The oxidation reactions of alkanes with hydrogen peroxide and peracids (peracetic acid (PAA) and m-chloroperoxybenzoic acid (mCPBA)) catalysed by two Fe(II) complexes of pentadentate {N5}-donor ligands have been investigated. Kinetic isotope effect experiments and the use of other mechanistic probes have also been performed. While the total yields of oxidized products are similar regardless of oxidant (e.g. 30–39% for oxidation of cyclohexane), the observed alcohol/ketone ratios and kinetic isotope effects differ significantly with different oxidants. Catalytic reactions in H2O2 medium are consistent with the involvement of hydroxyl radicals in the CH bond cleavage step, and resultant low kinetic isotope effect values. On the other hand, catalytic reactions performed using peracid media indicate the involvement of an oxidant different from the hydroxyl radical. For these reactions, the kinetic isotope effect values are relatively high (within a range of 4.2–5.1) and the C3/C2 selectivity parameters in adamantane oxidation are greater than 11, thereby excluding the presence of hydroxyl radicals in the CH bond cleavage step. A low spin Fe(III)-OOH species has been detected in the H2O2-based catalytic system by UV/Vis, mass spectrometry and EPR spectroscopy, while an Fe(IV)-oxo species is postulated to be the active oxidant in the peracid-based catalytic systems. Computational studies on the CH oxidation mechanism reveal that while the hydroxyl radical is mainly responsible for the H-atom abstraction in the H2O2-based catalytic system, it is the Fe(IV)-oxo species that abstracts the H-atom from the substrate in the peracid-based catalytic systems, in agreement with the experimental observations.
The reaction of N-unprotected methylesters of phenylglycine derivatives (1a-1f) with electron-rich internal alkynes (2a-2e), catalyzed by [Ru(cymene)Cl-2](2) (10%), gives the corresponding 3,4-disubstituted isoquinoline-l-carboxylates 3 through C-H/N-H oxidative coupling. The C-H bond activation step is assisted by carboxylates, and N-fluoro-2,4,6-trimethylpyridinium triflate works as the terminal oxidant. The process shows a remarkable tolerance to the presence of diverse electron-releasing and electron attracting functional groups at the phenyl ring of the amino acid. In addition, the reaction of phenylglycine derivatives (1a-if) with methyl acrylate (4a) catalyzed by [Ru(cymene)Cl-2](2) (10%) under the same experimental conditions, gives the corresponding 3,N-disubstituted isoindoline-1-carboxylates 5 through C-H/N-H coupling. Isoindolines 5 are obtained as a mixture of diastereoisomers, with moderate to high values of diastereomeric excess (up to 80%). (C) 2016 Elsevier B.V. All rights reserved.
Counteranion-catalysis represents an appealing but challenging approach for the development of enantioselective oxidative CH bond functionalization reactions. In this work, a new family of 3,3′-triazolyl BINOL-derived phosphoric acids was synthesized and employed in the intramolecular asymmetric CH bond functionalization of N-aryl substituted tetrahydroisoquinolines. As previously reported with related structures, the presence of the triazole groups on the catalysts was key to attain enantioselectivity. Our study also shows the importance of choosing the appropriate regioisomeric triazole groups at the BINOL backbone to achieve a more efficient chirality transfer. Moderate enantiomeric ratios were obtained with the N-benzamide substrates, whereas the change of the nature of the nucleophile fragment was translated to a dramatic loss of the enantioselectivity. Therefore, it can be foreseen that there is a need for designing further superior catalyst structures to develop future counter-anion organocatalyzed asymmetric CH bond functionalization reactions.
3d metal (Cu, Fe, Co, V) containing composite catalysts for the solvent-free microwave-assisted transformation of 1-phenylethanol to acetophenone with tert-butyl hydroperoxide (TBHP) as oxidant were prepared by ball milling. The influence of multiwalled carbon nanotubes (CNTs) and graphene oxide (GO) additives on the catalytic activity of the catalysts was studied. CNTs or GO were mixed by ball milling with the metal salts (CoCl2), oxides (CuO, Fe2O3, V2O5) or binary systems (Fe2O3-CoCl2, CoCl2-V2O5, CuO-Fe2O3). For CoCl2-based catalytic systems, addition of small amounts (0.1–5%) of CNTs or GO leads to significant improvement in catalytic activity, e.g. 1% of the CNTs additive allows to rise yields from 28 to 77%, under the same catalytic conditions. The CoCl2-5%CNTs composite is the most active among the studied ones with 85% yield and TON of 43 after 1h.
