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–NH2)­(CO)2(OEt) (19′) were independently synthesized
and tested in the ATH of acetophenone. The amido complex 17 and the borohydride complex 18 displayed similar activity
to 16 activated in basic 2-PrOH, but the anti NH OEt complex 19′ was completely inactive.
This result suggested that the NH effect, as described by Noyori,
was required to obtain catalytic activity. The syn NH BH4 manganese complex is one of the most active manganese
ATH catalysts to date and can hydrogenate a variety of aromatic ketones,
including base-sensitive substrates such as p-acetylbenzoate
ethyl ester.