Iron(II) Complexes Containing Unsymmetrical P–N–P′ Pincer Ligands for the Catalytic Asymmetric Hydrogenation of Ketones and Imines

After
their treatment with LiAlH4 and then alcohol,
new iron dicarbonyl complexes mer-trans-[Fe­(Br)­(CO)2(P–CHN–P′)]­[BF4] (where P–CHN–P′ = R2PCH2CHNCH2CH2PPh2 and R = Cy or iPr or P–CHN–P′
= (S,S)- Cy2PCH2CHNCH­(Me)­CH­(Ph)­PPh2) are catalysts for the hydrogenation
of ketones in THF solvent with added KOtBu at 50
°C and 5 atm H2. Complexes with R = Ph are not active.
With the enantiopure complex, alcohols are produced with an enantiomeric
excess of up to 85% (S) at TOF up to 2000 h–1, TON of up to 5000, for a range of ketones. An activated imine is
hydrogenated to the amine in 90% ee at a TOF 20 h–1and TON 99. This is a significant advance in asymmetric pressure
hydrogenation using iron. The complexes are prepared in two steps:
(1) a one-pot reaction of phosphonium dimers ([cyclo-(PR2CH2CH­(OH)−)2]­[Br]2), KOtBu, FeBr2, and
Ph2PCH2CH2NH2 (or (S,S)-Ph2PCH­(Ph)­CH­(Me)­NH2 for the enantiopure complex) in THF under a CO atmosphere
to produce the complexes cis- and trans-[Fe­(Br)2(CO)­(P–CHN–P′)];
(2) the reaction of these with AgBF4 under CO­(g) to afford
the dicarbonyl complexes in high yield (50–90%). NMR and DFT
studies of the process of precatalyst activation show that the dicarbonyl
complexes are converted first to hydride–aluminum hydride complexes
where the imine of the P–CHN–P′ ligand
is reduced to an amide [P–CH2N–P′]− with aluminum hydrides still bound to the nitrogen.
These hydride species react with alcohol to give monohydride amine
iron compounds FeH­(OR′)­(CO)­(P–CH2NH–P′),
R′ = Me, CMe2Et as well as the iron(0) complex Fe­(CO)2(P–CH2NH–P′) under certain
conditions.