Two polymorph modifications of tris(hexafluoroacetylacetonato)iron(III) revealed: is that common for other trivalent metals?

A long-standing issue about the correct identification of an important starting reagent, iron(III) hexafluoroacetylacetonate, Fe(hfac)3 (1), has been resolved. The tris-chelated mononuclear complex was found to crystallize in two polymorph modifications which can be assigned as the low-temperature (1-L) monoclinic P21/n and the high-temperature (1-H) trigonal P-3. Low-temperature polymorph 1-L was found to transform to 1-H upon sublimation at 44 °C. Two modifications are clearly distinguished by powder X-ray diffraction (PXRD), single-crystal X-ray diffraction, differential scanning calorimetry (DSC), and melting-point measurements. On the other hand, the two forms share similar characteristics in direct analysis in real-time mass spectrometry (DART-MS), attenuated total reflection (ATR) spectroscopy, and some physical properties, such as color, volatility, sensitivity, and solubility. Analysis of the literature and some of our preliminary data strongly suggest that the appearance of two polymorph modifications for trivalent metal (both transition and main group) hexafluoroacetylacetonates is a common case for several largely used complexes not yet accounted for in the crystallographic databases.

Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex

The epoxidation of olefins by substituting "air" for potentially harmful oxidants was achieved using an oxidation method that integrated a fluorous iron(III) salen catalyst derived from common metals and pivalaldehyde. Several aromatic disubstituted olefins were converted into their corresponding epoxides with high efficiency and quantitative yields. This reaction represents an environmentally friendly oxidation process that utilizes an abundant source of air and employs a readily available metal, iron, in the form of salen complexes, making it an environmentally conscious oxidation reaction.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Iron-Catalyzed Wacker-type Oxidation of Olefins at Room Temperature with 1,3-Diketones or Neocuproine as Ligands*

Herein, we describe a convenient and general method for the oxidation of olefins to ketones using either tris(dibenzoylmethanato)iron(III) [Fe(dbm)₃ ] or a combination of iron(II) chloride and neocuproine (2,9-dimethyl-1,10-phenanthroline) as catalysts and phenylsilane (PhSiH₃ ) as additive. All reactions proceed efficiently at room temperature using air as sole oxidant. This transformation has been applied to a variety of substrates, is operationally simple, proceeds under mild reaction conditions, and shows a high functional-group tolerance. The ketones are formed smoothly in up to 97 % yield and with 100 % regioselectivity, while the corresponding alcohols were observed as by-products. Labeling experiments showed that an incorporated hydrogen atom originates from the phenylsilane. The oxygen atom of the ketone as well as of the alcohol derives from the ambient atmosphere.

Synthesis of Sterically Hindered β-Diketones via Condensation of Acid Chlorides with Enolates

Bulky β-diketones have rarely exceeded dipivaloylmethane (DPM) in steric demand, largely due to synthetic limitations of the Claisen condensation. This work demonstrates hindered acid chlorides to be selective electrophiles in noncoordinating solvents for condensations with enolates. An improved synthesis of DPM is described (90% yield), and crowded β-diketones featuring bulky o-biphenyl or m-terphenyl fragments were prepared in good to excellent yields. These compounds are anticipated to have a steric profile far greater than that of DPM. General reaction conditions and mechanistic considerations are included.

Applications of the Marcus cross relation to inner sphere reduction of O2: implications in small-molecule activation

The Marcus cross relation is demonstrated to be applicable to inner sphere electron transfer from iron to molecular oxygen by incorporation of the Fe(iii)–O₂to Fe(iii) + superoxide BDFE inK_eq. A few case-studies are provided as working examples.

Ring-opening hydrofluorination of 2,3- and 3,4-epoxy amines by HBF4·OEt2: application to the asymmetric synthesis of (S,S)-3-deoxy-3-fluorosafingol

Treatment of a range of 2,3- and 3,4-epoxy amines with HBF(4)·OEt(2) at room temperature results in fast and efficient S(N)2-type ring-opening hydrofluorination to give stereodefined amino fluorohydrins. Operational simplicity, scalability, and short reaction time at ambient temperature are notable features of this method. The utility of this methodology is exemplified in a concise asymmetric synthesis of (S,S)-3-deoxy-3-fluorosafingol.

