Synthesis and activity of macrocyclized chiral Mn(III)-Schiff-base epoxidation catalysts

A series of chiral macrocyclic Mn(III)Salen complexes has been prepared with two salicylidene moieties linked in their 3 and 3′ positions by aliphatic polyether bridges of variable lengths or by a more rigid aromatic junction arm. X-ray structures of ligand precursors and of complex 8 have been performed. All complexes have been used in the asymmetric epoxidation of 1,2-dihydronaphthalene with NaOCl as oxygen atom donor and exhibited modest enantiomeric excesses. Complex 10 was selected to be tested with two cis-disubstituted olefins and several oxidants, namely NaOCl, PhIO and n-Bu₄NHSO₅. 2,2′-Dimethylchromene oxide was obtained from 2,2′-dimethylchromene with ee values of 56% and 74% when using 10 and NaOCl and PhIO, respectively.     

Synthesis and structure of V(V) and Mn(III) NHC complexes supported by a tridentate bis-aryloxide-N-heterocyclic carbene ligand

Vanadium (V) and manganese (III) metal complexes (2, [^tBu(OCO)]V(O)Cl; 3, [^tBu(OCO)]Mn(acac)), which are supported by a tridentate bis-aryloxide-N-heterocyclic carbene ligand ([^tBu(OCO)]²⁻[η³-O,C,O-{(3,5-di-tert-butyl-C₆H₂O)₂N₂C₃H₄}]²⁻) have been prepared and structurally characterized. Both complexes were efficiently synthesized in a straightforward and smooth manner involving the direct reaction of the imidazolinium proligand 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride 1, easily accessible in a two-step synthesis with an overall good yield, with (ⁱPrO)₃VO and Mn(acac)₃, respectively.     

Iron(II) complexes with bio-inspired N,N,O ligands as oxidation catalysts: olefin epoxidation and cis-dihydroxylation

The Rieske dioxygenases are a group of non-heme iron enzymes, which catalyze the stereospecific cis-dihydroxylation of its substrates. Herein, we report the iron(II) coordination chemistry of the ligands 3,3-bis(1-methylimidazol-2-yl)propionate (L1) and its neutral propyl ester analogue propyl 3,3-bis(1-methylimidazol-2-yl)propionate (PrL1). The molecular structures of two iron(II) complexes with PrL1 were determined and two different coordination modes of the ligand were observed. In [Fe(II)(PrL1)(2)](BPh(4))(2) (3) the ligand is facially coordinated to the metal with an N,N,O donor set, whereas in [Fe(II)(PrL1)(2)(MeOH)(2)](OTf)(2) (4) a bidentate N,N binding mode is found. In 4, the solvent molecules are in a cis arrangement with respect to each other. Complex 4 is a close structural mimic of the crystallographically characterized non-heme iron(II) enzyme apocarotenoid-15-15'-oxygenase (APO). The mechanistic features of APO are thought to be similar to those of the Rieske oxygenases, the original inspiration for this work. The non-heme iron complexes [Fe(II)(PrL1)(2)](OTf)(2) (2) and [Fe(II)(PrL1)(2)](BPh(4))(2) (3) were tested in olefin oxidation reactions with H(2)O(2) as the terminal oxidant. Whereas 2 was an active catalyst and both epoxide and cis-dihydroxylation products were observed, 3 showed negligible activity under the same conditions, illustrating the importance of the anion in the reaction.     

Evidence for involvement of cationic intermediate in epoxidation of chiral allylic alcohols and unfunctionalised alkenes catalysed by MnIII(quinazolinone) complexes

A pair of Mn^III(quinazolinone) complexes was prepared and evaluated in the catalytic epoxidation of the chiral allylic alcohols and unfunctionalised alkenes with iodosylbenzene. Epoxidation of chiral allylic alcohols with 1,3-allylic strain proceeded chemo- and diastereoselectively to give threo-epoxy alcohol (up to 99% d.r). Epoxidation of unfunctionalised alkenes by the proposed catalyst system proceeded nonselectively, as evidenced by the formation of isomerization and rearrangement products in the epoxidation of (Z)-stilbene. A three-step pathway involving a cationic intermediate is proposed for the formation of isomerization and rearrangement products. The fact that only products resulting from the cationic intermediate were detected in the oxidation of a mechanistic probe, (2-methoxy-3-vinylcyclopropyl)benzene, with Mn^III(quinazolinone) and PhIO substantiated the cationic oxygen transfer mechanism.     

