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.

A series of five (S,S)-trans-[Fe(CO)(Br)(PR2-CH2CHNCH(Ph)CH(Ph)NCHCH2-PR2)][X] compounds (1a–c, X = BPh4; 1d,e, X = BF4) were synthesized and tested for the asymmetric transfer hydrogenation (ATH) of acetophenone. Three of the complexes had methyl-substituted aryl groups (a, R = para-CH3C6H4; b, R = ortho-CH3C6H4; c, R = 3,5-(CH3)2C6H3), and two had trifluoromethyl-substituted aryl groups (d, R = para-CF3C6H4; e, R = 3,5-(CF3)2C6H3). Using both known and new phosphonium dimers, [cyclo-(PR2CH2CH(OH)−)2][Br]2 (2a–c), in a one-pot template reaction, the corresponding (S,S)-trans-[Fe(CH3CN)2(PR2-CH2CHNCH(Ph)CH(Ph)NCHCH2-PR2)][BPh4]2 complexes (3a–c) were generated and then converted to precatalysts 1a–c via CO addition reactions. While investigating compounds 1a–c, an alternative route for synthesizing phosphonium dimers was developed that allowed the facile introduction of tetrafluoroborate counterions. Compounds 1d and 1e could not be synthesized using previously developed methods; phosphinoacetaldehyde diethyl acetal precursors (5d, 5e) were isolated because trifluoromethyl-substituted phosphonium dimers did not form. Precursors 5d and 5e were incompatible with a base-catalyzed template approach, so a new acid-catalyzed template procedure was developed to generate the tetrafluoroborate salts (S,S)-trans-[Fe(CH3CN)2(PR2-CH2CHNCH(Ph)CH(Ph)NCHCH2–PR2)][BF4]2 (3d, 3e). Both 3d and 3e were converted to precatalysts 1d and 1e via CO addition reactions. Complexes 1b, 1d, and 1e were inactive for the ATH of acetophenone, while complexes 1a and 1c were active. Compound 1a showed very high activity, with a turnover frequency of 30 000 h–1 at 28 °C, and is currently the most active iron ATH catalyst. Compound 1c produced more enantiopure (R)-1-phenylethanol, with an ee of 90%, and is the most selective iron catalyst reported to date for the ATH of acetophenone. The activity of complexes 1a–e for ATH was compared to those of known complexes 1f (R = Ph), 1g (R = Et), 1h (R = i-Pr), and 1i (R = Cy), and the most active catalysts were defined by a narrow range of electronic (ν(CO)) as well as steric (Tolman cone angles) parameters.

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page.ABSTRACT: Our group previously reported the development of iron carbonyl catalysts bearing chiral tridentate P-N-P' ligands for the asymmetric hydrogenation of prochiral ketones in THF. An NMR study into the activation process identified the amine hydride alkoxide complexes Fe(P-NH-P')(CO)(H)(OR 1 ) with R 1 = Me, tBu or tAmyl and P-NH-P' = PPh2CH2CH2NHCH2CH2PiPr2 or (S,S)-PPh2CHPhCHMeNHCH2CH2PCy2. These still required treatment with excess KOtBu and H2(g) to be catalytically active in THF. Both experimental methods and Density Functional Theory (DFT) calculations were used to show that this treatment leads to the formation of a hydride amide complex Fe(P-N-P')(CO)(H) which reacts with dihydrogen to form cis and trans dihydride complexes Fe(P-NH-P')(CO)(H)2, identified by NMR spectroscopy. In the presence of KOtBu, NaOtBu or KOtBu/2,2,2-cryptand and H2(g), these species are active for the catalytic hydrogenation of acetophenone, while in the absence of H2(g), inactive Fe(0) complexes are formed. Ketone hydrogenation is proposed to occur in an outer sphere stepwise process and this enantio-determining step has been modeled by DFT. The calculations suggest that the energy barriers for either hydride attack on the ketone, or dihydrogen splitting either to the nitrogen of the amide complex in the inner coordination sphere or to the oxygen of an alkoxide group in the outer sphere are similar and that either hydride transfer or dihydrogen splitting could determine the turn-over frequency depending of the nature of the ketone.

The asymmetric reduction of ketones and imines by transfer of hydrogen from isopropanol as the solvent catalyzed by metal complexes is a very useful method for preparing valuable enantioenriched alcohols and amines. Described here is the development of three generations of progressively more active iron catalysts for this transformation. Key features of this process of discovery involved the realization that one carbonyl ligand was needed (as in hydrogenases), the synthesis of modular ligands templated by iron, the elucidation of the mechanisms of catalyst activation and action, as well as the rational synthesis of precursors that lead directly and easily to the species in the catalytic cycle. The discovery that iron, an abundant element that is essential to life, can form catalysts of these hydrogenation reactions is a contribution to green chemistry.

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