Susana L.H. Rebelo

Oxidation of alkylaromatics with hydrogen peroxide catalysed by manganese(III) porphyrins in the presence of ammonium acetate

The oxidation of toluene (1), ethylbenzene (2) and cumene (3) with hydrogen peroxide, in the presence of several manganese(III) porphyrins with electron-withdrawing substituents, was studied using ammonium acetate as a co-catalyst. All products were characterised and their formation was justified by studying the oxidation of primary precursors, under the same conditions. The formation of the nitrate compounds was shown to be dependent on the presence of ammonium acetate. The oxidation of cumene and ethylbenzene afford products resulting from dehydrogenation reactions.

An efficient approach for aromatic epoxidation using hydrogen peroxide and Mn(III) porphyrins

Efficient epoxidation, in very high conversions and selectivities, of aromatic hydrocarbons with hydrogen peroxide, in the presence of Mn(III) porphyrins [Mn(TDCPP)Cl, Mn(beta NO(2)TDCPP)Cl, Mn(TPFPP)Cl] as catalysts is described; naphthalene and anthracene afford the anti-1,2:3,4-arene dioxides whereas with phenanthrene the 9,10-oxide is obtained.

Gold-supported magnetically recyclable nanocatalysts: a sustainable solution for the reduction of 4-nitrophenol in water

In this work mesoporous silica-coated manganese(II) ferrite (MnFe2O4) magnetic nanoparticles functionalized with amine and thiol groups were prepared and used as supports for the in situ immobilization of gold nanoparticles (Au NPs). The resulting Au-supported magnetic nanocatalysts, denoted as Mn@SiO2_NH2@Au and Mn@SiO2_SH@Au, were tested in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), at room temperature in water, in the presence of NaBH4 reducing agent. This organic compound is typically used in the production of pesticides and dyes and commonly found in the resulting wastewaters. The chemical, morphological, textural and magnetic properties of the nanosupports and resulting Au-supported nanocatalysts were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherms at −196 °C and SQUID magnetometry. The influence of the type of organosilane linker between the magnetic nanosupport and the Au NPs on the in situ immobilization of the Au NPs was evaluated: Mn@SiO2_NH2@Au presented lower Au loading than Mn@SiO2_SH@Au, but the anchored Au NPs showed a higher degree of crystallinity. The magnetic Au-supported nanocatalysts led to almost 100% reduction of 4-NP to 4-AP, monitored by UV-vis spectroscopy, with the reaction time depending on the type of nanocatalyst/linker: 12 and 17 min, for the amine- and thiol-based nanocatalysts respectively, what corresponded to pseudo first-order rate constants normalized for Au loading of K = 6117 mmol−1 min−1 and 827 mmol−1 min−1, respectively. Both catalysts could be efficiently recovered by magnetic separation and were highly stable upon reuse in four further cycles, preserving their catalytic performance with negligible Au leaching.

Green oxidation catalysis with metal complexes: from bulk to nano recyclable hybrid catalysts

Transition metal complexes are efficient homogeneous catalysts for a plethora of liquid-phase reactions, but their low stability, high cost and difficult recovery from the reaction medium limit their implementation in industrial processes. Their immobilization onto solid supports emerged as a potential strategy to overcome these issues, giving rise to recyclable hybrid catalysts which have been conquering new horizons in Green Catalysis.This report reviews the progress on the design and catalytic activity of recyclable hybrid catalysts based on metallosalen, metal acetylacetonate and metalloporphyrin complexes for oxidation reactions, starting from metal complexes supported onto bulk supports – carbon materials, mesoporous silicas, clay based materials and metal-organic frameworks – and progressively scaling down to nanosupports – carbon nanotubes, nanosilicas, magnetic iron oxide nanoparticles and colloidal gold.An overview of the general methodologies developed for metal complex immobilization onto solid supports is presented as a function of the type of interaction between the complex and the support – covalent bonding, non-covalent interactions and encapsulation. For each type of support, the most common immobilization strategies are also delineated.Special emphasis is given to literature examples in which the immobilization strategy and catalytic performance of the hybrid catalysts, including activity and reusability, were analyzed; the influence of the support dimensions on the performance of the hybrid catalysts is also addressed.