Studies of the catalytic activity of Fe(III)(salen) complexes as epoxidation catalysts

Two new Fe(III)(salen) complexes, FeL¹ClO₄·2H₂O (1) and FeL²ClO₄ (2) [L¹ = N,N′-ethylenebis(3-formyl-5-methylsalicylaldimine) and L² = N,N′-cyclohexenebis(3-formyl-5-methylsalicylaldimine)], have been synthesized and characterized. The catalytic activity of the complexes for epoxidation of alkenes has been investigated in the presence of two terminal oxidants PhIO and NaOCl, with two solvents CH₃CN and CH₂Cl₂. As alkenes styrene and (E)-stilbene have been chosen for investigation; styrene is a better substrate than electron-rich (E)-stilbene. The study also suggests that unlike their Mn(III) counterparts, 1 and 2 are poor epoxidation catalysts; catalysis proceeds with formation of one intermediate, rather than forming more than one intermediate depending on the terminal oxidant used. Use of exogenous neutral donor ligands such as Py, PyNO and 1-MePy is effective to improve catalytic behavior.     

Catalytic activity of manganese(III)-oxazoline complexes in urea hydrogen peroxide epoxidation of olefins: The effect of axial ligands

The catalytic activity of two manganese(III)-oxazoline complexes [Mn(phox)₂(CH₃OH)₂]ClO₄ and Mn(phox)₃ (Hphox = 2-(2′-hydroxylphenyl)oxazoline), was studied in the epoxidation of various olefins. All of epoxidation reactions were carried out in (1:1) mixture of methanol:dichloromethane at room temperature using urea hydrogen peroxide (UHP) as oxidant and imidazole as co-catalyst. The epoxide yields clearly demonstrate the influence of steric and electronic properties of olefins, the catalysts and nitrogenous bases as axial ligand. [Mn(phox)₂(CH₃OH)₂]ClO₄ catalyst with low steric properties has higher catalytic activity than Mn(phox)₃. The highest epoxide yield (95%) was achieved for indene at the presence of [Mn(phox)₂(CH₃OH)₂]ClO₄ within 5 min. The proximal and distal interactions of strong π-donor axial ligands such as imidazole with the active intermediate are efficiently increased activity of the catalytic system.     

胺功能化的拟薄水铝石纳米颗粒负载的Mn-V复合物：高效的烯烃环氧化催化剂

将具有高比表面积和表面高度羟基化的拟薄水铝石纳米颗粒与3-(3甲氧基硅烷)-正丙胺进行共价结合而官能团化,再用于负载硫酸氧钒和六羰基钼络合物。所得样品采用红外光谱、粉末X射线衍射、热重-差热分析、X射线光电子能谱、元素分析、电感耦合等离子体和透射电镜等技术进行了表征,并用于顺-环辛烯的环氧化反应中,优化了诸如溶剂和氧化剂等反应条件。反应过程采用气-液色谱进行监测。重复使用实验表明,该纳米催化剂可重复使用多次,并保持顺-环辛烯接近完全环氧化。所得到的优化反应条件也成功用于其它的取代烯烃的环氧化反应中。     

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

Rhenium(I), (III) and (V) complexes of tridentate ONX (X = O,N, S)-donor Schiff bases

The reactions of the potentially tridentate Schiff bases 2-[(2-hydroxyphenyl)iminomethyl]phenol (H₂ono) and 2-(2-aminobenzylideneimino)phenol (H₃onn) with trans-[ReOBr₃(PPh₃)₂] were studied, and the complexes [Re^IIIBr(PPh₃)₂(ono)] (1) and [Re^VBr(PPh₃)₂(onn)]Br (2) were isolated. In 1ono acts as a dianionic tridentate ligand, and in 2onn is coordinated as a tridentate trianionic imido-imino-phenolate. The complex [Re^I(CO)₃(ons)(Hno)] was isolated from the reaction of [Re(CO)₅Br] with 2-[(2-methylthio)benzylideneimino]phenol (Hons; Hno = 2-aminophenol), with ons coordinated as a bidentate chelate with a free SCH₃ group. These complexes were characterized by X-ray crystallography, NMR and IR spectroscopy.     