In the present work, four mononuclear iron(III) complexes containing BMPA (BMPA=bis-(2-pyridylmethyl)amine) and derivative ligands, have been studied as catalyst in toluene oxidation, at 25°C and 50°C, using hydrogen peroxide as oxidant and acetonitrile as solvent. All catalysts were able to oxidize toluene with satisfactory yields, producing o-, m-, p-cresols, benzaldehyde and benzyl alcohol, as main products, and traces of 2-methylbenzoquinone and benzoic acid. The catalyst [Fe(BMPA)Cl3] presented the most promising results, reaching yields up to 30.2% at 50°C after 24h. Furthermore, [Fe(BMPA)Cl3] was applied in the oxidation of other aromatic compounds as benzene, ethylbenzene, cumene, n-propylbenzene, p-xylene and anisole. The reaction with H2O2 was monitored by electronic UV–vis spectroscopy in the presence and absence of toluene and its oxidation products, as well as by ESI-(+)-MS/Q-TOF mass spectrometry, in order to provide some information about the reaction mechanism.
Acceleration of palladium catalyzed CH activation by various Lewis Acids was demonstrated on the directed ortho-alkenylation and acylation of acetanilide and urea derivatives. The universality of this effect was investigated by the study of different palladium catalysts, directing groups in the aromatic substrates and versatile Lewis acids. Experiments were carried out to monitor the reactions and to compare the behavior and activity of different types of Lewis acids. Kinetic investigation revealed a rate determining CH activation step, and DFT studies were performed for the explanation of Lewis acid effect on CH activation.
The homogeneously rhodium catalyzed hydroformylation of 1-decene was studied using operando FTIR spectroscopy. The bulky chelating diphosphite ligand BiPhePhos was used for catalyst modification. Special emphasis was given to the transformation of the Rh-precursor Rh(acac)(CO)2 to the activated HRh(BiPhePhos)(CO)2 catalyst. Under hydroformylation conditions, this complex was found to be the most abundant catalyst species over a wide range of olefin conversion. Other inactive or non-selective rhodium species were not detectable. Analysis of the turnover frequency revealed a first order dependence of the hydroformylation rate with respect to the concentration of 1-decene. These findings indicate that the coordination of the olefin to the Rh-BiPhePhos catalyst is determining the hydroformylation rate of 1-decene.
Direct activation of CH4 to oxygenates and unsaturated light hydrocarbons was investigated using Fe-modified ZSM-5 and Ferrierite (FER) for a partial oxidation of CH4 with N2O oxidant. The amount of active α-oxygen sites and number of Bronsted acid sites on the Fe-modified zeolites were well correlated with CH4 conversion rate and product distributions. The amount of α-oxygen sites was largely changed according to preparation method such as wet impregnation or ion-exchange of iron precursor and types of zeolites. A large number of Bronsted acid sites and α-oxygen sites on the Fe-modified FER revealed a higher oxygenate formation such as methanol and dimethyl ether (DME) with COx, and a larger number of strong acid sites on Fe-modified ZSM-5 was also responsible for a higher selectivity to light hydrocarbons by a successive dehydration of oxygenates formed. The different catalytic performances were verified through proper measurements of the amount and type of acidic sites as well as the α-oxygen sites measured by N2O pulse chemisorption. The Fe-modified FER prepared by impregnation method possessed a larger amount of α-oxygen sites due to abundant Bronsted acid sites, which was responsible for a higher rate of CH4 conversion to oxygenates with the help of N2O decomposition on the α-oxygen sites originated from iron oxide nanoparticles.
Low temperature selective transformation of alkanes to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines in combination with H2O2 have been recently identified as powerful oxidation catalysts for these challenging reactions due to the formation of ultra-high valent diiron oxo species PcFe(IV)μNFe(IV)O(Pc+). This very strong two-electron oxidizing species is generated from peroxo complex PcFe(IV)μNFe(III)OOR(Pc) (RH in the case of H2O2) via heterolytic OO bond cleavage. Therein we show that the evolution of the peroxo diiron complex depends on the peroxide structure. Using tBuOOH we have demonstrated the formation of an one-electron oxidizing PcFe(IV)μNFe(IV)O(Pc) and tBuO radical via homolytic OO cleavage of the peroxocomplex. The reactivity of the μ-nitrido diiron tetra-t-butylphthalocyanine − tBuOOH catalytic system was investigated in the oxidation of different CH bonds in alkanes, olefins, aromatic and alkylaromatic compounds. The main products of cyclohexane oxidation were cyclohexanone and cyclohexanol whereas bicyclohexyl was formed in minor amounts even in the presence of O2 and tBuOOH. Under optimal conditions, the turnover numbers of almost 5300 have been achieved.
In our previous study, we applied Pd@SiO2 core-shell catalysts to hydrogen peroxide synthesis and obtained a higher yield of hydrogen peroxide than that obtained with the use of general supported catalysts (Pd/SiO2). As an extension of the previous study on Pd@SiO2 catalysts, the effects of the core-shell thickness on the hydrogen peroxide synthesis reaction were examined in this study. A shell below a certain thickness in the core-shell structure of the Pd nanocatalyst results in a decrease in the catalytic activity. Overall, a volcano curve is observed for the hydrogen peroxide production rate as a function of the shell thickness. Through N-2-adsorption and desorption, TEM, CO-chemisorption, and XRD analyses, we identify the causes for the improved direct hydrogen peroxide synthesis yields and later optimize the shell thickness for the efficient utilization of Pd. (C) 2016 Elsevier B.V. All rights reserved.
The complexes of copper [Cu(κONN’-HL)(NO3)(DMF)](NO3)∙H2O (1) and [Cu(κONN’-HL)Cl2]∙½DMSO (2), and of manganese [Mn(κON-HL)2Cl2]Cl (3) and [Mn(κON-HL)2(NO3)2](NO3)∙H2O (4) were synthesized by reactions of the respective chloride or nitrate salt with a non-aqueous solutions of the Schiff base aminoalcohol HL (product of condensation of salicylic aldehyde and aminoethylpiperazine) and characterized by X-ray diffraction analysis. The catalytic investigations disclosed a prominent activity of the copper compounds 1 and 2 towards oxidation of cyclohexane with hydrogen peroxide in the presence of various promoters (nitric, hydrochloric, oxalic acids and pyridine), under mild conditions. The unusual promoting effect of pyridine on the catalytic activity of the copper catalysts allowed to achieve yields up to 21% based on cyclohexane. Chromatographic studies revealed that cyclohexyl hydroperoxide is a main reaction product and chlorocyclohexane (in the presence of HCl as promoter) was also detected, suggesting a free radical reaction pathway with hydroxyl radicals as attacking species. Complexes 1 and 2 act also as catalysts in the oxidation of 1-phenylethanol with tert-butylhydroperoxide, showing acetophenone yields up to 62% and TON (turnover numbers) up to 620 in the presence of the K2CO3 promoter.
Magnetically recoverable copper oxide catalysts prepared by sol-immobilization method exhibited interesting properties for the allylic oxidation of cyclohexene with molecular oxygen as the sole oxidant. The catalysts were prepared by immobilization of pre-synthesized PVA (polyvinyl alcohol)-stabilized Cu2O nanoparticles (NPs) on a magnetically recoverable support; the catalyst was further oxidized to CuO NPs after calcination at 600 °C. Both catalysts can selectively oxidize cyclohexene through allylic oxidation to give 2-cyclohexene-1-one as the main product, but CuO was identified as the most active species providing 90% cyclohexene conversion and 96% selectivity for allylic products under 100 °C and 4 bar pressure of O2 for 6 h of reaction time. The catalysts were magnetically recovered without metal leaching and could be reused in at least six consecutive runs.
