This is an Account of our development of iron-based catalysts for the asymmetric transfer hydrogenation (ATH) and asymmetric pressure hydrogenation (AH) of ketones and imines. These chemical ...processes provide enantiopure alcohols and amines for use in the pharmaceutical, agrochemical, fragrance, and other fine chemical industries. Fundamental principles of bifunctional reactivity obtained by studies of ruthenium catalysts by Noyori’s group and our own with tetradentate ligands with tertiary phosphine and secondary amine donor groups were applied to improve the performance of these first iron(II) catalysts. In particular the correct positioning of a bifunctional H–Fe–NH unit in an iron hydride amine complex leads to exceptional catalyst activity because of the low energy barrier of dihydrogen transfer to the polar bond of the substrate. In addition the ligand structure with this NH group along with an asymmetric array of aryl groups orients the incoming substrate by hydrogen-bonding, and steric interactions provide the hydrogenated product in high enantioselectivity for several classes of substrates. Enantiomerically pure diamines or diphenylphosphino-amine compounds are used as the source of the asymmetry in the tetradentate ligands formed by the condensation of the amines with dialkyl- or diaryl-phosphinoaldehydes, a synthesis that is templated by Fe(II). The commercially available ortho-diphenylphosphinobenzaldehyde was used in the initial studies, but then diaryl-phosphinoacetaldehydes were found to produce much more effective ligands for iron(II). Once the mechanism of catalysis became clearer, the iron-templated synthesis of (S,S)-PAr2CH2CH2NHCHPhCHPhNH2 ligand precursors was developed to specifically introduce a secondary amine in the precatalyst structures. The reaction of a precatalyst with strong base yields a key iron–amido complex that reacts with isopropanol (in ATH) or dihydrogen (in AH) to generate an iron hydride with the Fe–H bond parallel to the secondary amine N–H. In the AH reactions, the correct acidity of the intermediate iron–dihydrogen complex and correct basicity of the amide are important factors for the heterolytic splitting of the dihydrogen to generate the H–Fe–N–H unit; the acidity of dihydrogen complexes including those found in hydrogenases can be estimated by a simple additive ligand acidity constant method. The placement of the hydridic–protonic Fe–H···HN interaction in the asymmetric catalyst structure influences the enantioinduction. The sense of enantioinduction is predictable from the structure of the H–Fe–N–H-containing catalyst interacting with the ketone in the same way as related H–Ru–N–H-containing catalysts. The modular construction of the catalysts permits large variations in order to produce alcohol or amine products with enantiomeric excess in the 90–100% range in several cases.
Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted–Lowry acids. A simple additive ligand acidity constant equation (LAC for short) ...allows the estimation of the acid dissociation constant K a LAC of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pK a LAC and pK a THF, pK a DCM, pK a MeCN for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pK a LAC −18) to least acidic (pK a LAC 50). Figures are constructed showing metal acids above the solvent pK a scales and organic acids below to summarize a large amount of information. Acid–base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.
The conventional homogeneous catalysts for the enantioselective hydrogenation or transfer hydrogenation of ketones are based on platinum metals and, in particular, ruthenium. This method provides ...valuable enantiopure alcohols for the fine chemical industries. This tutorial review summarizes recent successes in replacing expensive and toxic ruthenium in these catalysts with "greener" iron substitutes including my lab's recent progress in this area using iron complexes containing readily-prepared tetradentate ligands. It will enlighten chemists interested in homogeneous catalysis and asymmetric synthesis.
A simple equation (pK a THF = ∑A L + C charge + C nd + C d6) can be used to obtain an estimate of the pK a of diamagnetic transition metal hydride and dihydrogen complexes in tetrahydrofuran, and, by ...use of conversion equations, in other solvents. It involves adding acidity constants A L for each of the ligands in the 5-, 6-, 7-, or 8-coordinate conjugate base complex of the hydride or dihydrogen complex along with a correction for the charge (C charge = −15, 0 or 30 for x = +1, 0 or −1 charge, respectively) and the periodic row of the transition metal (C nd = 0 for 3d or 4d metal, 2 for 5d metal) as well as a correction for d6 octahedral acids (C d6 = 6 for d6 metal ion in the acid, 0 for others) that are not dihydrogen complexes. Constants A L are provided for 13 commonly occurring ligand types; of these, nine neutral ligands are correlated with Lever’s electrochemical ligand parameters E L. This method gives good estimates of the over 170 literature pK a values that range from less than zero to 50 with a standard deviation of 3 pK a units for complexes of the metals chromium to nickel, molybdenum, ruthenium to palladium, and tungsten to platinum in the periodic table. This approach allows a quick assessment of the acidity of hydride complexes found in nature (e.g., hydrogenases) and in industry (e.g., catalysis and hydrogen energy applications). The pK a values calculated for acids that have bulky or large bite angle chelating ligands deviate the most from this correlation. The method also provides an estimate of the base strength of the deprotonated form of the complex.
The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of ...a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the p
K
LAC
a
(LAC is ligand acidity constant) of the theoretical hydride complexes H-M-L
+
formed by the protonation of pair of valence
d
electrons on the metal in the M-L complex. Preliminary findings indicate that a negative p
K
LAC
a
indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive p
K
LAC
a
indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η
6
-arene.
The power of transition metal ions and their ligands to reverse the native reactivity of small molecules is highlighted by providing examples for 15 classes of ligands including types L, XL, X, X
2
, and X
3
.
A rational approach is needed to design hydrogénation catalysts that make use of Earth-abundant elements to replace the rare elements such as ruthenium, rhodium, and palladium that are traditionally ...used. Here, we validate a prior mechanistic hypothesis that partially saturated amine(imine) diphosphine ligands (P-NH-N-P) activate iron to catalyze the asymmetric reduction of the polar bonds of ketones and imines to valuable enantiopure alcohols and amines, with isopropanol as the hydrogen donor, at turnover frequencies as high as 200 per second at 28°C. We present a direct synthetic approach to enantiopure ligands of this type that takes advantage of the iron(ll) ion as a template. The catalytic mechanism is elucidated by the spectroscopic detection of iron hydride and amide intermediates.
This perspective reviews our efforts to use a mechanism-based approach to develop catalysts for the asymmetric hydrogenation of prochiral ketones and imines. A goal is to discover catalysts based on ...the abundant 3d transition metals, particularly iron.
Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many ...industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.
Abstract
The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of ...recent research efforts, “solar methane” can now be produced through the photocatalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane.
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