Asymmetric Transfer Hydrogenation of Ketones with Well-Defined Manganese(I) PNN and PNNP Complexes

Three new manganese complexes trans-[Mn(P–NH–NH–P)(CO)2][Br], (14) P–NH–NH–P = (S,S)-PPh2CH2CH2NH–CHPhCHPhNHCH2CH2PPh2), fac-[Mn(P′–NH–NH2)(CO)3][Br], (15) P′–NH–NH2 = (S,S)-PPh2(C6H4)NHCHPhCHPhNH2, and syn-mer-Mn(P–NH–NH2)(CO)2Br, (16) P–NH–NH2 = (S,S)-PPh2CH2CH2NHCHPhCHPhNH2 were synthesized and tested for the asymmetric transfer hydrogenation (ATH) of acetophenone in 2-PrOH. The ligands have stereogenic centers derived from the starting diamine, (S,S)-DPEN. Complex 16 was shown by NOE NMR experiments to have Mn–Br syn to the N–H of the secondary amine. Only the precatalyst 16, upon reaction with potassium tert-butoxide (KOtBu) in 2-PrOH, generated an active system at 80 °C for the ATH of acetophenone to 1-phenylethanol in an enantiomeric excess (ee) of 42% and thus was selected for further investigation into the mechanism of transfer hydrogenation. The corresponding amido complex Mn(P–N–NH2)(CO)2 (17), a borohydride complex syn-mer-Mn(P–NH–NH2)(CO)2(BH4) (18), and an ethoxide complex anti-mer-Mn(P–NH–N...