Competitive Epoxidation Research Articles

Nonheme Iron-Catalyzed Enantioselective cis -Dihydroxylation of Aliphatic Acrylates as Mimics of Rieske Dioxygenases

Open AccessCCS ChemistryRESEARCH ARTICLE17 Feb 2022Nonheme Iron-Catalyzed Enantioselective cis-Dihydroxylation of Aliphatic Acrylates as Mimics of Rieske Dioxygenases Jie Chen, Xiu Luo, Ying Sun, Si Si, Yuankai Xu, Yong-Min Lee, Wonwoo Nam and Bin Wang Jie Chen Google Scholar More articles by this author , Xiu Luo Google Scholar More articles by this author , Ying Sun Google Scholar More articles by this author , Si Si Google Scholar More articles by this author , Yuankai Xu Google Scholar More articles by this author , Yong-Min Lee Google Scholar More articles by this author , Wonwoo Nam Google Scholar More articles by this author and Bin Wang Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101780 SectionsSupplemental MaterialAboutPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Enantioselective cis-dihydroxylation of alkenes represents an ideal route to synthesize enantioenriched syn-2,3-dihydroxy esters that are important structural motifs in numerous biologically and pharmaceutically relevant molecules. Bioinspired nonheme iron-catalyzed enantioselective cis-dihydroxylation meets the requirement of the modern synthetic chemistry from atomic economy, green chemistry and sustainable development perspectives. However, nonheme iron-catalyzed enantioselective cis-dihydroxylation is much underdeveloped because of the formidable challenges of controlling chemo- and enantioselectivities and product selectivity caused from the competitive epoxidation, cis-dihydroxylation and overoxidation reactions. Herein, we disclose a biologically inspired nonheme iron complex-catalyzed enantioselective cis-dihydroxylation of multi-substituted acrylates using hydrogen peroxide (H2O2) as a terminal oxidant by controlling the non-ligating or weakly ligating counterions of iron(II) complexes; we have demonstrated a dramatic counteranion effect on the enantioselective cis-dihydroxylation of olefins by H2O2 catalyzed by nonheme iron complexes. A range of structurally disparate alkenes are transformed to the corresponding syn-2,3-dihydroxy esters in practically useful yields with exquisite chemo- and enantioselectivities (up to 99% ee). Given the mild and benign nature of this biologically inspired oxidation system and the ubiquity and synthetic utility of enantioenriched syn-2,3-dihydroxy esters in pharmaceuticals candidates and natural products, we expect that this strategy to some extent can serve as a promising compliment to the well-known Sharpless asymmetric dihydroxylation. Download figure Download PowerPoint Next article FiguresReferencesRelatedDetails Issue AssignmentNot Yet AssignedSupporting Information Copyright & Permissions© 2022 Chinese Chemical Society Downloaded 0 times Loading ...

Short time biomimetic oxidation of styrene with aqueous hydrogen peroxide: Crucial roles played by acetic acid

Short time oxidation of styrene with aqueous hydrogen peroxide catalyzed by a series of manganese porphyrins, MnT(aryl)PP(OAc), (aryl = phenyl, 2-chlorophenyl, 2-methylphenyl, 4-chlorophenyl or 4-methoxyphenyl) was studied under different conditions. In contrast to most previous catalytic systems containing manganese porphyrins and terminal oxidants other than hydrogen peroxide, no oxidation products were observed in the presence of the manganese porphyrins and various amounts of a nitrogen donor co-catalyst. Also, the addition of small and large alcohols to the reaction mixture had no positive effect on the efficiency of the catalyst, although the oxidative stability of the catalyst increased in the case of methanol. The oxidative stability and catalytic activity of the catalysts were remarkably increased in the presence of acetic acid. Furthermore, conducting the oxidation reaction in the presence of acetic acid and an optimized amount of imidazole decreased the reaction time from 120 to 20 min. However, the addition of bicarbonate had no effect on the catalytic performance of the catalysts. A mechanism was proposed to explain the crucial roles played by acetic acid in this catalytic system, using competitive epoxidation of cis-and trans-stilbene to elucidate the active oxidant species involved in the catalytic cycle of the oxidation reaction.

Probing Competitive and Co-operative Hydroxyl and Ammonium Hydrogen-Bonding Directed Epoxidations.

