Iron Catalysts Containing Amine(imine)diphosphine P-NH-N-P Ligands Catalyze both the Asymmetric Hydrogenation and Asymmetric Transfer Hydrogenation of Ketones

Abstract

When activated with base, the iron(II) complexes with tetradentate amine(imine)diphosphine ligands, (S,S)-trans-[FeCl(CO)(PAr 2 -NH-N-PAr′ 2 )]BF 4 (1: Ar, Ar′ = Ph; 2: Ar = Ph, Ar′ = 4-MeC 6 H 4 ; 3: Ar, Ar′ = 3,5-Me 2 C 6 H 3 ), are very active for the asymmetric transfer hydrogenation (ATH) of ketones in KO t Bu/2propanol. For ATH, better enantioselectivity, but lower catalytic activity, was observed in general when using catalyst precursors with the bulkier dixylylphosphino groups compared to those with diphenylphosphino groups. The complexes were much less active for the pressure hydrogenation of ketones, where 1 and 2 produced racemic product alcohols, while 3 yielded chiral alcohols with an enantiomeric excess of up to 70% (R) at turnover frequencies up to 80 h −1 and turnover numbers of 100 for a range of ketones at 50 °C and 20 atm H 2 . This is a rare example of asymmetric pressure hydrogenation using an iron complex. Unlike the case of ATH, there is no effect on the rate upon the addition of KO t Bu beyond the 2 equiv needed to convert the precursor complex to the active amido(ene-amido) and amine(ene-amido)hydrido forms. Both AH and ATH reactions share the same iron hydride intermediate formed by reaction of the amido(ene-amido) iron complex with either dihydrogen or 2-propanol. Kinetic studies on the H 2 hydrogenation of acetophenone catalyzed by 1, activated by base in benzene, using the method of initial rates indicated that the heterolytic splitting of the dihydrogen at the amido(ene-amido) iron complex is the turnover-limiting step of the catalytic cycle for hydrogenation. For 1 in benzene at 323 K over the ranges of concentrations [1] = (2.4−4.8) × 10 −4 M and [ketone] = (3.6−7.2) × 10 −2 M, and of H 2 pressures = 10−20 atm, the rate law is rate = k[1][H 2 ], with k = 0.16 ± 0.01 M −1 s −1 , ΔH ⧧ = 10.0 ± 0.2 kcal mol −1 , and ΔS ⧧ = −31.0 ± 0.5 cal mol −1 K −1. Detailed DFT calculations also support the finding that the barrier for H 2 splitting is the turnover-limiting step. The higher barrier for H 2 activation compared to isopropanol activation in order to generate the active amine(ene-amido)hydrido form explains why this system is biased toward ATH over AH.

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

“…Based on derivatives of Knçlker's iron complex, the groups of Berkessel (Fe-7), [30] Pignataro/ Piarulli/Gennari (Fe-11) [31,32] as well as Wills (Fe-12) [33] investigated the asymmetric hydrogenation of ketones.Comparable results were obtained using Fe-9. [34] Greatly improved results were achieved by Morris and co-workers using Fe-10. [35,36] Excellent enantioselectivity combined with very high activity was shown by Gao and co-workers using Fe-8, although the reaction was presumably catalyzed by ligandmodified iron nanoparticles.…”

“…[29] Based on this,the same group developed the more active and selective catalysts based on the (pre-)catalysts Fe-19, [53,54] Fe-20 [55] and Fe-22. [34,56] Mezzetti and co-workers were able to achieve extraordinary enantioselectivities using av ariety of macrocyclic P 2 N 2 ligand-stabilized Fe complexes,s uch as Fe-23. [57] Theg roup of Wills used derivatives of Knçlker'si ron complex such as Fe-21 [58] and Fe-12 [33] for the asymmetric transfer hydrogenation involving the use of formic acid (conversion of acetophenone:9 0%,w ith 24 %ee).…”

[(1,2,5,6-η)-Cycloocta-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-ylidene-κC2)(triphenylphosphane-κP)iridium(I) tetrafluoridoborate

“…Transfer hydrogenation of ketones and imines is an encouraging example of an efficient and benign chemical transformation that exemplifies some of the key aspects of green chemistry (Ruff et al, 2016;Zuo et al, 2014). The Nheterocyclic carbene (NHC) ligands can be tuned sterically and electronically by having different alkyl groups on the nitrogen atoms (Gusev, 2009).…”

Iron Group Hydrides in Noyori Bifunctional Catalysis

“…KOH or KO t Bu in 2‐PrOH (Scheme ) 85. This catalyst reduces a wide range of aryl ketones by hydrogen transfer from 2‐PrOH with TON of 5000 and TOF ranging from 4 to 200 s −1 depending on the structure, with electronegative substituents such as CF 3 , Cl on the ketone and pyridyl ketones giving the highest rates of reduction 85,87. While the ee of the alcohols produced often starts at over 90% at low conversions, by the time equilibrium is reached the ee tend to fall to between 70 and 90% ee ( R ).…”

“…While the ee of the alcohols produced often starts at over 90% at low conversions, by the time equilibrium is reached the ee tend to fall to between 70 and 90% ee ( R ). The ee can be increased for some ketone substrates to over 90% by using two Pxylyl 2 groups85,87 or one PCy 2 and one PPh 2 group88 in the ligand structure. The ee is greater than 99% ( R ) for the reduction of the diphenylphosphinoyl‐functionalized imines PhMeC=NPOPh 2 and ThMeC= NPOPh 2 , Th = 2‐thiophene.…”