A new square planar mononuclear Mn complex for catalytic epoxidation of stilbene

The manganese(III) complex (2) with a diamide ligand has been synthesized. This complex was found to catalyze both the epoxidation of (Z)- and (E)-stilbene with high conversion and the oxidation of benzyl alcohol to benzaldehyde.     

Oxovanadium(IV) complexes as molecular catalysts in epoxidation: Simple access to pyridylalkoxide derivatives

Reaction of bis(aryl)-2-pyridylmethanol ligands (1a-7a) with VO(SO₄) · 5H₂O results in the formation of metal-oxo complexes [VO(N-O)₂] (1-7), with N-O = bis(aryl)-2-pyridylmethanol. A molecular structure of (4) has been determined by single crystals X-ray diffraction study, which showed the expected square planar pyramidal geometry with the pyridine ring nitrogens in trans-position to each other. The metal-oxo complexes (1-4,6,7) demonstrated the ability to catalyse epoxidation reactions of alkenes with molecular oxygen.     

非血红素N4配体Mn(III)配合物催化烯烃的不对称环氧化反应

利用Mn(OAc)3与手性四氮配体制备了非血红素N4配体的Mn(Ⅲ)配合物,并将其应用于催化α,β-不饱和烯酮和简单烯烃的不对称环氧化反应,考察了氧化剂H2O2的量和添加剂HOAc的量等条件对反应结果的影响,研究了该催化剂的底物适用范围,获得了51%~94%的ee值.     

Reactivity of tris(acetylacetonato) iron(III) with tridentate [ONO] donor Schiff base as an access to newer mixed-ligand iron(III) complexes

Two new mixed-ligand iron(III) complexes, [Fe(L(n))(acac)(C(2)H(5)OH)] incorporating coordinated ethanol from the reaction solvent were accessed from the reaction of [Fe(acac)(3)] with [ONO] donor dibasic tridentate unsymmetrical Schiff base ligands derived from condensation of 2-hydroxy-1-napthaldehyde with 2-aminophenol (H(2)L(1)) or 2-aminobenzoic acid (H(2)L(2)). The thermal study (TGA-DTA) provided evidence for weakly bound ethanol which is readily substituted by neutral N-donor molecule imidazole, benzimidazole or pyridine to produce an array of newer complexes, [Fe(L(n))(acac)X] (n=1, 2; X=Im, Bim, Py). The compounds were characterized by elemental analyses, FT-IR, UV-vis, solution electrical conductivity, FAB mass, (1)H and (13)C NMR spectroscopy. Room temperature magnetic susceptibility measurements (μ(eff)～5.8 B.M.) are consistent with spin-free octahedral iron(III) complexes. Cyclic voltammetry of ethanol complexes revealed a quasi-reversible one electron redox response (ΔE(p)>100 mV) for the Fe(III)/Fe(II) couple. Low half wave redox potential (E(1/2)) values suggested easy redox susceptibility. The ground state geometries of the ethanol and imidazole complexes have been ascertained to be distorted octahedral by density functional theory using DMol3 program at BLYP/DNP level.     

Multi-wall carbon nanotube supported manganese(III)tetraphenylporphyrin: efficient catalysts for epoxidation of alkenes with NaIO4 under various reaction conditions

Manganese(III)tetraphenylporphyrin supported on multi-wall carbon nanotubes (MWCNTs) were developed as efficient catalysts for epoxidation of alkenes with sodium periodate under mild conditions. The catalysts were prepared by axial ligation of manganese to 2-aminobenzimidazole and 2-aminothiazole preanchored to MWCNTs. The formation of these heterogenized catalysts was followed using elemental analysis, FT-IR spectroscopy, diffuse reflectance UV-Vis spectrophotometry, and scanning electron microscopy. The reactivity of these catalysts under both magnetic stirring and ultrasonic irradiation and also their reusability were investigated.     

Enantioselective epoxidations of alkenes catalyzed by (salen)Mn(III) in aqueous surfactant systems

An aqueous reaction medium, based on a surfactant solution of diethyltetradecylamine N-oxide (AOE-14), was developed for the enantioselective epoxidation of 1,2-dihydronaphthalene and of various cis-β-alkyl styrenes with increasing hydrophobicity, using bleach as oxidant and the Jacobsen chiral (salen)Mn(III) as catalyst. AOE-14 is able to both solubilize all reactants in water and bind the metal of the salen complex acting as coligand. Its use leads to good yields (>75%) and to ee values ranging from 75% up to 91% even in the case of cis-β-alkyl styrenes where lower cis/trans epoxide ratios are observed. The ratio of surfactant/substrate used is 1:1 or 4:1, much lower than those generally used in the literature.     