The diastereoselectivities and rates of epoxidation (upon treatment with Cl3CCO2H then m-CPBA) of a range of cis- and trans-4-aminocycloalk-2-en-1-ol derivatives (containing five-, six-, and seven-membered rings) have been investigated. In all cases where the two potential directing groups can promote epoxidation on opposite faces of the ring scaffold, evidence of competitive epoxidation pathways, promoted by hydrogen-bonding to either the in situ formed ammonium moiety or the hydroxyl group, was observed. In contrast to the relative directing group abilities already established for the six-membered ring system (NHBn ≫ OH > NBn2), an N,N-dibenzylammonium moiety appeared more proficient than a hydroxyl group at directing the stereochemical course of the epoxidation reaction in a five- or seven-membered system. In the former case, this was rationalized by the drive to minimize torsional strain in the transition state being coupled with assistance from hydrogen-bonding to the ammonium moiety. In the latter case, this was ascribed to the steric bulk of the ammonium moiety disfavoring conformations in which hydrogen-bonding to the hydroxyl group results in direction of the epoxidation to the syn face. In cases where the two potential directing groups can promote epoxidation on the same face of the ring scaffold, an enhancement of epoxidation diastereoselectivity was not observed, while introduction of a second, allylic heteroatom to the substrate results in diminishment of the rate of epoxidation in all cases. Presumably, reduction of the nucleophilicity of the olefin by the second, inductively electron-withdrawing heteroatom is the dominant factor, and any assistance to the epoxidation reaction by the potential to form hydrogen-bonds to two directing groups rather than one is clearly unable to overwhelm it.

Self‐Folded Silyl Cavitands with In‐ and Outwardly Directed Allyl Groups

AbstractCavitands endowed with in‐ and outwardly directed allylsilanes are described; epoxidation reactions of the allyl groups with meta‐chloroperbenzoic acid are included. The synthesis of introverted and extroverted allylsilanes tethered to triquinoxaline‐spanned resorcinarenes was successfully achieved. Competitive epoxidation experiments between the two isomers disclosed that the introverted allyl was more reactive than the extroverted allyl, despite a clearly congested nuisance: the vase‐formed cavity would actively stabilize the reaction process.

Remarkable Solvent, Porphyrin Ligand, and Substrate Effects on Participation of Multiple Active Oxidants in Manganese(III) Porphyrin Catalyzed Oxidation Reactions

The participation of multiple active oxidants generated from the reactions of two manganese(III) porphyrin complexes containing electron-withdrawing and -donating substituents with peroxyphenylacetic acid (PPAA) as a mechanistic probe was studied by carrying out catalytic oxidations of cyclohexene, 1-octene, and ethylbenzene in various solvent systems, namely, toluene, CH(2) Cl(2) , CH(3) CN, and H(2) O/CH(3) CN (1:4). With an increase in the concentration of the easy-to-oxidize substrate cyclohexene in the presence of [(TMP)MnCl] (1a) with electron-donating substituents, the ratio of heterolysis to homolysis increased gradually in all solvent systems, suggesting that [(TMP)Mn-OOC(O)R] species 2a is the major active species. When the substrate was changed from the easy-to-oxidize one (cyclohexene) to difficult-to-oxidize ones (1-octene and ethylbenzene), the ratio of heterolysis to homolysis increased a little or did not change. [(F(20) TPP)Mn-OOC(O)R] species 2b generated from the reaction of [(F(20) TPP)MnCl] (1b) with electron-withdrawing substituents and PPAA also gradually becomes involved in olefin epoxidation (although to a much lesser degree than with [(TMP)Mn-OOR] 2a) depending on the concentration of the easy-to-oxidize substrate cyclohexene in all aprotic solvent systems except for CH(3) CN, whereas Mn(V)=O species is the major active oxidant in the protic solvent system. With difficult-to-oxidize substrates, the ratio of heterolysis to homolysis did not vary except for 1-octene in toluene, indicating that a Mn(V)=O intermediate generated from the heterolytic cleavage of 2b becomes a major reactive species. We also studied the competitive epoxidations of cis-2-octene and trans-2-octene with two manganese(III) porphyrin complexes by meta-chloroperbenzoic acid (MCPBA) in various solvents under catalytic reaction conditions. The ratios of cis- to trans-2-octene oxide formed in the reactions of MCPBA varied depending on the substrate concentration, further supporting the contention that the reactions of manganese porphyrin complexes with peracids generate multiple reactive oxidizing intermediates.