A novel catalytic reaction of alcohol etherification in the system ROH − PtCl42− ‐ PtCl62− was found. Methanol easily transforms into dimethyl ether in the presence of catalytic amounts of PtII chloro complexes at 70°C. Under the same conditions reaction of ethanol affords diethyl ether (catalytic) and π-ethylene PtII complex (stoichiometric). The reactions are accompanied by multiple H/D exchange, which is indicative of intermediacy of corresponding alkyl platinum derivatives. The plausible reaction mechanism involves oxidative addition of alcohol forming intermediate alkyl platinum(IV) derivative followed by decomposition of it via reductive elimination step under the action of alcohol giving the ether and regenerating catalyst. In the case of ethyl alcohol reaction, β-hydrogen abstraction from the intermediate Pt-ethyl species yields π-ethylene platinum(II) complex. Although it seems that the reaction does not involve the initial breaking of CH bonds of an alcohol, this system can be regarded as a model for studying of some peculiarities of Shilov chemistry, in particular, of isotope scrambling mechanisms in Shilov alkane activation.In contrast to reactions of dimethyl and diethyl ethers formation, tert-butyl ethers formation in CD3OH/t-BuOH medium is catalyzed by PtIV chloro complexes also and is not accompanied by isotope scrambling. These observations argue against intermediacy of alkyl platinum derivatives suggesting that acid-catalyzed mechanism operates in tert-butyl alcohol etherification.
Gadolinium (Gd)-doped silver orthophosphate (Ag3PO4) nanoparticles, with spherical morphology, were synthesized by a co-precipitation method and their photocatalytic activities were evaluated in degradation of Reactive Blue 19 (RB19), as a model organic pollutant. The synthesized Ag3PO4 and Gd-doped Ag3PO4 were characterized using X-ray diffraction pattern analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, diffuse reflectance UV–vis spectroscopy and photoluminescence spectroscopy. In comparison with Ag3PO4, the experimental results revealed a considerable enhancement in photocatalytic activity of Gd-doped Ag3PO4. The optimum mole fraction of Gd loading in Ag3PO4 was 3%. Lower photocatalytic degradation efficiency of RB19 in the presence of oxalate anion and p-benzoquinone indicated the crucial role of photogenerated holes and superoxide anion-radicals in photocatalysis on Gd-doped Ag3PO4. The enhancement of photocatalytic activity was attributed to scavenge the photoexcited electrons, and to suppress their recombination with holes and generation of active oxygen species. A nonlinear empirical kinetic model was introduced to predict the apparent first order rate constant (kapp) as a function of operational parameters, including initial RB19 concentration, Gd-doped Ag3PO4 dosage and light intensity. Degradation byproducts were identified using gas chromatography-mass spectrometry and a probable pathway was proposed. Moreover, Gd-doped Ag3PO4 samples displayed adequate reusability after three successive experiments.
•Temperature determines contributions of catalytic and gas-phase stages.•Reactor surface is important “catalytic” factor in gas-phase oxidation.•C2 hydrocarbon yield at catalytic OCM is limited by gas-phase process.•Why catalysts are unimportant in chain-branched gas-phase DMTM.•Catalytic control of fast gas-phase processes is possible.
Ordered mesoporous MCo2O4 (M=Cu, Zn and Ni) spinel catalysts were synthesized via nano-replication method using mesoporous silica KIT-6 as the hard template. They were applied to methane combustion, in comparison with bulk MCo2O4 spinel catalysts prepared by co-precipitation method. A combined N2 adsorption-desorption, XRD and TEM results clearly confirm that mesoporous MCo2O4 (m-MCo2O4) spinel catalysts contain ordered mesostructure, resulting in the higher BET surface area and pore volume than bulk ones (b-MCo2O4). Moreover, the former catalysts demonstrate the better thermal stability as indicated by larger amount of MCo2O4 phase and smaller size of crystallite domain after calcination at 550°C. Therefore, such excellent properties rationalize that the m-MCo2O4 spinel catalysts reveal higher catalytic activity for methane combustion than bulk counterparts. When it comes to the catalytic performance of the meso catalysts for methane combustion, the m-CuCo2O4 spinel catalyst has superior performance, which is related to the high normalized amount of Co3+ cations on the surface, as evidenced by XPS.
