Acceptor-substituted vinyl- (VCP) and arylcyclopropanes (ACP) are reactive substrates for nucleophilic activation using Bu₄N[Fe(CO)₃(NO)] (TBA[Fe]). In this account, the application of this catalytic strategy in traceless allylic substitution, [3+2]-cycloaddition, and Cloke-Wilson rearrangement will be presented. Additional information, derived from detailed spectroscopic and theoretical investigations, is discussed. These results not only lead to a deeper understanding of the reactivity of the catalyst in VCP and ACP activation, but also set the stage for a new type of catalyst activation using photochemical irradiation at 415 nm.

Der nukleophile Fe-Komplex Bu₄N[Fe(CO)₃(NO)] (TBA[Fe]) katalysiert die direkte intramolekulare C-H-Aminierung von α-Azidobiarylen oder (Azidoaryl)alkenen zu den entsprechenden Carbazolen und Indolen unter milden Reaktionsbedingungen mit geringen Mengen an Katalysator. Dies und die breite Toleranz für funktionelle Gruppen machen dieses Protokoll zu einer attraktiven Alternative zu den etablierten Edelmetall-basierenden Verfahren.

The nucleophilic iron complex Bu₄N[Fe(CO)₃(NO)] (TBA[Fe]) catalyzes the direct intramolecular C−H amination of α-azidobiaryls and (azidoaryl)alkenes into the corresponding carbazoles and indoles, respectively, under mild conditions and with low catalyst loadings. These features and the broad functional-group tolerance render this method a particularly attractive alternative to established noble-metal-based procedures.

Iron hydride complexes of the general formula P₂Fe(NO)CO)H are highly active catalysts for the hydrosilylation of aldehydes or ketones and phosphine oxides. Depending on the solvent, the in situ reduction of the phosphine oxide can be faster than the corresponding hydrosilylation of a carbonyl group. This unusual activity was used within the context of catalytic Wittig olefination.

A short total synthesis of (±)-garcinol and (±)-isogarcinol, two endo-type B PPAPs with reported activity against methiciline resistant Staphylococcus aureus (MRSA), is presented. The separation of framework-constructing from framework-decorating steps and the application of two highly regio- and stereoselective Pd-catalysed allylations, that is, the Pd-catalysed decarboxylative Tsuji–Trost allylation and the diastereoselective Pd-catalysed allyl–allyl cross-coupling, are key elements that allowed the total synthesis to be accomplished within 13 steps starting from acetylacetone. After separation of the enantiomers the absolute configurations of the four natural products (i.e., (−)-garcinol, (+)-guttiferone E (i.e., ent-garcinol), (−)-isogarcinol, and (+)-isoxanthochymol (i.e., ent-isogarcinol)) were assigned based on ECD spectroscopy.

Außer der wirtschaftlichen Energiegewinnung sind auch die effiziente Speicherung und Freisetzung von Energie wesentliche Anforderungen beim Aufbau einer nachhaltigen Energieversorgung. Wir berichten hier über die Entwicklung einer wiederaufladbaren Wasserstoffbatterie, die auf dem Prinzip einer Ru-katalysierten Hydrierung von CO₂ zu Ameisensäure (Aufladevorgang) und einer Ru-katalysierten Zersetzung der Ameisensäure zu CO₂ und H₂ (Entladevorgang) beruht. Beide Prozesse laufen mit dem gleichen Katalysator unter identischen Bedingungen bei hoher Temperatur unter Druck (Aufladevorgang) bzw. bei hoher Temperatur unter druckfreien Bedingungen (Entladevorgang) ab. Bis zu fünf Lade-Entlade-Zyklen können ohne Verlust an Speicherkapazität durchlaufen werden; das entstehende CO₂/H₂-Gemisch ist CO-frei und kann direkt in der Brennstoffzelle verwendet werden.

Fe-katalysierte Reaktionen haben sich in den vergangenen 10 Jahren fest in der organischen Synthese etabliert. Das ursprünglich von Hogsed und Hieber beschriebene komplexe Ferrat [Fe(CO)₃(NO)]⁻ zeigt katalytische Aktivität in zahlreichen organischen Reaktionen und wird allgemein als isoelektronisch zum [Fe(CO)₄]²⁻ betrachtet. Allerdings zeigt der letztgenannte Komplex kaum katalytische Aktivität. Die hier vorgestellten spektroskopischen und quantenchemischen Untersuchungen zeigen, dass das komplexe Ferrat [Fe(CO)₃(NO)]⁻ nicht wie angenommen als eine Fe^−II-Spezies, sondern vielmehr als eine Fe⁰-Spezies, in der das Metall kovalent über zwei π-Bindungen an ein NO⁻ gebunden ist, aufgefasst werden sollte. Eine Metall-N-σ-Bindung wird nicht beobachtet.