Stereoelectronic effects of the meso-substituents on the catalytic performance of iron(III) meso-tetraarylporphyrins: Pyridyl and N-methylated pyridyl groups compared to phenyl, 4-methoxyphenyl and 4-sulfonatophenyl ones

The activity of a series of iron(III) meso-tetraarylporphyrins (aryl=phenyl, 4-methoxyphenyl, 4-sulfonatophenyl, 2-pyridyl, 3-pyridyl, 4-pyridyl and N-methyl-pyridyl counterparts) in the catalysis of oxidation reactions of olefins with tetra-n-butylammonium periodate (TBAP) or NaIO4 in the presence of imidazole (ImH) has been studied. While the presence of ImH has no effect on the catalytic activity of the Fe-porphyrins containing pyridyl or N-methylated pyridyl substituents at the meso-positions, the catalytic performance of the other Fe-porphyrins significantly increased by the use of ImH in 5:1 molar ratio with respect to the catalyst. As was observed in the comparison of the Mn-porphyrins with pyridyl substituents, the oxidative stability of FeT(4-py)P(OAc) is remarkably higher than that of FeT(2-py)P(OAc) and FeT(3-py)P(OAc). Also, the same order of relative stability was found in the case of the N-methylated analogues. It is noteworthy that N-methylation of FeT(py)P(OAc) complexes led to the substantial decrease in their oxidative stability. Competitive oxidation of cis- and trans-stilbene, provides indirect evidence for the involvement of high valent Fe-oxo porphyrins as well as periodato iron porphyrins as the active oxidant in reactions catalyzed by the used iron porphyrins. However, the involvement of a high valent iron-oxo porphyrin species is more pronounced in the case of FeTPPS4(OAc), FeT(2-py)P(OAc), FeT(3-py)P(OAc), FeT(4-py)P(OAc) and the N-methylated analogues. Also, an unusual preference of trans-stilbene over cis-stilbene was observed in the competitive epoxidation reaction catalyzed by FeT(3-MePy)P(OAc) and FeT(4-MePy)P(OAc) which cannot be described by the cis/trans isomerization. Furthermore, significant cis/trans isomerization observed upon oxidation of cis-stilbene in the reaction catalyzed by FeT(2-MePy)P(OAc) indicates that steric effects due to the presence of methyl groups at the ortho positions of this complex are of minor importance. On the other hand, large differences were observed in the product distribution for oxidation reactions with TBAO performed in dichloromethane and aqueous solvent. In spite of the lack of close correlation between the catalytic activity of the Fe-porphyrins and the electronic properties of the meso-substituents, in comparison with the corresponding Mn-porphyrins, there is a better correlation between the catalytic activity of the Fe-porphyrins and the electronic effects of the meso-groups.

Conformationally restricted dynamic supramolecular catalysts for substrate-selective epoxidations

A second generation of a substrate-selective dynamic supramolecular catalytic system consisting of a catalyst part and a receptor part, connected by a hydrogen-bonding motif, has been realized based on rational design. The results from analyses of the equilibrium mixture of the species generated by the components of the first generation system led us to selectively lock the cisoid conformation of the catalyst part to increase the amount of the substrate-selective catalytic cavity in the equilibrium mixture. This was realized by strapping the catalyst part by organic synthesis. This strapping led to an increase in substrate selectivity in the pair-wise competitive epoxidations of pyridyl- vs. phenyl-appended styrenes and pyridyl- vs. phenyl-appended stilbenes of both Z- and E- configuration compared to the first generation system, reaching 3.4 : 1 as the highest substrate selectivity for Z-mono-pyridyl-stilbene (27a) vs. the corresponding all-carbon analogue (28a) and for E-dipyridyl-stilbene (26b) vs. the corresponding all-carbon analogue (28b), respectively.

ChemInform Abstract: Allylic Oxofunctionalization of Cyclic Olefins with Homogeneous and Heterogeneous Catalysts

Heterogeneous catalysis and heterogenization of homogeneous catalysts are catch phrases of modern chemistry. Here we attempt to trace the development from homogeneous to heterogeneous catalysis in the allylic oxofunctionalization of cyclic olefins. The utility of this oxidative conversion is highlighted succinctly in the allylic oxidation of isophorone to ketoisophorone, which is a useful intermediate in the synthesis of carotenoids and fragrances. Competitive allylic oxidation and epoxidation are discussed briefly for the structurally simpler cyclohexene. Allylic oxidation of bicyclic α-pinene to verbenone is used as an example to illustrate that upon progressing to more complex cyclic olefins, competitive isomerization and structural rearrangements often result in very poor selectivity to the desired α,β-unsaturated ketone.

