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

Abstract

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(-1)and 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.

Ketone Hydrogenation with Iridium Complexes with “non N–H” Ligands: The Key Role of the Strong Base

“…[5][6][7][8][9][10] Of these, the family of [RuCl2(diphosphine)(diamine)] molecules developed by Noyori, Ikariya, Ohkuma and coworkers stands out. [11][12][13][14] Efficient enantioselective catalysts based on rhodium, [15][16][17] iridium, [18][19][20][21][22][23][24][25][26] and iron [27][28][29][30][31][32][33][34][35][36] have also been reported in the literature. Interest in iridium as a catalytic metal is sparked by the observation that it outperforms rhodium for the ionic hydrogenation of particularly difficult substrates such as imines and industrial processes based on Ir-catalyzed ketone hydrogenation have been implemented.…”

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

“…The trans-H 2 configuration is supported by the characteristic 1 H NMR chemical shifts (−9.57 and −9.69 ppm for 3a and −9.29 and −9.37 ppm for 3b) and the mutual coupling constant of the hydride signals ( 2 J HH = 9.7 Hz for 3a and 9.5 Hz for 3b). 13 In the case of 1a, smaller amounts of the cis-H 2 complex 3a′ (Scheme 1) are also observed, based on the hydride chemical shifts (−8.63 and −21.13 ppm), hyperfine structure ( 2 J HH = 15 Hz), and NOESY pattern. 17 1 H EXSY NMR spectroscopy indicates intramolecular hydride exchange within 3a′ and intermolecular exchange of 3a and 3a′.…”

Iron-, Cobalt-, and Nickel-Catalyzed Asymmetric Transfer Hydrogenation and Asymmetric Hydrogenation of Ketones

“…46 13 When iron precatalysts containing unsymmetrical P−N−P′ pincer ligands were used, the hydrogenation of ketones proceeded at TOF up to 2000 h −1 and with up to 85% ee. Notably, the catalyst system must be activated by treatment with LiAlH 4 and then with alcohol.…”