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

Abstract

AbstractThe 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