Biomimetic Iron‐Catalyzed Asymmetric Epoxidation of Aromatic Alkenes by Using Hydrogen Peroxide

A novel and general biomimetic non-heme Fe-catalyzed asymmetric epoxidation of aromatic alkenes by using hydrogen peroxide is reported herein. The catalyst consists of ferric chloride hexahydrate (FeCl(3)6 H(2)O), pyridine-2,6-dicarboxylic acid (H(2)(pydic)), and readily accessible chiral N-arenesulfonyl-N'-benzyl-substituted ethylenediamine ligands. The asymmetric epoxidation of styrenes with this system gave high conversions but poor enantiomeric excesses (ee), whereas larger alkenes gave high conversions and ee values. For the epoxidation of trans-stilbene (1 a), the ligands (S,S)-N-(4-toluenesulfonyl)-1,2-diphenylethylenediamine ((S,S)-4 a) and its N'-benzylated derivative ((S,S)-5 a) gave opposite enantiomers of trans-stilbene oxide, that is, (S,S)-2 a and (R,R)-2 a, respectively. The enantioselectivity of alkene epoxidation is controlled by steric and electronic factors, although steric effects are more dominant. Preliminary mechanistic studies suggest the in situ formation of several chiral Fe-complexes, such as [FeCl(L*)(2)(pydic)]HCl (L*=(S,S)-4 a or (S,S)-5 a in the catalyst mixture), which were identified by ESIMS. A UV/Vis study of the catalyst mixture, which consisted of FeCl(3)6 H(2)O, H(2)(pydic), and (S,S)-4 a, suggested the formation of a new species with an absorbance peak at lambda=465 nm upon treatment with hydrogen peroxide. With the aid of two independent spin traps, we could confirm by EPR spectroscopy that the reaction proceeds via radical intermediates. Kinetic studies with deuterated styrenes showed inverse secondary kinetic isotope effects, with values of k(H)/k(D)=0.93 for the beta carbon and k(H)/k(D)=0.97 for the alpha carbon, which suggested an unsymmetrical transition state with stepwise O transfer. Competitive epoxidation of para-substituted styrenes revealed a linear dual-parameter Hammett plot with a slope of 1.00. Under standard conditions, epoxidation of 1 a in the presence of ten equivalents of H(2) (18)O resulted in an absence of the isotopic label in (S,S)-2 a. A positive nonlinear effect was observed during the epoxidation of 1 a in the presence of (S,S)-5 a and (R,R)-5 a.

Heterogeneous Baeyer–Villiger oxidation of ketones with H 2O 2/nitrile, using Mg/Al hydrotalcite as catalyst

We synthesized a magnesium–aluminium hydrotalcite and used it as a catalyst in the Baeyer–Villiger (BV) oxidation of cyclohexanone with a mixture of 30% aqueous hydrogen peroxide and benzonitrile as oxidant. The hydrotalcite proved an excellent catalyst for the process. The influence of experimental variables was examined in depth in order to bring the working conditions as close as possible to those usable on an industrial scale. We optimized the cyclohexanone/hydrogen peroxide/benzonitrile proportion and used various nitriles, solvents and amounts of catalyst, benzonitrile and methanol proving the most effective nitrile and solvent, respectively, for the intended purpose. The reaction was found to occur to an acceptable extent with other carbonyl compounds as substrates; by exception, α,β-unsaturated carbonyl compounds provided poor results by effect of their undergoing competitive epoxidation of their double bonds.

Kinetics of (Porphyrin)manganese(III)‐Catalyzed Olefin Epoxidation with a Soluble Iodosylbenzene Derivative