Apart from energy generation, the storage and liberation of energy are among the major problems in establishing a sustainable energy supply chain. Herein we report the development of a rechargeable H₂ battery which is based on the principle of the Ru-catalyzed hydrogenation of CO₂ to formic acid (charging process) and the Ru-catalyzed decomposition of formic acid to CO₂ and H₂ (discharging process). Both processes are driven by the same catalyst at elevated temperature either under pressure (charging process) or pressure-free conditions (discharging process). Up to five charging–discharging cycles were performed without decrease of storage capacity. The resulting CO₂/H₂ mixture is free of CO and can be employed directly in fuel-cell technology.

During the past 10 years iron-catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)₃(NO)]⁻, which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)₄]²⁻, which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)₃(NO)]⁻ cannot be regarded as a Fe^−II species, but rather is predominantly a Fe⁰ species, in which the metal is covalently bonded to NO⁻ by two π-bonds. A metal–N σ-bond is not observed.

A series of defined iron–hydrogen complexes was prepared in a straightforward one-pot approach. The structure and electronic properties of such complexes were investigated by means of quantum-chemical analysis. These new complexes were then applied in the dehydrogenative silylation of methanol. The complex (dppp)(CO)(NO)FeH showed a remarkable activity with a TOF of more than 600 000 h⁻¹ of pure hydrogen gas within seconds.

A quantum chemical investigation of the Bu₄N[Fe(CO)₃(NO)]-catalyzed Cloke–Wilson rearrangement of vinyl cyclopropanes is reported. It was found that allylic CC bond activation can proceed through a S_N2′ or S_N2-type mechanism. The application of the recently reported intrinsic bond orbital (IBO) method for all structures indicated that one FeN π bond is directly involved. Further analysis showed that during the reaction oxidation occurs at the NO ligand exclusively.

A defined (P^N^N^P)–Ru complex possessing tertiary amines within the ligand backbone proved to be highly active both in transfer hydrogenations and hydrogenations of a variety of ketones. As compared to the existing catalytic systems, no bifunctional activation of H₂ or of the substrate by the metal center and a secondary amine within the ligand backbone is required to obtain high activities at catalyst loadings of down to 10 ppm.

Triazolium-derived N-heterocyclic carbene (aNHC) ligands, which are readily accessible by deprotonation of the corresponding triazolium salts, proved to be versatile ligands in diverse allylic substitution reactions. The corresponding triazolium salts are formed from azides and alkynes through the application of 1,3-dipolar cycloaddition and N-alkylation reactions. The unique property of these ligands is to be zwitterionic in their liberated form and to act as strong redox-active σ-donor ligands. By virtue of these qualities, these ligands enable the development of an unprecedented Fe-catalyzed regioselective aryloxylation of allylic carbonates.

Phosphonates are useful building blocks for organic synthesis, and are also bioactive compounds. Traditionally, they are prepared by nucleophilic substitution under thermal conditions. The preparation of more densely functionalized phosphonates is still a synthetic challenge. In this paper, we present a convenient one-pot three-component phosphono-allylation of activated olefins to give phosphonates bearing a variety of functional groups.

A growing world population and increasing energy demands are without a doubt the most important challenges for mankind within this century. Catalysis is a key technology in various fields of chemistry and offers the potential to increase the material output of chemical synthesis without an unreasonable increase in the energy necessary for the production of new materials. Hence, the various disciplines of catalysis are “sustainable” by definition. However, for a wide range of catalytic transformations to take the necessary next step from academic research to industrial application, the catalytic processes do not only have to be sustainable with regard to the substrate-to-product conversions, but they also have to be sustainable with regard to parameters such as solvent, energy source, and the nature of the catalyst. Currently, the vast majority of transition-metal-catalyzed homogenous reactions are based upon late and expensive transition-metal complexes. In the present review we summarize the current state of the art in sustainable metal catalysis, that is, catalysis based upon inexpensive biorelevant metals such as Ca, Mg, V, Mo, Mn, Fe, Co, Ni, Cu, Zn, B, Si, and Se. These metals are part of nature’s catalytic toolbox and have experienced a tremendous comeback in metal catalysis within the past 10 years.