Mesoporous SBA-15 materials modified with oxodiperoxo tungsten complexes as efficient catalysts for the epoxidation of olefins with hydrogen peroxide

A hybrid mesoporous SBA-15 material (2a) containing an oxodiperoxo tungsten complex of the type [WO(O₂)₂L] (L = pyrazolylpyridine) was synthesized by a post-grafting route. The organic–inorganic hybrid catalyst was characterized by means of XRD, N₂ adsorption–desorption and FT-IR. The catalytic property of 2a in the epoxidation of cyclooctene with H₂O₂ as the oxidant was investigated in comparison with other three kinds of hybrid tungsten containing SBA-15 materials bearing ethylenediamine, imidazole or 4,4′-bipyridine ligands. It was found that all oxodiperoxo tungsten catalysts were active at the reaction temperature of 55 °C with CH₃CN as solvent. However, only the catalyst with the pyrazolylpyridine ligand showed good recoverability and relatively high stability against leaching of active tungsten species. Moreover, this catalyst showed very high efficiency for H₂O₂ utilization, and its catalytic activity could be further improved by using solvent mixtures of CH₃CN and CH₃COOH.     

Aminopropyl group-modified SBA-15 covalent attachment Mn(salen) complexes as catalysts for styrene epoxidation

A series of aminopropyl group-modified ordered mesoporous silica materials impregnated with Mn(salen) were prepared using successive grafting procedures. The prepared composite catalysts were well characterized by inductively coupled plasma atomic emission spectroscopy, Fourier transform-infrared, UV–Vis diffuse reflectance spectroscopy, X-ray diffraction analysis, and transmission electron microscopy in order to confirm the structure integrities of the Mn(salen) units after the incorporation, to evidence the formation of a covalent bond between the starting Mn(salen) units and the aminopropyl group-modified SBA-15 matrix in the presence of NaOH by abstraction of an HCl molecule. These heterogeneous catalysts exhibited comparable catalytic activity and selectivity to those of the homogeneous counterpart in the epoxidation of styrene by using NaClO as oxidant. In addition, the effects of key reaction parameters, including the loadings of the neat Mn(salen), molar ratios of NaClO to styrene, and PPNO amount on the reactivity and selectivity, were also studied. Finally, the reusability of the prepared heterogeneous catalyst was evaluated.     

Self-assembled biomimetic catalysts: studies of the catalase and peroxidase activities of Mn(III)-Schiff base complexes

Five Mn(III) nitrate complexes have been synthesized from dianionic hexadentate Schiff bases obtained by the condensation of 3-ethoxy-2-hydroxybenzaldehyde with different diamines. The complexes have been characterized by elemental analysis, ESI mass spectrometry, IR and ¹H NMR spectroscopy, r. t. magnetic, and molar conductivity measurements. Parallel-mode EPR spectroscopy of 1 is also reported. Ligand H₂L³ and complexes [MnL¹(H₂O)₂](NO₃)(CH₃OH) (1), [MnL³(H₂O)₂]₂(NO₃)₂(CH₃OH)(H₂O) (3), and [MnL⁴(H₂O)₂](NO₃)(H₂O)₂ (4) were crystallographically characterized. The X-ray structures show the self-assembly of the Mn(III)–Schiff base complexes through µ-aquo bridges between neighboring axial water molecules and also by π–π stacking interactions, establishing dimeric and polymeric structures. The peroxidase and catalase activities of the complexes have been studied. Complexes with the shorter spacer between the imine groups (1–2) behave as better peroxidase and catalase mimics, probably due to their ability to coordinate the hydrogen peroxide substrate to manganese.     

Synthesis,characterization and pronounced epoxidation activity of cis-dioxo-molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide

The synthesis and catalytic performance of novel cis-dioxo-Mo(VI) complexes containing simple ONO tridentate Schiff base ligands in the epoxidation of various olefins using tert-butyl hydroperoxide in desired times with excellent chemo- and stereoselectivity have been described. The study of turnover numbers and the UV–Vis spectra of the Mo complexes in the present epoxidation system indicate well the high efficiency and stability of the catalysts during the reaction. The electron-deficient and bulky groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst.