Olefin Epoxidation Reactions Research Articles

Mononuclear Nonheme High-Spin Iron(III)-Acylperoxo Complexes in Olefin Epoxidation and Alkane Hydroxylation Reactions.

Mononuclear nonheme high-spin iron(III)-acylperoxo complexes bearing an N-methylated cyclam ligand were synthesized, spectroscopically characterized, and investigated in olefin epoxidation and alkane hydroxylation reactions. In the epoxidation of olefins, epoxides were yielded as the major products with high stereo-, chemo-, and enantioselectivities; cis- and trans-stilbenes were oxidized to cis- and trans-stilbene oxides, respectively. In the epoxidation of cyclohexene, cyclohexene oxide was formed as the major product with a kinetic isotope effect (KIE) value of 1.0, indicating that nonheme iron(III)-acylperoxo complexes prefer C═C epoxidation to allylic C-H bond activation. Olefin epoxidation by chiral iron(III)-acylperoxo complexes afforded epoxides with high enantioselectivity, suggesting that iron(III)-acylperoxo species, not high-valent iron-oxo species, are the epoxidizing agent. In alkane hydroxylation reactions, iron(III)-acylperoxo complexes hydroxylated C-H bonds as strong as those in cyclohexane at -40 °C, wherein (a) alcohols were yielded as the major products with high regio- and stereoselectivities, (b) activation of C-H bonds by the iron(III)-acylperoxo species was the rate-determining step with a large KIE value and good correlation between reaction rates and bond dissociation energies of alkanes, and (c) the oxygen atom in the alcohol product was from the iron(III)-acylperoxo species, not from molecular oxygen. In isotopically labeled water (H2(18)O) experiments, incorporation of (18)O from H2(18)O into oxygenated products was not observed in the epoxidation and hydroxylation reactions. On the basis of mechanistic studies, we conclude that mononuclear nonheme high-spin iron(III)-acylperoxo complexes are strong oxidants capable of oxygenating hydrocarbons prior to their conversion into iron-oxo species via O-O bond cleavage.

Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts

We report a remarkable Brønsted acid effect in the epoxidation of olefins by nonheme manganese catalysts and aqueous hydrogen peroxide. More specifically, a mononuclear nonheme manganese complex bearing a tetradentate N4 ligand, Mn(II)(Dbp-MCP)(OTf)2 (Dbp-MCP = (1R,2R)-N,N'-dimethyl-N,N'-bis((R)-(3,5-di-tert-butyl-phenyl)-2-pyridinylmethyl)cyclohexane-1,2-diamine; OTf(-) = CF3SO3(-)), is a highly efficient catalyst in the epoxidation of olefins by aqueous H2O2 in the presence of H2SO4 (1-3 mol %). The yields of epoxide products as well as the chemo- and enantioselectivities increase dramatically in the presence of H2SO4; no formation of epoxides is observed in the absence of H2SO4. In addition, the product yields and enantioselectivities are dependent significantly on the manganese catalysts and Brønsted acids. The catalytic epoxidation of olefins by other oxidants, such as peracids, alkyl hydroperoxides, and iodosylbenzene, is also affected by the presence of H2SO4; product yields and enantioselectivities are high and similar irrespective of the oxidants in the presence of H2SO4, suggesting that a common epoxidizing intermediate is generated in the reactions of [Mn(II)(Dbp-MCP)](2+) and the oxidants. Mechanistic studies, performed with (18)O-labeled water (H2(18)O) and cumyl hydroperoxide, reveal that a high-valent manganese-oxo species is formed as an epoxidizing intermediate via O-O bond heterolysis of Mn-OOH(R) species. The role of H2SO4 is proposed to facilitate the formation of a high-valent Mn-oxo species and to increase the oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin epoxidation reactions. Density functional theory (DFT) calculations support experimental results such as the formation of a Mn(V)-oxo species as an epoxidizing intermediate.

Design and synthesis of hybrid solids based on the tetravanadate core toward improved catalytic properties

Abstract Five inorganic–organic hybrid vanadates based on tetravanadate cores, transition metals and N-donor ligands have been designed and synthesized under hydrothermal conditions, namely, [Zn(eIM)3]2V4O12 (1), [Zn(pIM)3]2V4O12·H2O (2), [Zn(ipIM)3]2V4O12 (3), [Co(eIM)3]2V4O12·H2O (4), [Cu(eIM)2(H2O)]2V4O12 (5) (eIM = 1-ethylimidazole, pIM = 1-propylimidazole, ipIM = isopropylimidazole). All compounds were fully characterized by single-crystal XRD, powder XRD, elemental analysis, TGA, and FT-IR spectroscopy. The hybrid zinc vanadates (1–3) and cobalt vanadate (4) exhibit interesting 2D folded structures and the hybrid copper vanadate (5) presents a 1D chain configuration. All compounds can catalyze olefin epoxidation reactions when using TBHP (TBHP = tert-butyl hydroperoxide) as an oxidant in acetonitrile. The introduction of transition metal ions into tetravanadate cores not only improved the catalytic activity but also fulfilled the heterogeneous catalytic behavior. 1–5 all exhibit extraordinary efficiency in converting olefins to the corresponding epoxides with high conversion and selectivity (particularly, conv. up to 97.1%, sele. up to 100% for 1). Leaching test was also carried out to prove the heterogeneous behavior.

ChemInform Abstract: Mononuclear Nonheme Iron(III)‐Iodosylarene and High‐Valent Iron‐Oxo Complexes in Olefin Epoxidation Reactions.

AbstractA non‐heme Fe(III)‐complex, generated from Fe(II)‐complex (ITT) and chiral iodosoarene (CIB) in the presence of HClO4, is successfully applied to the catalytic enantioselective epoxidation of di‐ and trisubstituted α,β‐unsaturated enones.

Kinetic studies of fluorinated aryl molybdenum(ii) tricarbonyl precursors in epoxidation catalysis

Benzyl substituted molybdenum tricarbonyl complexes displaying CF3 groups are synthesized and applied as catalyst precursors in olefin epoxidation reactions.

A discrete {Co4(μ3-OH)4}(4+) cluster with an oxygen-rich coordination environment as a catalyst for the epoxidation of various olefins.

Using the sterically hindered terphenyl-based carboxylate, the tetrameric Co(ii) complex [Co4(μ3-OH)4(μ-O2CAr(4F-Ph))2(μ-OTf)2(Py)4] () with an asymmetric cubane-type core has been synthesized and fully characterized by X-ray diffraction, UV-vis spectroscopy, and electron paramagnetic resonance spectroscopy. Interestingly, the cubane-type cobalt cluster with 3-chloroperoxybenzoic acid as the oxidant was found to be very effective in the epoxidation of a variety of olefins, including terminal olefins which are more challenging targeting substrates. Moreover, this catalytic system showed a fast reaction rate and high epoxide yields under mild conditions. Based on product analysis and Hammett studies, the use of peroxyphenylacetic acid as a mechanistic probe, H2(18)O-exchange experiments, and EPR studies, it has been proposed that multiple reactive cobalt-oxo species Co(V)[double bond, length as m-dash]O and Co(IV)[double bond, length as m-dash]O were involved in the olefin epoxidation.

One-pot epoxidation of alkenes using aerobic photoperoxidation of toluenes

We developed an aerobic photooxidative synthesis of peroxybenzoic acids from toluenes using anthraquinone-2-carboxylic acid (AQN-2-CO2H) as a photocatalyst. We also found a one-pot epoxidation reaction of alkenes using 4-tert-butylperoxybenzoic acid produced in situ by aerobic photooxidative synthesis.

Manganese Complexes of Pyrrole‐ and ­Indolecarboxamide Ligands: Synthesis, Structure, Electrochemistry, and Applications in Oxidative and Lewis‐Acid‐­Assisted Catalysis

AbstractThis work shows the synthesis and structural, spectroscopic, and electrochemical properties of MnIII complexes supported with pyrrole‐ and indolecarboxamide ligands. In all cases, the respective ligand constitutes a N4 coordination sphere about the MnIII ion. The MnIII complexes of pyrrolecarboxamide ligands are square pyramidal with a fifth Cl atom, whereas analogous complexes of indolecarboxamide ligands are essentially square planar. Electrochemical studies reveal highly negative MnIII/II and moderately positive MnIV/III redox potentials. In situ generated Mn4+ species of the pyrrolecarboxamide ligands were characterized by absorption and electron paramagnetic resonance spectroscopy. All complexes functioned as catalysts in olefin epoxidation reactions by using PhIO as the oxo‐transfer agent. All complexes also acted as Lewis acid catalysts for ring‐opening reactions of assorted epoxides with various nucleophiles. We also show a one‐pot, two‐step epoxidation reaction followed by ring opening, which illustrates the catalytic significance of the present complexes. Importantly, MnIII complexes carrying electron‐donating substituents on the ligand were found to be better catalysts.

Mononuclear Nonheme Iron(III)‐Iodosylarene and High‐Valent Iron‐Oxo Complexes in Olefin Epoxidation Reactions

AbstractHigh‐spin iron(III)‐iodosylarene complexes are highly reactive in the epoxidation of olefins, in which epoxides are formed as the major products with high stereospecificity and enantioselectivity. The reactivity of the iron(III)‐iodosylarene intermediates is much greater than that of the corresponding iron(IV)‐oxo complex in these reactions. The iron(III)‐iodosylarene species—not high‐valent iron(IV)‐oxo and iron(V)‐oxo species—are also shown to be the active oxidants in catalytic olefin epoxidation reactions. The present results are discussed in light of the long‐standing controversy on the one oxidant versus multiple oxidants hypothesis in oxidation reactions.

Mononuclear Nonheme Iron(III)-Iodosylarene and High-Valent Iron-Oxo Complexes in Olefin Epoxidation Reactions.

High-spin iron(III)-iodosylarene complexes are highly reactive in the epoxidation of olefins, in which epoxides are formed as the major products with high stereospecificity and enantioselectivity. The reactivity of the iron(III)-iodosylarene intermediates is much greater than that of the corresponding iron(IV)-oxo complex in these reactions. The iron(III)-iodosylarene species-not high-valent iron(IV)-oxo and iron(V)-oxo species-are also shown to be the active oxidants in catalytic olefin epoxidation reactions. The present results are discussed in light of the long-standing controversy on the one oxidant versus multiple oxidants hypothesis in oxidation reactions.

Interplay of Experiment and Theory in Elucidating Mechanisms of Oxidation Reactions by a Nonheme Ru(IV)O Complex.

A comprehensive experimental and theoretical study of the reactivity patterns and reaction mechanisms in alkane hydroxylation, olefin epoxidation, cyclohexene oxidation, and sulfoxidation reactions by a mononuclear nonheme ruthenium(IV)-oxo complex, [Ru(IV)(O)(terpy)(bpm)](2+) (1), has been conducted. In alkane hydroxylation (i.e., oxygen rebound vs oxygen non-rebound mechanisms), both the experimental and theoretical results show that the substrate radical formed via a rate-determining H atom abstraction of alkanes by 1 prefers dissociation over oxygen rebound and desaturation processes. In the oxidation of olefins by 1, the observations of a kinetic isotope effect (KIE) value of 1 and styrene oxide formation lead us to conclude that an epoxidation reaction via oxygen atom transfer (OAT) from the Ru(IV)O complex to the C═C double bond is the dominant pathway. Density functional theory (DFT) calculations show that the epoxidation reaction is a two-step, two-spin-state process. In contrast, the oxidation of cyclohexene by 1 affords products derived from allylic C-H bond oxidation, with a high KIE value of 38(3). The preference for H atom abstraction over C═C double bond epoxidation in the oxidation of cyclohexene by 1 is elucidated by DFT calculations, which show that the energy barrier for C-H activation is 4.5 kcal mol(-1) lower than the energy barrier for epoxidation. In the oxidation of sulfides, sulfoxidation by the electrophilic Ru-oxo group of 1 occurs via a direct OAT mechanism, and DFT calculations show that this is a two-spin-state reaction in which the transition state is the lowest in the S = 0 state.

Stereochemical Communication within a Chiral Ion Pair Catalyst.

Ionic interactions are increasingly appreciated as a key, asymmetry-inducing factor in enantioselective catalytic transformations, including those involving Brønsted acid or base catalysis, phase-transfer catalysis, and related processes. However, a detailed understanding of these interactions is often lacking. Herein, we show how an enantiopure anion enforces a chiral conformation onto a catalytically relevant achiral cation. Specifically, we use vibrational circular dichroism (VCD) spectroscopy to monitor the transmission of stereochemical information from a chiral phosphate anion to a flexible manganese(III)-salen cation. We show that VCD can be used to study solvent effects and that the obtained chiroptical data directly and quantitatively correlate with the experimentally observed enantioselectivity in an asymmetric olefin epoxidation reaction.

Self-Supported Chiral Polymeric MnIII Salen Complexes as Highly Active and Recyclable Catalysts for Epoxidation of Nonfunctionalized Olefins.

A series of self-supported chiral polymeric MnIII N,N'-ethylenebis(salicylimine) (salen) complexes were synthesized through metalation of the corresponding salen ligands obtained by condensation of several bis/tris-aldehydes with (1R,2R)-1,2-diaminocyclohexane. Upon employment in the asymmetric epoxidation reaction of nonfunctionalized olefins, all complexes showed enhanced activity and enantioselectivity relative to the classical Jacobsen's monomeric salen complex. However, 1,3,5-triazole-based polymeric MnIII salen complex 7 was noticeably preferred over others owing to its ability to render higher enantioselectivity at the expense of lower catalyst loading. Furthermore, complex 7 was recycled and reused in eight recycling experiments with marginal loss in catalytic activity.

Manganese- and cobalt-based coordination networks as promising heterogeneous catalysts for olefin epoxidation reactions.

We demonstrate the synthesis of Mn(2+)- and Co(2+)-based coordination networks using two Co(3+)-based metalloligands differing by the position of the appended arylcarboxylate groups. The structural analyses reveal topologically distinct networks with pores of variable dimensions allowing facile diffusion of substrates and/or reagents. All four networks function as heterogeneous catalysts for the olefin epoxidation reactions using tert-butyl-hydroperoxide without any requirement of solvent or additive. Control and optimization experiments illustrate recyclable network-based catalysts that make them attractive candidates for the "greener" oxidation chemistry processes.

Highly efficient aerobic epoxidation of cyclic olefins in mild conditions by a novel binuclear manganese(II) complex containing N-(4-nitrophenyl)picolinamide ligand

A novel binuclear Mn(II) complex based on a para-nitro substituted amidic ligand (N-(4-nitrophenyl)picolinamide) has been synthesized and characterized by X-ray crystallography. This complex shows a high degree of conversion and epoxide selectivity in Mukaiyama aerobic epoxidation reactions of cyclic olefins in mild conditions.

Reusable manganese compounds containing pyrazole-based ligands for olefin epoxidation reactions.

We describe the synthesis of new manganese(ii) and manganese(iii) complexes containing the bidentate ligands 2-(3-pyrazolyl)pyridine, pypz-H, and 3(5)-(2-hydroxyphenyl)pyrazole, HOphpz-H, with formula [MnX2(pypz-H)2] (X = Cl(-), 1, CF3SO3(-), 2, OAc(-), 3 or NO3(-) (4)), [MnCl2(pypz-H)(H2O)2], 5, or [MnCl(Ophpz-H)2], 6. All the complexes have been characterized through analytical, spectroscopic and electrochemical techniques. Single X-ray structure analysis revealed a six-coordinated Mn(ii) ion in complexes 1-5, and a five-coordinated Mn(iii) ion in complex 6. Compound 5 is the first co-crystal of Mn(ii) containing Cl and H2O ligands together with bidentate nitrogen ligands. The catalytic activity of complexes 1-6 has been tested with regard to the epoxidation of styrene and, in the case of 1, 5 and 6, other alkenes have been epoxidized using peracetic acid as oxidant in different media, among which glycerol, a green solvent never used in epoxidation reactions using peracetic acid as oxidant. The catalysts show moderate to high conversions and selectivities towards the corresponding epoxides. For complexes 1, 5 and 6, a certain degree of cis→trans isomerization is observed in the case of cis-β-methylstyrene. These observations have been explained through computational calculations. The reutilization of catalysts 1 and 6 for the epoxidation of alkenes has been evaluated in [bmim] : acetonitrile mixture (bmim = 1-butyl-3-methylimidazolium), allowing the effective recyclability of the catalytic system and keeping high conversion and selectivity values up to 12 successive runs, in all cases.

New Oxomolybdenum(VI) Hybrid Materials

The chemistry of MovIoxides is dominated by complexes containing the cis-dioxomolybdenum(VI) structural units. Many of these complexes have been investigated as epoxidation catalysts and a number of papers have been published on the effect of nitrogen donor ligands. The motivation to study these discrete, homogeneous catalysts in the form of monomeric or dimeric species stemmed from the fact that they are well-defined, single site catalysts, which usually favour high reaction activities and product selectivities. Our research group has investigated the reaction of dichlorodioxomolybdenum(VI) complexes with excess water using three distinct heating methods: hydrothermal, reflux or microwave-assisted synthesis. The main challenge was to design and prepare novel heterogeneous MovIcatalysts with the same catalytic properties and kinetics usually attained from the homogeneous counterparts. The results led to the isolation of several higher nuclearity oxomolybdenum(VI) compounds, namely the octanuclear complex [Mo8O22(OH)4(di-t-Bu-bipy)4] (di-tert-butyl-2,2'-bipyridine),[1] and the polymeric materials {[MoO3(bipy)][MoO3(H2O)]}n (2,2'-bipyridine)[2] and {[Mo3O9(pzpy)}n (2-[3(5)-pyrazolyl]pyridine).[3] These compounds where characterized by using solid-state techniques, namely elemental analyses, thermogravimetry, FT-IR, solid-state NMR, electron microscopy and powder X-ray diffraction (both from laboratory and/or synchrotron sources). The catalytic behaviour of the discrete complexes and the polymeric compounds was tested in olefin epoxidation reactions. Fundação para a Ciência e a Tecnologia (FCT, Portugal; PTDC/EQUEQU/121677/2010; PTDC/QEQ-SUP/1906/2012; SFRH/BPD/97660/2013; EXPL/CTM-NAN/0013/2013 - FCOMP-01-0124-FEDER-041282), the European Union, QREN, FEDER, COMPETE, and Associate Laboratory CICECO (PEst-C/CTM/LA0011/2013) are gratefully acknowledged for funding.

Advance of Glucosamine and Their Derivatives as Chiral Ligands in Asymmetric Syntheses

Glucosamine is a chiral compound degraded from widely existed natural product chitosan. It is becoming an ideal raw material for preparation chiral amine, amine/phosphine ligands due to its more chiral centers and strong coordination ability. The application of glucosamine derivatives as chiral ligands in asymmetric organic reactions has become a very active research area. In this paper, the application of glucosamine derivatives in asymmetric allylic alkylation, Heck reaction, 1,4conjugate addition reaction, epoxidation reaction of olefins and cyclopropanation of olefin has been reviewed.

Asymmetric Epoxidation of Olefins with Homogeneous Chiral (Salen) Manganese (III) Complex

Optically active epoxides are known as versatile chiral building blocks in organic synthesis, and many efforts have been directed toward developing asymmetric epoxidation (AE) reaction of olefins. The first major breakthrough in AE using synthetic catalysts was realized by Katsuki and Sharpless in 1980, they achieved enantiomeric excesses (ee’s) greater than 90% in the titanium tartrate-based epoxidation of a variety of allylic alcohols. However, the substrates must bear specific functional groups to achieve the precoordination required for high enantioselectivity. In 1990 Jacobsen et al. and Katsuki et al. announced their independently developed systems for the catalytic AE of unfunctionalized olefins using chiral manganese (III) salen complexes. Since that time, the area of chiral manganese (III)- catalyzed AE has expanded greatly. In this review we will focus on the ligand development and the application of homogeneous chiral (salen) manganese (III)-promoted AE of olefins in recent years. Keywords: Asymmetric epoxidation, Chiral, Manganese, Salen, homogeneous, Ligand, Olefin.

Dioxidomolybdenum(VI) Complexes Containing Ligands with the Bipyrrolidine Backbone as Efficient Catalysts for Olefin Epoxidation

AbstractAir‐stable chiral dioxidomolybdenum(VI) complexes of the type [MoO2(L)] (L = L12–; 1, L = L22–, 2) were prepared by the reaction of [MoO2Cl2] with the bis(phenol) ligands 1,4‐bis(2‐hydroxy‐benzyl)‐(S,S)‐2,2′‐bipyrrolidine [(S,S)‐H2L1] and 1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)‐(S,S)‐2,2′‐bipyrrolidine [(S,S)‐H2L2). They were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single‐crystal X‐ray diffraction analysis, which revealed a distorted octahedral coordination geometry around the molybdenum(VI) center with a cis‐α configuration. The Mo–Ooxo bond lengths are almost equal and vary only between 1.701 and 1.7091 Å. Complexes 1 and 2 were found to be efficient and selective catalysts for various olefin epoxidation reactions. Catalyst loadings of 0.5 mol‐%, the use of 2 equiv. of oxidant (tert‐butyl hydroperoxide), as well as unchanged catalyst performance over three consecutive catalytic runs demonstrate the excellent stability of the catalysts. Various challenging substrates including 1‐octene or limonene were converted selectively to their corresponding epoxides. However, no asymmetric induction was observed.