Oxidation Of Adamantane Research Articles

Stereoelectronic Tuning of Bioinspired Nonheme Iron(IV)-Oxo Species by Amide Groups in Primary and Secondary Coordination Spheres for Selective Oxygenation Reactions.

Two mononuclear iron(II) complexes, [(6-amide2-BPMEN)FeII](OTf)2 (1) and [(6-amide-Me-BPMEN)FeII(OTf)](OTf) (2), supported by two BPMEN-derived (BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridine-2-yl-methyl)ethane-1,2-diamine) ligands bearing one or two amide functionalities have been isolated to study their reactivity in the oxygenation of C-H and C═C bonds using isopropyl 2-iodoxybenzoate (iPr-IBX ester) as the oxidant. Both 1 and 2 contain six-coordinate high-spin iron(II) centers in the solid state and in solution. The 6-amide2-BPMEN ligand stabilizes an S = 1 iron(IV)-oxo intermediate, [(6-amide2-BPMEN)FeIV(O)]2+ (1A). The oxidant (1A) oxygenates the C-H and C═C bonds with a high selectivity. Oxidant 1A, upon treatment with 2,6-lutidine, is transformed into another oxidant [{(6-amide2-BPMEN)-(H)}FeIV(O)]+ (1B) through deprotonation of an amide group, resulting in a stronger equatorial ligand field and subsequent stabilization of the triplet ground state. In contrast, no iron-oxo species could be observed from complex 2 and [(6-Me2-BPMEN)FeII(OTf)2] (3) under similar experimental conditions. The iron(IV)-oxo oxidant 1A shows the highest A/K selectivity in cyclohexane oxidation and 3°/2° selectivity in adamantane oxidation reported for any synthetic nonheme iron(IV)-oxo complexes. Theoretical investigation reveals that the hydrogen bonding interaction between the -NH group of the noncoordinating amide group and Fe═O core smears out the equatorial charge density, reducing the triplet-quintet splitting, and thus helping complex 1A to achieve better reactivity.

Thermal decomposition behaviors of adamantane and 1-methyladamantane in oxygen atmosphere: ReaxFF molecular dynamics simulation

High energy density fuels (HEDFs) have garnered considerable interest in the aerospace applications due to their high density and effective heat dissipation capabilities. However, there is still no clear understanding of the pyrolysis and oxidation mechanisms of HEDFs. In this work, a computational strategy based on ReaxFF molecular dynamics reveals dehydrogenation and ring-opening reaction events for adamantane and 1-methyladamantane. A comparative analysis of the initial decomposition behavior, intermediate reaction mechanism, and final product distribution demonstrates that the decay of the reactants is dependent on temperature and oxygen concentration. The main decomposition pathways of adamantane and 1-methyladamantane involve multi-step dehydrogenation and ring opening by the following reaction: i. C10H16 → C10H15 → C7H10 + C3H5, ii. C10H16 → C10H12, iii. C11H18 → C11H16, iv. C11H18 → C11H15 → C2H4 + C9H11. C7H10, C3H5, and C9H11 undergo further bond-breaking reactions to form linear hydrocarbon chains. At low temperatures, C10H16 and C11H18 exhibit a preference for reacting with OH and HO2, accounting for 15 %–35 % of the reactions. At high temperatures, the preferred reaction for C10H16 and C11H18 is with O2, accounting for 33 %–64 % of the reactions. The bridge carbon atom acts as the initial dehydrogenation site for both adamantane and 1-methyladamantane, as well as the active site for ring opening. The decomposition energy barrier for 1-methyladamantane (108.16–130.39 kJ/mol) is lower than that for adamantane (130.72–142.61 kJ/mol). Our findings shed light on the complicated interplay of oxidation of adamantane and 1-methyladamantane.

Selective synthesis of cyclic alcohols from cycloalkanes using nickel(ii) complexes of tetradentate amidate ligands.

Selective functionalisation of hydrocarbons using transition metal complexes has evoked significant research interest in industrial chemistry. However, selective oxidation of unactivated aliphatic C-H bonds is challenging because of the high bond dissociation energies. Herein, we report the synthesis, characterisation and catalytic activity of nickel(ii) complexes ([Ni(L1-L3)(OH2)2](ClO4)2 (1-3)) of monoamidate tetradentate ligands [L1: 2-(bis(pyridin-2-ylmethyl)amino)-N-phenylacetamide, L2: 2-(bis(2-pyridin-2-ylmethyl)amino)-N-(naphthalen-1-yl)acetamide, L3: N-benzyl-2-(bis(pyridin-2-ylmethyl)amino)acetamide] in selective oxidation of cycloalkanes using m-CPBA as the oxidant. In cyclohexane oxidation, catalysts showed activity (TON) in the order 1 (654) > 2 (589) > 3 (359) with a high A/(K + L) ratio up to 23.6. Using catalyst 1, the substrate scope of the reaction was broadened by including other cycloalkanes such as cyclopentane, cycloheptane, cyclooctane, adamantane and methylcyclohexane. Further, the Fenton-type reaction in the catalytic cycle was discarded based on the relatively high 3°/2° ratio of 8.6 in adamantane oxidation. Although the formation of chlorinated products during the reactions confirmed the contribution of the 3-chlorobenzoyloxy radical mechanism, the high alcohol selectivity obtained for the reactions indicated the participation of nickel-based oxidants in the oxidation process.