Alternative selective oxidation pathways for aldehyde oxidation and alkene epoxidation on a SiO2-supported Ru-monomer complex catalyst

We have prepared a novel Ru-mononer complex supported on a SiO(2) surface by using a Ru-monomer complex precursor with a p-cymene ligand, which was found to be highly active for the selective oxidation of aldehydes and the epoxidation of alkenes using O(2). The structure of the supported Ru catalyst was characterized by means of FT-IR, solid-state NMR, diffuse-reflectance UV/vis, XPS, Ru K-edge EXAFS, and DFT calculations, which demonstrated the formation of isolatedly located, unsaturated Ru centers behind a p-cymene ligand of the Ru-complex precursor. The site-isolated Ru-monomer complex on SiO(2) achieved tremendous TONs (turnover numbers) for the selective oxidation of aldehydes and alkenes; e.g. TONs of 38,800,000 for selective isobutyraldehyde (IBA) oxidation and 2,100,000 for trans-stilbene epoxidation at ambient temperature, which are among the highest TONs in metal-complex catalyzes to our knowledge. We also found that the IBA sole oxidation with an activation energy of 48 kJ mol(-1) much more facile than the trans-stilbene epoxidation with an activation energy of 99 kJ mol(-1) was completely suppressed by the coexistence of trans-stilbene. The switchover of the selective oxidation pathways from the IBA oxidation to the trans-stilbene epoxidation was explained in terms of energy profiles for the alternative selective oxidation pathways, resulting in the preferential coordination of trans-stilbene to the Ru-complex at the surface. This aspect gives an insight into the origin of the efficient catalysis for selective epoxidation of alkenes with IBA/O(2).

Facile Aerobic Photo-Oxidation of Aldehydes in the Presence of Catalytic Lithium Bromide

Aldehydes were found to be oxidized to the corresponding carboxylic acid in the presence of catalytic lithium bromide under photo-irradiation.

Mechanistic Studies on the Mukaiyama Epoxidation

A detailed mechanistic study on the Mukaiyama epoxidation of limonene with dioxygen as oxidant, bis(acetylacetonato)nickel(II) as catalyst, and an aldehyde as co-reagent is reported. All major products of the reaction have been quantitatively identified, both with isobutyraldehyde and 2-methylundecanal as co-reacting aldehydes. Limonene epoxide is formed in good yield. The main products evolving from the aldehyde are carboxylic acid, CO(2), CO, and lower molecular weight ketone and alcohol (K + A). A mechanism is proposed in which an acylperoxy radical formed by the autoxidation of the aldehyde is the epoxidizing species. The observation of carbon dioxide and (K + A) in a 1:1 molar ratio supports this mechanism. CO(2) and (K + A) are formed in molar amounts of 50-60% with respect to the amount of epoxide produced, indicating that epoxidation takes place not only via acylperoxy radicals but also via a peracid route. Cyclohexene epoxidation was also investigated with a number of different metal complexes as catalysts. Cyclohexene is very sensitive for allylic oxidation, which provides information about the action of the catalyst, e.g., metals that form strongly oxidizing stable high-valence complexes are more likely to induce allylic oxidation. Color changes in the reaction mixture indicate the presence of such high-valence species. In the case of nickel, it was found that low-valence compounds predominate during the reaction, which is in line with the fact that this metal displays the highest selectivity for epoxide. A mechanism that accounts for the observations is presented.

Metal Complex-Catalyzed Epoxidation of Olefins by Dioxygen with Co-Oxidation of Aldehydes. A Mechanistic Study

Mechanistic studies of the oxidation of olefins by dioxygen plus aldehyde in the presence of metal complexes such as metalloporphyrins and metal cyclam complexes have been carried out. Epoxides were the predominant products, with trace amounts of allylic oxidation products. cis-Stilbene was oxidized to a mixture of cis- and trans-stilbene oxides. It is concluded from this study that the principal role of the metal complexes is to aid in the initiation step for the free radical autoxidation of the aldehyde and that acylperoxy radicals generated in the autoxidation reaction (or metal complexes formed by complexation of the acylperoxy radicals) are the active epoxidizing agents.