AbstractWe examined the kinetics of a well‐behaved system for homogeneous porphyrin‐catalyzed olefin epoxidation with a soluble iodosylbenzene derivative 1 as the terminal oxidant and Mn(TPFPP)Cl (2) as the catalyst. The epoxidation rates were measured by using the initial rate method, and the epoxidation products were determined by gas chromatography. The epoxidation rate was found to be first order with respect to the porphyrin catalyst and zero order on the terminal oxidant. In addition, we found the rate law to be sensitive to the nature and concentration of olefin substrates. Saturation kinetics were observed with all olefin substrates at high olefin concentrations, and the kinetic data are consistent with a Michaelis–Menten kinetic model. According to the observed saturation kinetic results, we propose that there is a complexation between the active oxidant and the substrate, and the rate‐determining step is thought to be the breakdown of this putative substrate–oxidant complex that generates the epoxidation products and the resting state porphyrin catalyst. Competitive epoxidations further indicate a reversible complexation of the active oxidant and the olefin substrate. The activation parameters ΔH‡ and ΔS‡ for the oxygen‐transfer process (k2) in the cis‐cyclooctene epoxidation were determined to be 12.3 ± 0.9 kcal mol–1 and–15.6 ± 3.2 cal mol–1 K–1, respectively. In addition, the Hammett constant ρ+ was measured for the epoxidation of para‐substituted styrenes, and the value of –0.27 ± 0.04 is too low to be consistent with the involvement of a discrete carbocation in the transition state. We also prepared a (porphyrin)manganese catalyst immobilized on silica support, and found the epoxidation of cis‐cyclooctene catalyzed by this heterogeneous catalyst proceeds at virtually the same turnover frequency as by the homogeneous porphyrin catalyst. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis, Structural Characterization, and Catalytic Performance of Dititanium-Substituted γ-Keggin Silicotungstate

A novel titanium-substituted silicotungstate cluster of [{gamma-SiTi2W10O36(OH)2}2(mu-O)2]8- (1) is synthesized by the introduction of titanium(IV) ions into a divacant lacunary gamma-Keggin-type silicotungstate of [gamma-SiW10O36]8-. This titanium-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the gamma-Keggin fragment of 1 contains a dinuclear titanium center bridged by two hydroxo groups, and the resulting Ti2(mu-OH)2 core connects to the other Ti2(mu-OH)2 core of the paired gamma-Keggin subunit through Ti-O-Ti linkages. The Ti2(mu-OH)2 core of 1 reacts with MeOH to form the corresponding alkoxo derivative, [{gamma-SiTi2W10O36(OH)(OMe)}2(mu-O)2]8- (2). Two of four hydroxo groups of the Ti2(mu-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(mu-OH)(mu-OMe) core, and the Ti-O-Ti linkages connecting two gamma-Keggin subunits are maintained in 2. The gamma-Keggin dititanium-substituted silicotungstate 1 catalyzes mono-oxygenation reactions, such as the epoxidation of olefins and sulfoxidation of sulfides with hydrogen peroxide under mild conditions, while the monotitanium-substituted silicotungstate, [alpha-SiTiW11O39]4- (3), and the fully occupied silicododecatungstate, [gamma-SiW12O40]4-, are inactive. The epoxidation with 1 is stereospecific; the configurations around the C=C double bonds of the cis- and trans-olefins are completely retained in the corresponding epoxides. For the competitive epoxidation of cis- and trans-2-octenes, the ratio of the formation rate of cis-2,3-epoxyoctane to that of the trans isomer (R(cis)/R(trans)) is relatively high (21.3) in comparison with those observed for the tungstate catalysts, including [gamma-SiW10O34(H2O)2]4-. The epoxidation of 3-methyl-1-cyclohexene is highly diastereoselective and gives the corresponding epoxide with an anti configuration. The molecular structure of 1 is preserved during the catalysis because the 29Si and 183W NMR spectra of the catalyst recovered after completion of the oxidation are consistent with those of as-prepared compound 1. All these facts suggest the contribution of rigid nonradical oxidants generated on the multinuclear titanium center of 1.

Modulation of the reactivity, stability and substrate- and enantioselectivity of an epoxidation catalyst by noncovalent dynamic attachment of a receptor functionality—aspects on the mechanism of the Jacobsen–Katsuki epoxidation applied to a supramolecular system

The synthesis of the components of the dynamic supramolecular hydrogen-bonded catalytic system 2 + 3 is described. The catalytic performance and substrate- and enantioselectivity of Mn(salen) catalyst 2 were investigated in the presence and absence of the Zn(porphyrin) receptor unit 3. The effects of pyridine and pyridine N-oxide donor ligands were also studied. Some aspects on the mechanism of the Jacobsen-Katsuki epoxidation, based on literature observations, are introduced as a means to analyse the behaviour of 2 and its modulation by the formation of macrocycle 1 with 3. A complete association model of the metal-free system 4 + 5 refutes the earlier assumption that macrocycle 1 is the predominant form of catalyst 2 under the standard epoxidation reaction conditions with 2 + 3. Evidence are provided that receptor-binding substrates and nonbinding substrates, respectively, are epoxidised by two different catalytic species, or two distinct distributions of species in competitive epoxidations using catalytic system 2 + 3. The two species are assigned to the endo and exo faces of the Mn(salen) catalyst in macrocycle 1, and to equivalently folded oligomeric structures with monomers 2 and 3 in adjacent positions.

