Reports: AC3
46845-AC3 Polar Bond Hydrogenation Catalyzed by Iron Complexes
We built on our discovery last year of iron complexes that are catalysts under mild conditions for the hydrogenation and asymmetric reduction of ketones to alcohols. This begins to achieve our objective of replacing toxic and expensive platinum metal catalysts with cheap and benign iron catalysts.
Six complexes of the type [Fe(NCMe)2(P-N-N-P)]2+ containing diiminodiphosphine ligands and the complexes [Fe(NCMe)2(P-NH-NH-P)]2+ with a diaminodiphosphine ligand were obtained by the reaction of Fe(II) salts with achiral and chiral P-N-N-P or P-NH-NH-P ligands, respectively, in acetonitrile at ambient temperature. The P-N-N-P ligands are derived from reaction of ortho-diphenylphosphinobenzaldehyde with the diamines 1,2-ethylenediamine, 1,3-propylenediamine, (R,R)-1,2-diphenyl-1,2-diaminoethane and (S,S)-1,2-disopropyl-1,2-diaminoethane. Some complexes could also be obtained conveniently in a one-pot template synthesis. Single crystal X-ray diffraction studies of the complexes revealed a trans distorted octahedral structure around the iron. The iPr or Ph substituents on the diamine were found to be axial in the five-membered Fe-N-CHR-CHR-N- ring of the chiral P-N-N-P ligands and this was explained as a steric effect. The new iron compounds were used as precatalysts for the hydrogenation of acetophenone. The complexes without axial substituents on the diamine had moderate catalytic activity while that with axial Ph substituents had low activity but fair (61%) enantioselectivity for the asymmetric hydrogenation of acetophenone. The fact that the diaminodiphosphine complex has a slightly higher activity than the corresponding diiminodiphosphine complex suggests that hydrogenation of the imine groups in the P-N-N-P ligand may be important for catalyst activation.
Iron(II) carbonyl compounds of the type trans-[Fe(NCMe)(CO)(P-N-N-P)]2+ bearing diiminodiphosphine ligands derived from ethylenediamine, (S,S)- or (R,R)-1,2-diphenyl-1,2-diaminoethane were synthesized and characterized, including crystal structures. The new complexes are pre-catalysts for the transfer hydrogenation of ketones at room temperature, giving turnover frequencies up to 2600 h-1, with low catalyst loadings (0.025-0.17%). Screening of a wide range of substrates showed that the precatalysts are able to produce alcohols from a wide range of simple ketones. Excellent enantioselectivities of up to 96% ee were obtained in the reduction of sterically demanding prochiral ketones.