A series of new hexacoordinated {Ru^II(NNNN,P)} complexes was prepared from [RuCl₂(R₃P)₃]. Their structure was determined by X-ray crystallography. The catalytic potential of this new class of complexes was tested in the alkylation of aniline with benzyl alcohol. In this test reaction, the influence of the counteranion plus electronic influences at the tetradentate ligand and the phosphine ligand were examined. The electrochemistry of all complexes was studied by cyclic voltammetry. Depending on the substituent at the ligand backbone, the complexes showed a different behavior. For all N-benzyl substituted complexes, reversible Ru^II/III redox potentials were observed, whereas the N-methyl substituted complex possessed an irreversible oxidation event at small scan rates. Furthermore, the electronic influence of different substituents at the ligand scaffold and at the phosphine on the Ru^II/III redox potential was investigated. The measured E₀ values were correlated to the theoretically determined HOMO energies of the complexes. In addition, these HOMO energies correlated well with the reactivity of the single complexes in the alkylation of aniline with benzyl alcohol. The exact balance of redox potential and reactivity appears to be crucial for synchronizing the multiple hydrogen-transfer events. The optimized catalyst structure was applied in a screening on scope and limitation in the catalytic dehydrative alkylation of anilines by using alcohols.

We present herein a versatile and broadly applicable Fe-catalyzed regioselective alkoxy allylation of activated double bonds. Substituted allylic carbonates are converted into the corresponding σ-enyl Fe complexes by reaction with Bu₄N[Fe(CO)₃(NO)] (TBAFe) at 30 °C. The liberated alkoxide adds to an activated double bond with the generation of a C-nucleophile, which is trapped by the σ-enyl Fe complex in a regioselective manner. Alternatively, the alkoxide acts as a base in deprotonating an external pronucleophile, which undergoes Michael addition. The method is characterized by a broad functional group tolerance, mild reaction conditions, low catalyst loadings, and high regioselectivities in favor of the ipso-substitution product.

The nucleophilic Fe complex Bu₄N[Fe(CO)₃(NO)] (TBAFe) has been used as a highly active catalyst for the mild hydrosilylation of a variety of functionalized aldehydes and ketones using inexpensive PMHS as stoichiometric reductant. The corresponding alcohols were obtained in good-to-excellent yields at low catalyst loadings of only 1 mol-% and reaction temperatures of 30–50 °C.

Allyl sulfones are versatile intermediates in organic chemistry. The presence of two distinct functional groups sets the stage for a plethora of subsequent transformations. However, despite these advantages the preparation of regioisomerically enriched sulfones is not easy. The use of sulfinate salts as nucleophiles in substitutions is frequently accompanied by side reactions such as π-bond migration, β-elimination, and so on. Herein we present a preparatively simple way to synthesize a variety of different aryl or alkyl allyl sulfones starting from readily accessible allylic carbonates. By employing aryl or alkyl α-sulfonyl succinimides as sulfinate synthons, mild and regioselective ipso substitution of diverse allylic carbonates was realized.

By using a readily available, air- and moisture-stable dihydrido–Ru complex, a variety of Z olefins are accessible under transfer-hydrogenation conditions with formic acid as the hydrogen source in excellent yields and Z/E selectivities.

Nowadays demand for selective, energy-efficient, and sustainable chemical transformations has spurred an increasing interest in the development of “sustainable metal catalysis”. This expression defines a type of catalytic transformation in which non-toxic, readily available and inexpensive, stable metal complexes are used for catalysis. The increasing prices for energy and noble metals, which are commonly used for catalysis, represent an economical and ecological dilemma. If the price for a catalyst exceeds the savings on the energy side an industrial application does not make sense. As a consequence of this dilemma, chemists are looking for exit strategies with catalysis by small organic molecules (organocatalysis) or by inexpensive, readily available metal complexes (sustainable metal catalysis) being the most prominent ones. It is an irony that these two major catalytic strategies are based on research that had been initiated several decades ago but was somehow forgotten. In the present Microreview, the story of the reincarnation of another forgotten metal complex species, that celebrates its 50th birthday this year, will be told, the [Fe(CO)₃(NO)]^– anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)