Disproportionation of hydrogen peroxide into singlet oxygen catalyzed by lanthanum-exchanged zeolites

Lanthanum(III) was immobilized on various zeolite supports by simple cation exchange. The resulting materials were tested in the disproportionation of H 2O 2 into singlet molecular oxygen ( 1O 2). The rate of H 2O 2 disproportionation and the reaction of the formed 1O 2 with citronellol as a typical olefinic substrate were strongly dependent on the zeolite topology and composition. Zeolites with a large crystal size, small pores, or a one-dimensional pore system showed low activity in the disproportionation of H 2O 2. In contrast, zeolites with a small crystal size or zeolites possessing large intersecting pores were active 1O 2 generators. La supported on ultrastable Y zeolite (USY) and zeolite Beta were identified as the most active and efficient catalysts. Typically, within 24 h at 40 °C, 100 mol of citronellol hydroperoxides is produced per mol of La supported on USY zeolite. Within this time, 800 mol of H 2O 2 is disproportionated into 1O 2. In comparison with unsupported La-hydroxide, La-USY shows a significantly higher activity and a slightly more efficient use of H 2O 2. The heterogeneous nature of the catalysis by La-USY was verified by filtration tests, and its stability was proved by X-ray diffraction and recycle experiments. Finally, the catalyst was used in the peroxidation of various olefinic compounds. Compared with conventional molybdate catalysts, La-USY is particularly useful for the selective peroxidation of allylic alcohol derivatives, with very little competitive epoxidation and no alcohol oxidation being observed.

Oxoiron(IV) porphyrin π-cation radical complexes with a chameleon behavior in cytochrome P450 model reactions

There is an intriguing, current controversy on the involvement of multiple oxidizing species in oxygen transfer reactions by cytochromes P450 and iron porphyrin complexes. The primary evidence for the "multiple oxidants" theory was that products and/or product distributions obtained in the catalytic oxygenations were different depending on reaction conditions such as catalysts, oxidants, and solvents. In the present work, we carried out detailed mechanistic studies on competitive olefin epoxidation, alkane hydroxylation, and C=C epoxidation versus allylic C-H hydroxylation in olefin oxygenation with in situ generated oxoiron(IV) porphyrin pi-cation radicals (1) under various reaction conditions. We found that the products and product distributions were markedly different depending on the reaction conditions. For example, 1 bearing different axial ligands showed different product selectivities in competitive epoxidations of cis-olefins and trans-olefins and of styrene and para-substituted styrenes. The hydroxylation of ethylbenzene by 1 afforded different products, such as 1-phenylethanol and ethylbenzoquinone, depending on the axial ligands of 1 and substrates. Moreover, the regioselectivity of C=C epoxidation versus C-H hydroxylation in the oxygenation of cyclohexene by 1 changed dramatically depending on the reaction temperatures, the electronic nature of the iron porphyrins, and substrates. These results demonstrate that 1 can exhibit diverse reactivity patterns under different reaction conditions, leading us to propose that the different products and/or product distributions observed in the catalytic oxygenation reactions by iron porphyrin models might not arise from the involvement of multiple oxidizing species but from 1 under different circumstances. This study provides strong evidence that 1 can behave like a "chameleon oxidant" that changes its reactivity and selectivity under the influence of environmental changes.

Epoxidation of cyclohexene catalyzed by transition-metal substituted $alpha;-titanium arsenate using tert-butyl hydroperoxide as an oxidant

Epoxidation of cyclohexene, using transition-metal substituted α-titanium arsenate {α-TiMAs, where MCu(II), Co(II), Mn(II), Fe(III), Cr(III) and Ru(III)} as a catalyst and dry tert-butyl hydroperoxide as an oxidant, was studied. In the epoxidation reaction, cyclohexene was oxidized to cyclohexene oxide, cyclohexenol and cyclohexenone. It was found that the reactivity of the epoxidation reaction decreased in the order α-TiRuAs>α-TiMnAs>α-TiFeAs>α-TiCrAs>α-TiCoAs>α-TiCuAs. A maximum selectivity for epoxidation of cyclohexene (89.89%) was observed for α-TiRuAs/dryTBHP system after 4 h of reaction when concentrations of the catalyst and the substrate were 0.20 and 20 mmol, respectively. The competitive epoxidation of cyclic olefins were studied and the reactivity-order in the relative rates of epoxidation of cyclic olefins was found to decrease in the order norbornene>cyclooctene>cyclohexene>cyclopentene. The mechanism for epoxidation of cyclohexene catalyzed by α-TiMAs/dryTBHP system is proposed.

Epoxidation of cyclohexene catalyzed by transition-metal substituted α-titanium arsenate using tert-butyl hydroperoxide as an oxidant

Epoxidation of cyclohexene, using transition-metal substituted α-titanium arsenate {α-TiMAs, where MCu(II), Co(II), Mn(II), Fe(III), Cr(III) and Ru(III)} as a catalyst and dry tert-butyl hydroperoxide as an oxidant, was studied. In the epoxidation reaction, cyclohexene was oxidized to cyclohexene oxide, cyclohexenol and cyclohexenone. It was found that the reactivity of the epoxidation reaction decreased in the order α-TiRuAs>α-TiMnAs>α-TiFeAs>α-TiCrAs>α-TiCoAs>α-TiCuAs. A maximum selectivity for epoxidation of cyclohexene (89.89%) was observed for α-TiRuAs/dryTBHP system after 4 h of reaction when concentrations of the catalyst and the substrate were 0.20 and 20 mmol, respectively. The competitive epoxidation of cyclic olefins were studied and the reactivity-order in the relative rates of epoxidation of cyclic olefins was found to decrease in the order norbornene>cyclooctene>cyclohexene>cyclopentene. The mechanism for epoxidation of cyclohexene catalyzed by α-TiMAs/dryTBHP system is proposed.

Competitive oxidative cleavage and epoxidation of 4,4′-diaminostilbene-2,2′-disulfonic acid by Fe(iii) aqua complexes

4,4′-Diaminostilbene-2,2′-disulfonic acid (DSD) has been investigated as a model compound of fluorescent whitening agents. The reaction of DSD with Fe(III) aqua complexes has been studied in the dark at room temperature: a fast degradation of DSD was observed under our experimental conditions. HPLC analysis was used to follow the kinetics of the redox reaction. The process involves epoxidation, leading to DSD epoxide (1), and the oxidative cleavage of the double bond, leading to 5-amino-2-formyl-benzenesulfonic acid (2) together with the reduction of Fe(III) into Fe(II). The dependence of the redox process on [Fe(H2O)5(OH)]2+ concentration indicates that the dominant reaction pathway is a reaction between DSD and the Fe(III) monomeric species, [Fe(H2O)5(OH)]2+. The complete degradation of DSD is observed with an excess of oxidant; the ratio [Fe(H2O)5(OH)]2+∶[DSD] has to be higher than 2. A mechanism giving rise to the two degradation products is proposed.

Multiple active oxidants in competitive epoxidations catalyzed by porphyrins and corroles.

We demonstrate the existence of multiple active oxygenating species in porphyrin and corrole-catalyzed competitive epoxidations of styrene and cis-cyclooctene.

A proposed mechanism of tamoxifen in breast cancer prevention

Recent clinical trials suggest that tamoxifen (TAM) is a preventive agent for breast cancer, however, the mechanism is unknown. Previously, we found that both 17β-estradiol (E 2) and estrone (E 1) could be activated by epoxidation resulting in their ability to bind to DNA, forming DNA adducts both in vitro and in vivo, and to inhibit nuclear DNA-dependent RNA synthesis. Since epoxidation is required for the activation of many well-known chemical carcinogens including benzo(a)pyrene, 7,12-dimethylbenz(a)anthracene, aflatoxins, etc., we proposed that estrogen epoxidation is the underlying mechanism for the initiation of breast cancer (Carcinogenesis 17 (1996) 1957). Here, we report that TAM is able to dramatically inhibit the formation of E 2 and E 1 epoxides as measured by both the loss of their ability to inhibit nuclear DNA-dependent RNA synthesis and to bind to nuclear DNA. These findings suggest that the breast cancer preventive effect of TAM may be through a competitive epoxidation inhibition mechanism that prevents the formation of E 2 and E 1 epoxides and consequently, the initiation of breast cancer.