In order to obtain monomers from vegetable source, the castor oil epoxidation process was investigated. The catalytic system used in this work, H2O2/alumina/ethyl acetate, can be considered as a green system, free of heavy metals and toxic solvents. These characteristics make the system appropriate for the purpose of this study since they increase the probabilities of obtaining a biomaterial with the desired specifications regarding toxicity. Reaction conditions of castor oil epoxidation were optimized using methyl ricinoleate as a model compound. In order to identify the operating region, it was developed an experimental design 23 with 17 assays (6 axial points and central point in triplicate) in which, methyl ricinoleate, hydrogen peroxide and catalyst initial quantities in the reaction mixture were the studied variables. The system showed great efficiency with 100% of selectivity in the methyl ricinoleate epoxide production. In optimized conditions, it showed conversion of 99% in 6h. It was obtained a conversion of 94%, an epoxidation percentage of 84 and a selectivity of 89% toward the epoxides for the castor oil epoxidation. These results show the efficacy of the catalytic system used in this work. Epoxidized castor oil structure was confirmed by FTIR, Raman and 1H NMR techniques.
Complexes [M(P,SR)(diene)X] where (P,SR)=CpFe[1,2-C5H3(PPh2)(CH2SR)] (M=Ir, R=tBu or Bn diene=cod, X=Cl; M=Rh, diene=cod or nbd; X=BF4 or Cl) were used as precatalysts for the redox isomerization of various allylic alcohols (7a–e) to the corresponding saturated ketones (8a–e) and or hydrogenation to the saturated alcohol (9a–e). In optimization studies using 1-phenyl-2-propen-1-ol (7a) in THF and in iPrOH/MeONa, the only observed product was the saturated alcohol 1-phenyl-1-propanol (9a) when working under a 30bar H2 pressure, but activation for only 1min under H2 pressure and then continuation under 1bar of H2 or Ar led to increasing amounts of the allylic isomerization product propiophenone (8a). Continued reaction under H2 converted (8a) into (9a). The Rh precatalysts were more active than the Ir analogues. For the rhodium precatalysts (3) and (4), the redox isomerization reaction could be carried out after precatalyst activation in iPrOH/MeONa under Ar at 82°C (without H2) with complete conversion in 1h (1% catalyst loading). However, longer reaction times resulted in slow transfer hydrogenation of (8a) leading to (9a) with low enantiomeric excess. Extension of the H2-free activation of the Rh precatalysts in iPrOH to other allylic alcohol substrates (7b–d) yielded the corresponding ketones with good to excellent yields and excellent chemoselectivities under appropriate conditions.
CH bond catalytic activation/functionalization has been matter of great interest, since these transformations could establish new grounds from the synthetic perspective, and could serve to propose alternative and greener methods for the production of organic molecules. In this sense, the development of efficient catalytic systems for CH bond activation becomes of great importance, and understanding the principles that govern such transformations, either stoichiometric or catalytic, is essential. To study such processes from the mechanistic or catalytic point of view requires of the design of ligands and catalysts able of performing CH bond activation. (P,N)-chelates have arisen as alternative ligands to develop new catalytic systems, due to their combination of different donor atoms, and the ability to tune their steric and electronic characteristics and to create potential vacant sites at the metal center due to hemilability.This mini review summarizes the advances from 2005 to 2015 in the field of CH bond activation at transition metal centers from groups 8–10 bearing (P,N)-chelating ligands. Even when the examples of inter- or intramolecular CH bond activation using transition metal complexes with the (P,N) ligands described herein do not engage yet in catalytic applications, their understanding becomes of great importance and sets the basis for future developments in this field.
We report the iridium-catalysed desilylative acylation of styryl and dienyl silanes by acid anhydrides to afford (E)-α,β-unsaturated ketones. The [{Ir(μ-Cl)(cod)}2] catalyst is the first non-rhodium complex successfully applied for this type of transformation. Stoichiometric reaction of [{Ir(μ-Cl)(cod)}2] with (E)-trimethyl(4-chlorostyryl)silane was carried out to gain insight into the reaction mechanism.
