Iron(II) complexes containing unsymmetrical P-N-P′ pincer ligands for the catalytic asymmetric hydrogenation of ketones and imines

Article abstract of DOI:10.1021/ja4082233

After their treatment with LiAlH₄ and then alcohol, new iron dicarbonyl complexes mer-trans-[Fe(Br)(CO)₂(P-CH=N-P′)][BF ₄] (where P-CH=N-P′ = R₂PCH₂CH=NCH ₂CH₂PPh₂ 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)-Ph ₂PCH(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 AgBF ₄ 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-CH ₂NH-P′) under certain conditions.