Cover Picture: Mn(I) and Fe(II)/PN(H)P Catalysts for the Hydrogenation of Ketones: A Comparison by Experiment and Calculation (Adv. Synth. Catal. 20/2019)

Abstract

The P‐stereogenic PN(H)P tridentate ligand HN(CH 2 CH 2) 2 P(Me)R (R = Cy, 1a; R = t Bu, 1b) gives the Mn(I) complexes [Mn(CO) 3 (PN(H)P)]Br, which were tested in the asymmetric H 2 hydrogenation of ketones. The amido species [Mn(CO) 2 (PNP)], hydrides [MnH(CO) 2 (PN(H)P)], and the alkoxide complex [Mn(OR)(CO) 2 ( 1a)] were detected by NMR spectroscopy. The manganese(I) derivative [Mn(CO) 3 ( 1a)]Br was compared to its iron(II) analogue [FeHBr(CO)( 1a)] 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 ( 1a)] that follows from the larger π delocalization onto the additional CO ligand as compared to [FeH(OCH(Me)Ph))(CO)( 1a)].