Olga V. Zalomaeva

Metal–organic frameworks of the MIL-101 family as heterogeneous single-site catalysts

In this short review paper, we survey our recent findings in the catalytic applications of mesoporous metal–organic frameworks of the MIL-101 family (Fe- and Cr-MIL-101) and demonstrate their potential in two types of liquid-phase processes: (i) selective oxidation of hydrocarbons with green oxidants—O2 and tert-butyl hydroperoxide—and (ii) coupling reaction of organic oxides with CO2. A comparison with conventional single-site catalysts is made with special attention to issues of the catalysts resistance to metal leaching and the nature of catalysis.

Kinetics and mechanism of the oxidation of alkyl substituted phenols and naphthols with tBuOOH in the presence of supported iron phthalocyanine

2,3,5-Trimethylbenzoquinone (precursor of vitamin E) and 2-methylnaphthoquinone (vitamin K3) were obtained in good yields by oxidation of 2,3,6-trimethylphenol and 2-methyl-1-naphthol, respectively, with tBuOOH catalyzed by supported iron tetrasulfophthalocyanine. The mechanism of this heterogeneous oxidation was studied using 18O2labeling experiments, EPR spectroscopy with spin traps, kinetic studies, and complete analysis of reaction products including minor ones. 18O2labeling experiments did not indicate the involvement of O2 in the oxidative process. EPR study of reaction mixtures of 2,3,6-trimethylphenol and 2-methyl-1-naphthol oxidations in the presence of 3,5-dibromo-4-nitrosobenzenesulfonic acid spin trap showed no formation of any radical intermediates. Besides the target quinones, epoxyquinones and formyldimethyl-1,4-benzoquinones, as over-oxidation minor products have been found. C–C and C–O coupling products relevant to one-electron oxidation pathways were detected in trace amounts. Based on the experimental results, a mechanism of oxidation of alkyl-substituted phenols and naphthols mediated by the supported iron phthalocyanine catalyst has been proposed which involves two successive electron transfers without escape of radical species in solution.

Synthesis of cyclic carbonates from epoxides or olefins and CO2 catalyzed by metal-organic frameworks and quaternary ammonium salts

Abstract Catalytic properties of the metal-organic framework Cr-MIL-101 in solvent-free cycloaddition of CO 2 to epoxides to produce cyclic carbonates using tetrabutylammonium bromide as co-catalyst have been explored under mild reaction conditions (8 bar CO 2 , 25 0 C). Styrene and propylene carbonates were formed with high yields (95% and 82%, respectively). Catalytic performance of Cr-MIL-101 was compared with other MOFs: Fe-MIL-101, Zn-MOF-5 and HKUST-1. The catalytic properties of different quaternary ammonium bromides, Cr-MIL-101 as well as PW 12 /Cr-MIL-101 composite material have been assessed in oxidative carboxylation of styrene in the presence of both tert -butyl hydroperoxide and H 2 O 2 as oxidants at 8–100 bar CO 2 and 25–80 0 C with selectivity to styrene carbonate up to 44% at 57% substrate conversion.

Access to functionalized quinones via the aromatic oxidation of phenols bearing an alcohol or olefinic function catalyzed by supported iron phthalocyanine

The controlled oxidation at only one position of compounds with several oxidizable sites, while keeping the other sites intact, has been demonstrated for phenols bearing alcohol or olefinic functional groups. Iron tetrasulfophthalocyanine supported on silica was found to be an efficient catalyst for the preparation of functionalized quinones under mild conditions, with tert-butylhydroperoxide as the oxidant. A novel rapid and mild one-pot procedure for the covalent grafting of iron tetrasulfophthalocyanine onto silica has been developed. The supported catalyst was characterized by chemical analysis, a specific surface study, UV-vis spectroscopy and XPS. A non-radical mechanism for this unusual selective oxidation has been revealed by 18O labelling experiments.

Efficient epoxidation of olefins by H2O2 catalyzed by iron “helmet” phthalocyanines

High yields of epoxides were obtained in the oxidation of a large range of olefins using 1.2-2 equiv. of H2O2 in the presence of iron helmet phthalocyanines. The involvement of high-valent iron oxo species was evidenced using cryospray mass spectrometry.

Mechanistic Insights into Oxidation of 2-Methyl-1-naphthol with Dioxygen: Autoxidation or a Spin-Forbidden Reaction?

Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step.

Clean catalytic oxidation of 8-hydroxyquinoline to quinoline-5,8-dione with tBuOOH in the presence of covalently bound FePcS–SiO2 catalysts

The clean catalytic oxidation of 8-hydroxyquinoline (8-HQ) with tert-butyl hydroperoxide to quinoline-5,8-dione (QD), a molecular framework fragment of antitumor compounds, over silica-supported iron tetrasulfophthalocyanine catalysts (FePcS) is reported. The pronounced influence of the FePcS state (monomer vs. dimer) and the support (amorphous SiO2vs. mesoporous MCM-41) on the catalytic activity and selectivity is revealed. Depending on the catalyst structure, turnover frequency values determined from the initial rates of 8-HQ consumption varied from 215 to 3570 h−1.The effects of solvent, temperature, reagent concentrations and catalyst amounts on the substrate conversion and QD selectivity were studied to optimize the reaction conditions. With an optimal catalyst, the yield of the target product reached 66%. The truly heterogeneous nature of the catalysis was also demonstrated.