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.

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.

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By using a copper transmetalation reagent [Cu­(Kaibene)2]­I, the NHC ligand (S,S)-MeNC3H2NCHPhCHPhNH2 “Kaibene” was transferred to ruthenium to make a precatalyst [RuCp*­(Kaibene)­(MeCN)]­(PF6) (Cp* = 1,2,3,4,5-pentamethyl­cyclopenta­dienyl), 7, in high yield as a mixture of two diastereomers. Under relatively mild conditions (0.02 mol % Ru, 0.16 mol % KOtBu, iPrOH, 50 °C, 25 bar of H2), this compound catalyzes the hydrogenation of aryl ketones and one alkyl ketone effectively with excellent activity and productivity (TOF up to 48 s–1, TON up to 104). At higher hydrogenation pressure (46 bar), the catalytic hydrogenation of N-phenyl-benzylimine to the corresponding amine is efficiently achieved. The hydrogenation of prochiral ketones resulted in low ee (35% for 4-chloroacetophenone). NMR spectroscopy was used to observe diastereomeric hydrides RuCp*­(Kaibene)­(H) 13- R/S that were generated by reaction of 7 with H2 and base in THF-d 8. Complementary DFT studies suggest that either the heterolytic splitting of dihydrogen to form 13- R/S or the hydride transfer to the substrate can be rate-determining depending on the substrate. Experimental and computational results support mechanisms that involve the heterolytic splitting of dihydrogen to the nitrogen of the amide-ligated form of Kaibene in THF or the heterolytic splitting to an outer-sphere alkoxide derived from the product alcohol or 2-PrOH solvent. An unusual feature is the rapid drop in ee of the product alcohol from as high as 60% (R) to 0% in some cases; this might be due to racemization of the Kaibene ligand in THF caused by the strong base or competitive inhibition of one diastereomer of the catalyst by reaction with the product (R)-alcohol.

The imidazolium salt [(S,S)-tBuNC3H3NCHPhCHPhNH2]­PF6, (S,S)-11·HPF6 is a precursor to the enantiopure “Kaibene” ligand, tBu-Kaibene, (S,S)-11 featuring a tert-butyl group on the N-heterocyclic carbene (NHC) ring-nitrogen atoms. It has been prepared in high yield and purity by refluxing a chiral cyclic sulfamidate with 1-tert-butylimidazole. Similarly (S,S)-12·HPF6 with a mesityl group at the imidazolium ring-nitrogen atom has been prepared in the same fashion and serves as a source of Mes-Kaibene, (S,S)-12. These bidentate Kaibene ligands feature an NHC and a primary amine separated by a chiral linker. Salts (S,S)-11·HPF6 or (S,S)-12·HPF6 react with base and AgI or CuI to give a total of four M­(Kaibene)2I compounds (M = Ag or Cu). At 22 °C, the amine-functionalized imidazolium cations undergo oxidative addition to iridium­(I) in [IrCl­(cod)]2 (cod = 1,5-cyclooctadiene) to generate iridium­(III) hydride R-Kaibene compounds [IrHCl­(cod)­((S,S)-11)]­(PF6) (17) and [IrHCl­(cod)­((S,S)-12)]­(PF6) (18), respectively, each as a mixture of six configurational isomers. In contrast, the salt (S,S)-11·HPF6 reacts with [Ir­(OtBu)­(cod)]2 to produce a bimetallic iridium compound with (S,S)-11 as the bridging ligand. This compound contains interesting NH···Cl and NH···Ir noncovalent intramolecular interactions. Salt (S,S)-12·HPF6 reacts with silver oxide to yield [Ag2((S,S)-12)2]­(PF6)2 (20). Reagent 20 serves as an efficient transmetalation reagent to deliver to each rhodium in [RhCl­(cod)]2 1 equiv of (S,S)-12 as a bidentate ligand to give [Rh­(cod)­((S,S)-12)]­(PF6). In the reaction between [IrCl­(cod)]2 and 20, (S,S)-12 ends up coordinated in an iridium­(III) hydride complex (22) as a tridentate ligand via the NHC, NH2, and a cyclometalated phenyl group. The two iridium hydride compounds, 18 and 22, are catalysts for the hydrogenation of a range of ketones (turnover number up to 499, turnover frequency up to 249 h–1, with er (enantiomeric ratio) up to 35:65 R:S).

Iron(II) Complexes Containing Unsymmetrical P-N-P' Pincer Ligands for the Catalytic Asymmetric Hydrogenation of Ketones and Imines. -The designed title complexes are successfully applied in the reduction of several aliphatic and aromatic substituted ketones and imines with moderate to high enantioselectivities. -(LAGADITIS, P. O.; SUES, P. E.; SONNENBERG, J. F.; WAN, K. Y.; LOUGH, A. J.; MORRIS*, R. H.; J. Am. Chem. Soc. 136 (2014) 4, 1367-1380, http://dx.doi.org/10.1021/ja4082233 ; Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 3H6, Can.; Eng.) -B. Voigt 34-093