Iron Complexes for the Catalytic Reduction of Polar Double Bonds

A series of new iron complexes featuring multi-dentate ligands containing phosphorus and nitrogen donor atoms were synthesized using a multi-component metal template synthesis. This was achieved by using convenient air-stable phosphonium dimers, [cyclo-(PR2CH2CH(OH)-)2][Br]2, which upon addition of base are cleaved to produce useful phosphino-aldehyde synthons. The phosphonium dimers were versatile in the template synthesis with KOtBu, [Fe(H2O)6][BF4]2 and the appropriate amine of iron(II) complexes bearing mertridentate P-N-N or P-N-S or P-N-Pʹ ligands and tetradentate P-N-N-P ligands. Crude solutions of trans-[Fe(NCMe)2(P-N-N-P)][BPh4]2 were stirred with KBr under a CO atmosphere overnight to produce pre-catalysts trans-[Fe(CO)(Br)(P-N-N-P)][BPh4]. The complexes were tested for the catalytic transfer hydrogenation of acetophenone in basic isopropanol. Only catalysts with R = Et and a CO ligand were found to be active. Examination of the role of base in pre-catalyst activation, revealed a five-coordinate iron(II) complex in which the tetradentate P-N-N-P ligand has been doubly deprotonated specifically at the carbons adjacent to the phosphorus atoms. These iron(II) ene-amido complexes were used in the transfer hydrogenation of acetophenone in isopropanol in base free conditions and had identical catalytic activity to that of the pre-catalysts with added base. A new iron pre-catalyst, [Fe(CO)(Br)(P-NH-N-P)][BPh4], was synthesized that features an imine-amine moiety on the tetradentate ligand such that this pre-catalyst produces intermediates even closer to the actual catalytic cycle. New iron complexes mer-trans-[Fe(Br)(CO)2(P-N-Pʹ)][BF4] were prepared by the template synthesis under a CO atmosphere using a phosphino-amine followed by reaction with AgBF4. These complexes were then reacted with LiAlH4 and alcohol to form mono-hydride iron pre-catalysts. The iron pre-catalysts that had alkyl or phenyl substituents on the phosphorus atoms of the P-N-Pʹ ligand were active for the H2 hydrogenation of ketones, aldehydes and imines under mild basic conditions (T = 50 °C, p(H2) = 5 atm). Catalytic activities of TOF up to 2000 h-1, TON up to 4000 and enantioselectivity up to 85% (S) were observed.Ph

Asymmetric catalysis using iron complexes : 'ruthenium lite'?

A review of recent developments in the use of iron catalysts for asymmetric transformations, including hydrogenations, transfer hydrogenation, hydrosilylation and oxidation reactions

Synthesis of new late transition metal P,P-, P,N-, and P,O- complexes using phosphonium dimers as convenient ligand precursors

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/ic4003753.The phosphonium dimer [-Cy2PCH(OH)CH2-]2(X)2, X = Cl(-), Br(-) was used to synthesize and characterize a variety of late transition metal complexes containing chelating phosphino-enolate (PCy2CH═CHO(-)), imine (PCy2CH2CH═NR, R = Ph, (S)-CHMePh), and oxime (PCy2CH2CH═NOH) ligands. The phosphonium dimer, when deprotected with base, generates the phosphine aldehyde PCy2CH2CHO in situ, which, in the presence of [M(COD)Cl]2, M = Rh, Ir, and a PF6(-) salt, or [Ni(H2O)6][BF4]2, facilitates a condensation reaction with an amine or hydroxylamine to form phosphino-imine or phosphino-oxime metal complexes [M(COD)(P-N)][PF6] or [Ni(P-N)2][X]2, X = ClO4(-), BF4(-), respectively. In the absence of an amine, phosphino-enolate containing complexes are formed. A neutral Ni(II) complex Ni(PCy2CH═CHO)2 with trans-bis(phosphino-enolate) ligands which resemble ligands used on nickel for olefin oligomerization, as well as neutral Rh(I) and Ir(I) 1,5-cyclooctadiene complexes M(COD)(PCy2CH═CHO) are characterized. Both the rhodium and iridium complexes are active olefin hydrogenation catalysts. Reaction of the phosphino-aldehyde with Pt(COD)Cl2 results in the formation of trans-PtCl2(PCy2CH2CHO)2 with pendant aldehyde groups, and under certain conditions, they undergo an intraligand aldol condensation to form a disphosphine ligand

Exploring the decomposition pathways of iron asymmetric transfer hydrogenation catalysts

The final published version of this article is available from the Royal Society of Chemistry at http://dx.doi.org/10.1039/C4DT02799JOur group has developed a series of iron-based asymmetric transfer hydrogenation (ATH) catalysts for the reduction of polar double bonds. The activation of the precatalysts as well as the catalytic mechanism have been thoroughly investigated, but the decomposition pathways of these systems are poorly understood. Herein, we report a study of the deactivation pathways for an iron ATH catalyst under catalytically relevant conditions. The decomposition pathways were examined using experimental techniques and density functional theory (DFT) calculations. The major decomposition products that formed, Fe(CO)((Et)2PCH2CH2CHCHNCH2CH2P(Et)2) (3a) and Fe(CO)((Et)2PCH2CH2C(Ph)C(Ph)NCH2CH2P(Et)2) (3b), had two amido donors as well as a C=C bond on the diamine backbone of the tetradentate ligand. These species were identified by NMR studies and one was isolated as a bimetallic complex with Ru(II)Cp*. Two minor iron hydride species also formed concurrently with 3a, as determined by NMR studies, one of which was isolated and contained a fully saturated ligand as well as a hydride ligand. None of the compounds that were isolated were found to be active ATH catalysts

Developing asymmetric iron and ruthenium-based cyclone complexes : complex factors influence the asymmetric induction in the transfer hydrogenation of ketones

The preparation of a range of asymmetric iron and ruthenium-cyclone complexes, and their application to the asymmetric reduction of a ketone, are described. The enantioselectivity of ketone reduction is influenced by a single chiral centre in the catalyst, as well as by the planar chirality in the catalyst. This represents the first example of asymmetric ketone reduction using an iron cyclone catalyst