Solvent Effects on Catalytic Epoxidation of Alkenes by Tetra‐n‐butylammonium Periodate and (Tetraarylporphyrinato)manganese(III)

Periodate-Mediated Aerobic Oxidation of Sulfides over a Bifunctional Porphyrin-polyoxometalate Catalyst: Photosensitized Singlet Oxygen Oxidation of Iodate to Periodate

Regeneration of terminal oxidants by molecular oxygen in metal-catalyzed oxidations of organic substrates has the advantage of avoiding the use of stoichiometric amounts of hazardous and/or expensive reagents to meet (some of) the green chemistry requirements. In the present study, photosensitized singlet oxygen oxidation of iodate to periodate has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed oxidation of sulfides to sulfoxides with periodate in water. To the best of our knowledge, it is the first report on singlet oxygen oxidation of iodate to periodate. In order to determine the contribution of photooxidation and oxidation pathways in the formation of sulfoxide, the oxidation of diphenyl sulfide with a very low reactivity toward aerobic photooxidation was studied; a sevenfold increase in the conversion of the sulfide to the diphenyl sulfoxide was observed for the reaction conducted in the presence of H2TMPyP-PW12O40/IO3-/O2/hν compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions, a ca. 1.5-fold increase was observed in the case of methyl phenyl sulfide, which shows high reactivity toward both the oxidation and photooxidation reactions. A porphyrin-POM nanocomposite formed by the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on PW12O40 was employed for the one-pot oxidation and photooxidation reactions. Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), diffuse-reflectance UV-vis spectroscopy, thermal gravimetric analysis, and Fourier transform infrared were used to characterize the formation of the hybrid compound. An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer equation. Also, FESEM images showed the formation of nearly spherical nanoparticles with a size of ca. 200 nm. The redshift of the Soret band of H2TMPyP upon immobilization on POM was attributed to strong N-H···O hydrogen-bond interactions between POM and porphyrin.

Kinetics and mechanistic studies on the formation and reactivity of high valent MnO porphyrin species: mono-ortho or para-substituted porphyrins versus a di-ortho-substituted one

High valent Mn(O) species of a series of electron-rich and -deficient meso-tetra(aryl)porphyrins (aryl = phenyl, 2-Cl-phenyl, 2-nitrophenyl, 2-Me-phenyl, 2-Br-phenyl, 2,6-di-Cl-phenyl 4-OMe-phenyl, 4-Me-phenyl, 4-Cl-phenyl and 4-pyridyl) were prepared at 273 K.

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide. These species are composed of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates. These complexes have been studied to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this Review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. This Review is organized by structural type of metal-oxygen intermediate and broken into subsections based on the metal (manganese and iron) and porphyrinoid ligand (porphyrin, corrole, and corrolazine).

Biomimetic oxidation of cyclic and linear alkanes: high alcohol selectivity promoted by a novel manganese porphyrin catalyst

A novel β-brominated Mn-porphyrin acts as a good catalyst for alkane (cyclohexane, adamantine and n-hexane) oxidation in biomimetic systems.

Ultrasound promoted green oxidation of alkenes with hydrogen peroxide in the presence of iron porphyrins supported on multi-walled carbon nanotubes

In the present work, oxidation of alkenes with hydrogen peroxide in the presence of meso-tetrakis(4-hydroxyphenyl)porphyrinatoiron(III) chloride supported onto surface of functionalized multi-wall carbon nanotubes (FMWCNT), [ Fe ( THPP ) Cl@MWCNT ], is reported. The simple heterogeneous catalyst was characterized by FT-IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and also thermal analysis. The amount of the catalyst loaded on the nanotubes was determined by atomic absorption spectroscopy. This heterogeneous catalyst proved to be an efficient and green catalyst and was successfully able to activate hydrogen peroxide without any additive toward the oxidation of alkenes in ethanol as a green solvent. Performance of the catalyst in oxidation of various alkenes was inspected under reflux, ultrasonic irradiation and mechanical stirring. Moreover, the catalyst can be reused several times under similar conditions.

Investigation of axial ligand effects on catalytic activity of manganese porphyrin, evidence for the importance of hydrogen bonding in cytochrome-P450 model reactions

Various nitrogenous bases, such as imidazoles, pyridines and amines were employed as axial ligands in epoxidation reaction of cyclooctene bytetra-n-butylammonium hydrogen monopersulfate (n- Bu 4 NHSO 5), in the presence of Mn ( III )-tetrakis(2,3-dimethoxyphenyl)porphyrin-acetate ( T (2,3- OMeP ) PorMnOAc ). T (2,3- OMeP ) PorMnOAc is a fairly stable catalyst, with the ability of producing hydrogen bonding. High epoxidation yield of 85 ± 6% was obtained in the presence of imidazole axial ligand with 100% selectivity in 30 min. Higher conversion of around 100% was obtained by pyridine axial base, while selectivity was reduced to 69%. Further epoxidation reactions were also performed using Mn ( III )-Tetrakis(2,3-dihydroxyphenyl)porphyrin-acetate ( T (2,3- OHP ) PorMnOAc ) as catalyst. In addition to the usual electronic and steric effects, it is proposed that the catalytic activity depends on the existence and kind of hydrogen bonding between the axial base and the ortho-methoxy or hydroxy groups on the phenyl rings of manganese porphyrin. The cis to trans ratio of cis-stilbene oxide formed by imidazole and pyridine axial bases were obtained as 7.5 and 2.5 respectively. In addition GC-Ms and UV-vis studies were employed to find the nature of active species and product formation. Our DFT calculations disclosed that pyridine hydrogen bonding with moiety of the macrocycle rings strongly affects the relative energies of S/Q spin states in [ T (2,3- OMeP ) PorMn V ( O )( Py )]+, in that it results in the longer Mn – O bond and reactivity toward substrates.

A UV-vis study of the effects of alcohols on formation and stability of Mn(por)(O)(OAc) complexes

Interactions of three different (acetato) (tetraarylporphyrinato) manganese (III) Mn(III)(por) with tetra-n-butylammonium hydrogen monopersulfate (n-Bu(4)NHSO(5)), in the presence of excess tetra-n-butylammonium acetate (n-Bu(4)NOAc) and in the absence or presence of various alcohols (alcohols=CH(3)OH, C(2)H(5)OH, i-C(3)H(7)OH, t-C(4)H(9)OH) in CH(2)Cl(2), were monitored by their UV-vis spectral changes, under identical conditions, at room temperature. (Acetato) (tetrakispentafluorophenylporphyrinato) manganese (III) Mn(III)(tpfpp)(OAc) and (acetato) (tetramesitylporphyrinato) manganese (III) Mn(III)(tmp)(OAc) produced their corresponding high valent Mn(tpfpp)(O)(OAc) and Mn(tmp)(O)(OAc) both in the absence or presence of alcohols. Whereas, (acetato) (tetraphenylporphyrinato) manganese (III) Mn(III)(tpp)(OAc) only generated Mn(tpp)(O)(OAc) in the presence of less bulky alcohols. In the absence of alcohols or in the presence of t-C(4)H(9)OH, the UV-vis spectra displayed a very weak sign of formation of Mn(tpp)(O)(OAc) complex. It was observed that alcohols generally increased the rate of formation of Mn-oxo species in accordance with their acidity or hydrogen bonding strength, and enhanced the stability of Mn-oxo complexes, as their size increases. Attempts are made to explain these effects. A mechanistic scheme is also suggested for the decomposition of HSO(5)(-) to O(2) and HSO(4)(-), through the formation and dimerization of Mn-oxo species.