Mn(I) and Fe(II)/PN(H)P Catalysts for the Hydrogenation of Ketones: A Comparison by Experiment and Calculation

Abstract

The P‐stereogenic PN(H)P tridentate ligand HN(CH2CH2)2P(Me)R (R=Cy, 1 a; R=tBu, 1 b) gives the Mn(I) complexes [Mn(CO)3(PN(H)P)]Br, which were tested in the asymmetric H2 hydrogenation of ketones. The amido species [Mn(CO)2(PNP)], hydrides [MnH(CO)2(PN(H)P)], and the alkoxide complex were detected by NMR spectroscopy. The manganese(I) derivative [Mn(CO)3(1 a)]Br was compared to its iron(II) analogue [FeHBr(CO)(1 a)] by kinetic and DFT studies. The DFT study suggests that both Mn(I) and Fe(II) operate via a bifunctional mechanism for H+/H− transfer with structurally similar enantiodetermining transition states and hence comparable enantioselectivity. The Mn(I) catalyst is less active than its Fe(II) analogue (k(Fe)/k(Mn)=ca. 30), which we attribute to the higher stability of the Mn(I) resting species, the off‐cycle alkoxo complex [Mn(OCH(Me)Ph))(CO)2(1 a)] that follows from the larger π delocalization onto the additional CO ligand as compared to [FeH(OCH(Me)Ph))(CO)(1 a)].magnified image

Retracted: The Manganese(I)‐Catalyzed Asymmetric Transfer Hydrogenation of Ketones: Disclosing the Macrocylic Privilege

“…An induction period was observed for the reaction, and was suppressed by pre‐activating 2 with base (2 h at 50 °C in 2‐propanol, see the Supporting Information). Most probably, the deprotonation of one NH group by base triggers the dissociation of a CO ligand and forms an amido complex that is the starting point of the catalytic cycle, as suggested for other Mn I catalysts . No hydrogenation occurs in the absence of base.…”

“…Other aryl substituents ( 5 d – f ) also give good enantioselectivities (93 % ee ). This reaction is the first example of asymmetric Mn I ‐catalyzed hydrogenation of ketones containing large aryl groups such as 1‐acetylpyrene . Several acyl‐substituted heterocycles such as acetylthiophenes ( 5 g – h ), furan ( 5 i ), pyrrole ( 5 j ), indole ( 5 k ), and pyridines ( 5 l – m ) were reduced to the corresponding alcohols without catalyst poisoning.…”

“…The catalyst is less active with ortho ‐substituted substrates ( 3 b , 3 e , 3 i ), probably because of steric effects, but increasing the reaction time from 3 to 6 hours gives good yields (88–98 % yield). Notably, the 2,6‐methoxy‐disubstituted ketone 3 h is reduced at 80 °C within 9 hours with 61 % yield and 97 % ee , which is unprecedented with Mn I . In addition to alkyl and alkoxy substituents, halogen substituents ( 3 i – n ) are tolerated (90–97 % ee ).…”

“…Tertiary amines ( 3 r – u ) are also well tolerated (90–96 % ee ). The complex 2 is the first chiral Mn I catalyst that hydrogenates acetylanilines ( 3 r , s ) and 4′‐(imidazol‐1‐yl)acetophenone ( 3 u ) . Ketones are reduced chemoselectively in the presence of C≡C triple bonds ( 3 v ) and sulfone ( 3 w ).…”

“…The enantioselectivity was rationalized by DFT calculations on the enantiodetermining hydride/proton transfer step assuming a Noyori bifunctional mechanism in which acetophenone forms a hydrogen bond to the NH group of the ligand (Figure ). The sense of induction and the calculated enantioselectivity are in fair agreement with experiment ( R , 99 vs. 94 % ee , respectively).…”