenylphosphinoyl (Scheme 46)[125]. At the exception of dialkyl derivatives (35% ee), yields (up to 85%) and enantioselectivities (up to 99% ee) were high.…”

Ni-catalyzed asymmetric hydrogenation of N-aryl imino esters for the efficient synthesis of chiral α-aryl glycines

“…1b) [28][29][30] . In view of current interest in the development of earthabundant metal catalysts in asymmetric hydrogenation [31][32][33][34][35][36][37][38][39][40][41] , which are inexpensive and environmentally friendly, the use of nickel has drawn increasing attention 24,[31][32][33][34][35][36][37][38][39][40][41] . Although Nicatalyzed asymmetric reduction has seen rapid development, the asymmetric hydrogenation of imines is still in its infancy 24,[42][43][44] .…”

α-Imino Iridium Carbenes from Imidoyl Sulfoxonium Ylides: Application in the One-Step Synthesis of Indoles

“…To develop an alternative synthetic route to obtain these imidoyl sulfoxonium ylides, we explored a strategy for imine group formation from β-keto sulfoxonium ylides ( 3 ) and anilines ( 4 ) (method B, Scheme ). To perform this transformation, TiCl 4 was applied as a Lewis acid, since this titanium complex is already known to efficiently promote the formation of imines . The reaction between dimethyloxosulfonium benzoylmethylide ( 3a ) and aniline ( 4a ) provided compound 2a in 83% yield after 40 h at room temperature in CH 2 Cl 2 as the solvent.…”