A Predictably Selective Palladium-Catalyzed Aliphatic C-H Oxygenation.

Direct oxygenation of nonactivated aliphatic C(sp3)-H groups with peroxycarboxylic acids in the presence of palladium tris(pyridylmethyl)amine complex (0.6 mol %) is reported, providing the corresponding hydroxylated derivatives in up to 94% yields. The oxidation of 3° C-H groups occurs stereospecifically, with the catalyst system demonstrating extremely high sensitivity to electronic effects (adamantane oxidation: 3°:2° up to >300). This suggests potential applications for the 3°-regioselective oxidative functionalization of complex molecules of natural origin.

Revisiting Alkane Hydroxylation with m-CPBA (m-Chloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes.

Mechanistic studies are performed on the alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid) catalyzed by nickel(II) complexes, NiII (L). In the oxidation of cycloalkanes, NiII (TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [NiII (L)] but is independent on [m-CPBA]; vobs =k2 [substrate][NiII (L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect (KIE) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, NiII (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the NiII (L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O2 and CCl4 on the product distribution pattern, possible contributions of (L)NiII -O. and the aroyloxyl radical as the reactive oxidants are discussed.

One-Stage Catalytic Oxidation of Adamantane to Tri-, Tetra-, and Penta-Ols

Tertiary tetraols of adamantane (C10H16, Tricyclo[3.3.1.1(3,7)]decan) have been widely used for the synthesis of highly symmetric compounds with unique physical and chemical properties. The methods for one-stage simultaneously selective, deep, and cheap oxidation of adamantane to tetraols of different structures have not yet been developed. In this research, chemically simple, cheap, and environmentally friendly reagents are used and that is the first step in this direction. The conditions, under which the impact of a hydrogen peroxide water solution on adamantane dissolved in acetonitrile results in full conversion of adamantane and formation of a total 72% mixture of its tri-, tetra-, and penta-oxygenated products, predominantly poliols, have been found. Conversion and adamantane oxidation depth are shown to depend on the ratio of components of the water-acetonitrile solution and the method of oxidizer solution introduction when using the dimer form of 1:1 dimethylglyoxime and copper dichloride complex as a catalyst. Under the conditions of mass-spectrometry ionization by electrons (70 eV), fragmentation across three C–C bonds of the molecular ions framework of adamantane tertiary alcohols Ad(OH)n in the range n = 0–4 increases linearly with the rise of n.

Ni(II) complexes of tripodal N4 ligands as catalysts for alkane hydroxylation and O-arylation of phenol: Structural and reactivity effects induced by fluoro substitution

Nickel(II) complexes [NiII(L1-2)(OAc)(H2O)][BPh4] (1–2) and [NiII(L3)(OAc)][BPh4] (3) derived from fluorinated tripodal ligands viz. N-((6-fluoropyridin-2-yl)methyl)(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L1 or FTPA), N,N′-bis((6-fluoropyridin-2-yl)methyl)(pyridin-2-yl)methanamine (L2 or F2TPA) and tris((6-fluoropyridin-2-yl)methyl)amine (L3 or F3TPA) have been synthesized and characterized by spectroscopic (UV–visible, FT-IR, paramagnetic NMR), elemental analysis, electrochemistry and X-ray diffraction techniques. In structurally similar complexes 1 and 2, Ni(II) center has a distorted octahedral coordination geometry constituted by all the four N atoms of the ligands, one acetate group and a water molecule. Complex 3 has different structural aspects. It does not have the water molecule in the coordination sphere and contains one acetate group bound with metal center in a bidentate mode. All the complexes exhibit a one-electron oxidation corresponding to the NiII/NiIII redox couple, the potential of which is influenced by the donor functionalities of ligand. These complexes catalyze the oxidation of cyclohexane efficiently (turn over number: 586–698) and selectively (alcohol to ketone ratio: 7.9:1 to 8.4:1). The study also includes the catalysis of adamantane oxidation to a mixture of ketones and alcohols. Catalytic potential of all the three complexes (1–3) has also been screened for CO coupling reactions of phenol with aryl halides. Among them, complex 1 is more efficient than 2 and 3 for such reactions.

Direct Oxidation of Csp3 -H bonds using in Situ Generated Trifluoromethylated Dioxirane in Flow.

A fast, scalable, and safer Csp 3 -H oxidation of activated and un-activated aliphatic chains can be enabled by methyl(trifluoromethyl)dioxirane (TFDO). The continuous flow platform allows the in situ generation of TFDO gas and its rapid reactivity toward tertiary and benzylic Csp3 -H bonds. The process exhibits a broad scope and good functional group compatibility (28 examples, 8-99 %). The scalability of this methodology is demonstrated on 2.5 g scale oxidation of adamantane.

Selective Alkane C–H Bond Oxidation Catalyzed by a Non-Heme Iron Complex Featuring a Robust Tetradentate Ligand

An iron complex, [FeII(BpyPY2Me)(CH3CN)2](OTf)2 (1-Fe, where BpyPY2Me is 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf is triflate), supported by an oxidatively rugged tetradentate ligand is reported for the catalytic oxidation of unactivated C–H bonds in cyclohexane and adamantane. With the use of m-chloroperbenzoic acid (mCPBA) as the terminal oxidant, the iron catalyst shows an alcohol-to-ketone (A/K) ratio of 7.5 for cyclohexane oxidation with conversion percentages as high as 90% with respect to oxidant. Moreover, catalysis toward adamantane oxidation shows high regioselectivity (3°/2° = 45) favoring tertiary C–H bonds with yields up to 87%. Results, including electrospray ionization mass spectrometry and UV–vis spectroscopy, indicate that a molecular non-heme iron(IV)-oxo intermediate is the catalytically active species.

Zirconia-supported 11-molybdovanadophosphoric acid catalysts: effect of the preparation method on their catalytic activity and selectivity.

The oxidation of adamantane with hydrogen peroxide catalyzed by zirconia-supported 11-molybdovanadophosphoric acid is shown to be a suitable green route for the synthesis of adamantanol and adamantanone. This work evaluates how the catalyst activity and selectivity are affected by some of its preparative parameters, such as the method for supporting the catalytically active heteropoly acid over the zirconia matrix or the pretreatments applied to the resulting materials before being used as heterogeneous catalysts. Our results indicate that the most effective catalysts able to maintain their activity after several reaction runs are those prepared by following the sol-gel route, whereas the most selective catalysts are those obtained by impregnation methods. Moreover, the calcination temperature has also been identified as a relevant parameter influencing the performance of catalysts based on supported heteropoly acids. The increasing catalytic activity observed over several consecutive reaction runs has been attributed to the formation of peroxo derivatives of polyoxometalate clusters at the surface of the catalyst and their accumulation after each reaction cycle.

Oxidation of Adamantane with H2O2–CF3COCF3 · 1.5 H2O in the Presence of VO(acac)2

The system hydrogen peroxide–hexafluoroacetone sesquihydrate effectively oxidizes adamantane in the presence of VO(acac)2 to afford 64% of adamantan-1-ol in tert-butyl alcohol or 76% of adamantan-2-one in a mixture of acetic acid with pyridine.

On the nature of the active intermediates in iron-catalyzed oxidation of cycloalkanes with hydrogen peroxide and peracids

Iron complexes of the PDP family [((S,S)-PDP)FeII(OTf)2] (1) and [((S,S)-PDP*)FеIII(μ-OH)2FеIII((S,S)-PDР*)](ОTf)4 (3), (S,S)-PDР = N,N′-bis(2-pyridylmethyl)-(S,S)-2,2′-biрyrrоlidine, (S,S)-PDР* = N,N′-bis(3,5-dimеthyl-4-mеthоxypyridyl-2-mеthyl)-(S,S)-2,2′-biрyrrоlidine, and of the TPA family [(TPА)FeII(CH3СN)2](СlO4)2 (4) and [(TPА*)FеIII(μ-OH)2FеIII(TPА*)](ОTf)4 (5), TPА = tris(2-pyridylmethyl)amine, TPA* = tris(3,5-dimеthyl-4-mеthoxyрyridyl-2-methyl)аmine, catаlyze the selеctive hydrоxylation of alkаnes with hydrogen peroxide and peroxycarboxylic acids as terminal oxidants. The nature of the active species of these catalytic systems has been evaluated by combined EPR spectroscopic and catalytic studies. To this end, the catalytic systems Fe complex/oxidant/RCOOH (catalyst: 1, 3, 4, 5; oxidant: H2O2, CH3CO3H, m-chloroperoxybenzoic acid = m-CPBA; RCOOH: acetic acid = AA, 2-ethylhexanoic acid = EHA), exhibiting EPR spectra of iron-oxo and/or iron-acylperoxo intermediates, have been systematically studied in the chemoselective oxidation of cyclohexane and regioselective oxidation of adamantane. In the latter case, high yield of oxidation products (up to 67 TN per Fe atom, or 67%) and high regioselectivity (3°/2° up to 41) were observed. Depending on the nature of the catalyst, oxidant and catalytic additive, various iron-oxygen intermediates have been observed in the catalytic systems studied. Iron(V)-oxo intermediates have been suggested to be the major active species of C–H hydroxylation by the systems catalyst/H2O2/RCOOH and catalyst/CH3CO3H/RCOOH. In contrast to the catalytic systems with H2O2 and CH3CO3H as oxidants, in the systems relying on m-CPBA, the contribution of iron-acylperoxo intermediates into the oxidation may be significant.

Partial oxidation of alkanes by dioxiranes formed in situ at low temperature.

Partial oxidation catalysts capable of efficiently operating at low temperatures may limit the over-oxidation of alkane substrates and thereby improve selectivity. This work focuses on examining alkane oxidation using completely metal-free organocatalysts, dioxiranes. The dioxiranes employed here are synthesized by oxidation of a ketone using a terminal oxidant, such as hydrogen peroxide. Our work generates the dioxirane in situ, so that the process can be catalytic with respect to the ketone. To date, we have demonstrated selective partial oxidation of adamantane using ketone catalysts resulting in yields upwards of 60% towards 1-adamantanol with greater than 99% selectivity. Furthermore, we have demonstrated that changing the electrophilic character of the ketone R groups to contain more electron-donating ligands facilitates the dioxirane ring formation and improves overall oxidation yields. Isotopic labelling studies using H218O2 show the preferential incorporation of an 18O label into the parent ketone, providing evidence for a dioxirane intermediate formed in situ The isotopic labelling studies, along with solvent effect studies, suggest the formation of peracetic acid as a reactive intermediate.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Selective C-H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications.

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C-H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using mCPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C-H oxidation. Mechanistic studies, which include correlation of the product formed with the FeV(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C-H bonds by FeV(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of 18O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of mCPBA and O2 by Fe-bTAML, leading to formation of the FeV(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation.

Catalytic C-H oxidations by nonheme mononuclear Fe(II) complexes of two pentadentate ligands: Evidence for an Fe(IV) oxo intermediate

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.

Preparative aerobic oxidations with basidiomycetous enzymes: CH-functionalization of adamantane

The potential of basidiomycetous enzymes for selective alkane CH-functionalizations in aqueous media has been disclosed utilizing surface and submerged cultures. A screening displayed the high catalytic activity of Dichomitus albidofuscus, Pholiota squarrosa, and Abortiporus biennis in aerobic oxidations of adamantane as a model hydrocarbon. With isopropanol as a co-solvent, the oxidation was accelerated significantly without changes of the fungal growth. 1-Adamantanol was obtained in 40% preparative yield from the oxidation of adamantane by D. albidofuscus. The CH-positional selectivity (3°/2°=3.6) and the deuterium kinetic isotopic effect (kH/kD=2.25) values provide evidence for the participation of fungal metalloenzymes in the CH-activation step.

Catalytic Oxidation of Alkanes and Alkenes by H2O2 with a μ‐Oxido Diiron(III) Complex as Catalyst/Catalyst Precursor

AbstractA new μ‐oxo diiron(III) complex of the lithium salt of the pyridine‐based unsymmetrical ligand 3‐[(3‐{[bis(pyridin‐2‐ylmethyl)amino]methyl}‐2‐hydroxy‐5‐methylbenzyl)(pyridin‐2‐ylmethyl)amino]propanoate (LiDPCPMPP), [Fe2(μ‐O)(LiDPCPMPP)2](ClO4)2, has been synthesized and characterized. The ability of the complex to catalyze oxidation of several alkanes and alkenes has been investigated by using CH3COOH/H2O2 (1:1) as an oxidative system. Moderate activity in cyclohexane oxidation (TOF = 33 h–1) and good activity in cyclohexene oxidation (TOF = 72 h–1) were detected. Partial retention of configuration (RC = 53 %) in cis‐ and trans‐1,2‐dimethylcyclohexane oxidation, moderate 3°/2° selectivity (4.1) in adamantane oxidation, and the observation of a relatively high kinetic isotope effect for cyclohexane oxidation (KIE = 3.27) suggest partial metal‐based oxidation, probably in tandem with free‐radical oxidation. Low‐temperature UV/Vis spectroscopy and mass spectrometric studies in the rapid positive detection mode indicate the formation of a transient peroxido species, [Fe2(O)(O2)(LiDPCPMPP)2]2+, which might be an intermediate in the metal‐based component of the oxidation process.

Biomimetic alkane oxidation by iodosylbenzene and iodobenzene diacetate catalyzed by a new manganese porphyrin: Water effect

This work describes the synthesis and characterization of the novel third-generation catalyst 5,10-(3,5-bromo,4-aminophenyl)-15,20-(phenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III) chloride, cis-[MnIIIBr12DAPDPP]Cl, and compares the catalytic activity of this compound with the catalytic activity of the first- and second-generation manganese porphyrins [MnIIITPP]Cl and cis-[MnIIIDAPDPP]Cl, respectively, in cyclohexane, adamantane and n-hexane, oxidation by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2). This work also investigates how addition of water and imidazole influences the catalytic systems in the adamantane and cyclohexane oxidation. In the absence of water and imidazole, cis-[MnIIIBr12DAPDPP]Cl leads to higher product yields as compared with [MnIIITPP]Cl and cis-[MnIIIDAPDPP]Cl in cyclohexane oxidation. The third-generation (β-octabrominated) cis-[MnIIIBr12DAPDPP]Cl was not fully destroyed in reactions with PhI(OAc)2 as oxidant. In the presence of imidazole, [MnIIITPP]Cl and cis-[MnIIIDAPDPP]Cl give superior cyclohexanol yields as compared with cis-[MnIIIBr12DAPDPP]Cl. Addition of water during adamantane oxidation by PhI(OAc)2 increases 1-adamantanol yield. As for cyclohexane oxidation by PhIO or PhI(OAc)2, the presence of water raises product yields and diminishes catalyst destruction, especially in the case of cis-[MnIIIDAPDPP]Cl. The presence of water in systems employing PhI(OAc)2 as oxidant affords higher product yields as compared with systems that use PhIO as oxidant.

Oxidation of adamantane with 2,3,4,5,6-pentafluoroperoxybenzoic acid in the presence of RuCl3 · 3H2O

Adamantan-1-ol (1, 1-AdOH) is the most important adamantane derivative. It is used as starting compound in the synthesis of memantine and rimantadine, and 1-adamantyl ethers are efficient antioxidant additives to oils and transmission fluids, which simultaneously improve their rheological characteristics at low temperatures [1]. A known procedure for the synthesis of alcohol 1 is based on the catalytic oxidation of adamantane (2) with m-chloroperoxybenzoic acid in the presence of metal complexes. In particular, the yield of 1 in the presence of Ni, Fe, Mn, and Co complexes [2–5] ranges from 1 to 12%. Ruthenium-containing catalysts ensured preparation of adamanatan-1-ol in 66% yield [6]. However, in the latter case the reaction was accompanied by side processes leading to adamantan2-ol, adamantan-2-one, and 1-chloroadamantane. In this communication we report for the first time the oxidation of adamantane (2) with 2,3,4,5,6-pentafluoroperoxybenzoic acid (3) catalyzed by RuCl3 · 3 H2O. 2,3,4,5,6-Pentafluoroperoxybenzoic acid (3) is a stable compound whose synthesis and application were reported for the first time in [7, 8]. Peroxy acid 3 efficiently oxidizes ∆-steroids [7] and α,β-unsaturated esters [8] to the corresponding epoxy derivatives in almost quantitative yield even in the absence of a catalyst. Taking into account unusually high activity of acid 3 in reactions with olefins, we anticipated that it can be used for the selective transformation of adamantane (2) into practically important alcohol 1.

Nickel(II) complexes of pentadentate N5 ligands as catalysts for alkane hydroxylation by using m-CPBA as oxidant: a combined experimental and computational study.

A new family of nickel(II) complexes of the type [Ni(L)(CH(3)CN)](BPh(4))(2), where L=N-methyl-N,N',N'-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N',N'-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L(3))(CH(3)CN)](BPh(4))(2) reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni(II) /Ni(III) redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH(3)CN)](BPh(4))(2) by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH(3)CN)](BPh(4))(2) or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH(3)CN)](BPh(4))(2) and the quinolylmethyl arms ([Ni(L4)(CH(3)CN)](BPh(4))(2), both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni(II)-O(.)] intermediate (ts1(hs), ts2(doublet)), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni(II) (L5)(OOCOC(6)H(4)Cl)](+) (5 a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni(II) leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.