Iron(III) Fluorinated Porphyrins: Greener Chemistry from Synthesis to Oxidative Catalysis Reactions

Iron(III) fluorinated porphyrins play a central role in the biomimetics of heme enzymes and enable cleaner routes to the oxidation of organic compounds. The present work reports significant improvements in the eco-compatibility of the synthesis of 5,10,15,20-tetrakis-pentafluorophenylporphyrin (H2TPFPP) and the corresponding iron complex [Fe(TPFPP)Cl], and the use of [Fe(TPFPP)Cl] as an oxidation catalyst in green conditions. The preparations of H2TPFPP and [Fe(TPFPP)Cl] typically use toxic solvents and can be made significantly greener and simpler using microwave heating and optimization of the reaction conditions. In the optimized procedure it was possible to eliminate nitrobenzene from the porphyrin synthesis and replace DMF by acetonitrile in the metalation reaction, concomitant with a significant reduction of reaction time and simplification of the purification procedure. The Fe(III)porphyrin is then tested as catalyst in the selective oxidation of aromatics at room temperature using a green oxidant (hydrogen peroxide) and green solvent (ethanol). Efficient epoxidation of indene and selective oxidation of 3,5-dimethylphenol and naphthalene to the corresponding quinones is observed.

Direct access to polycyclic peripheral diepoxy-meso-quinone derivatives from acene catalytic oxidation

The unprecedented epoxidation on the peripheral positions of tetracene and pentacene with hydrogen peroxide-urea adduct catalyzed by the metalloporphyrin [Mn(TDCPP)Cl] is reported under mild conditions, affording the direct polyfunctionalization of the acenes to anti-diepoxy-meso-quinone derivatives.

Biomimetic one-pot route to acridine epoxides.

The first direct epoxidation of acridine on the edge positions is reported. The reaction proceeds under mild conditions using a biomimetic catalytic system based on a Mn(III) porphyrin. The successive oxyfunctionalization to mono-, di-, and tetraepoxy derivatives is accomplished using hydrogen peroxide as a green oxidant at room temperature. Computed optimized geometries showed only slight shifts to the base planarity upon dearomatization by epoxidation, which is an important feature for DNA intercalation and bioactivity. NMR studies and comparison with theoretical values allowed the assignment of the stereochemistry of the anti- and syn-diepoxy and -tetraepoxy derivatives as well as compounds resulting from epoxide ring opening, exemplified by epoxydiol. The diepoxide is formed in an anti:syn ratio of ∼4, and the attack by nucleophiles, exemplified by ethylaniline, occurs selectively and with full conversion, using a microwave process with acetonitrile reflux for 10 min. Finally, studies of the electrostatic potential allowed the mechanisms of the formation of 4-hydroxyacridine and the regioselective reaction of diepoxyacridine with nucleophiles to be rationalized.

Synthesis and coordination studies of 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative

The introduction of a carboxylate function into porphyrins allows a variety of modifications, including coordination and conjugation, which are central to enhance the efficiency of macrocycles in photonic materials and biological applications. Herein, a synthetic strategy to obtain 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative was developed by the 1,3-dipolar cycloaddition of a carbomethoxyphenyl substituted porphyrin with an azomethine ylide, followed by hydrolysis under thermal acidic conditions. The 1,3-dipolar cycloaddition of the carbomethoxyphenyl porphyrin with N-methyl nitrone was also performed to give an isomeric mixture of isoxazolidine-fused chlorins, revealing lower selectivity and lower yields; in addition, retrocycloaddition of the isoxazolidine-fused chlorins was observed under the hydrolysis conditions. The resulting carboxyphenyl macrocycles were characterized using 1H and 19F NMR, ESI-MS and SC-XRD for 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin. In order to study the influence of coordination to a metal ion on the electronic properties of carboxyphenyl substituted porphyrins, a series of metal complexes of 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative were synthesized by microwave-mediated metallation with Fe(III), Cu(II) and Zn(II) salts. EPR spectroscopy was particularly relevant to the characterization of the Cu(II) complexes of both macrocycles and to study the coordination chemistry of these ligands with Cu(II) ions.