Mn(III) complexes with tridentate N,N,O-ligands as catalysts for the epoxidation of alkenes

Mn(III) complexes with tridentate Schiff bases have been prepared and applied as catalyst precursors in epoxidation of alkenes using iodosobenzene as an oxidant providing high conversions and high selectivities when cyclohexene derivatives were studied.     

Redox isomerisation of allylic alcohols catalysed by water-soluble ruthenium complexes in aqueous systems

The process of catalytic isomerisation of various allylic alcohols (alk-1-en-3-ols) into saturated ketones under mild conditions is reported. The water-soluble Na₄[{RuCl₂(mtppms)₂}₂] complex, previously reported by us as a precursor to very active hydrogenation catalysts was also found an active catalyst of the redox isomerisation of allylic alcohols in aqueous media. The new Na[Ru(CO)Cp(mtppms)₂] as well as Na₄[{RuCl(μ-Cl)(CCCPh₂)(mtppms)₂}₂] and Na₂[RuClCp(mtppms)₂] also showed good to excellent catalytic activities for redox isomerisations in aqueous systems at 50-80 °C under inert atmosphere.     

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, physico-chemical studies and solvent-dependent enantioselective epoxidation of 1,2-dihydronaphthalene catalysed by chiral Ruthenium(II) Schiff base complexes. I

Ruthenium(II) chiral Schiff base complexes 1–10 and their precursor ligands derived from -amino acids viz. -leucine, -histidine with salicylaldehyde, 3-tertiary-butyl-, 3,5-di-tertiary-butyl-, 3,5 dichloro- and 3,5-dinitrosalicylaldehyde are reported. The characterization of the ligands and complexes was accomplished by various appropriate physico-chemical studies, namely, microanalysis, IR-, UV/Vis-, ¹H, ³¹P{¹H} NMR, CD spectroscopy, optical rotation, conductance measurement and cyclic voltammetry. The complexes thus synthesised were used as catalysts for enantioselective epoxidation of 1,2-dihydronaphthalene. The effect on enantioselectivity and chemical conversions to epoxide were studied in different solvents viz. acetonitrile, dichloromethane and fluorobenzene along with change of the substituents on ligands and different terminal oxidants. The less polar nature of solvent as well as the donating group attached on the catalysts favours enantioselectivity, while PhIO was the oxidant of choice. The enantiomeric excess of the resulting epoxide was evaluated by chiral cyclodex BDA capillary column.     

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.     

Rapid and highly selective epoxidation of alkenes by tetrabutylammonium monopersulfate in the presence of manganese meso-tetrakis(pentafluorophenyl)porphyrin and tetrabutylammonium salts or imidazole co-catalysts

Epoxidation of various alkenes in low to high yields (29-100%) and good to excellent selectivities (75-100%) was performed with tetrabutylammonium monopersulfate in the presence of meso-tetrakis(pentafluorophenyl)porphyrin as catalyst and tetrabutylammonium acetate or fluoride or imidazole as co-catalysts in CH₂Cl₂, in less than 10 min at room temperature (∼25 °C).     

Highly selective, recyclable epoxidation of allylic alcohols with hydrogen peroxide in water catalyzed by dinuclear peroxotungstate

The highly chemo-, regio-, and diastereoselective and stereospecific epoxidation of various allylic alcohols with only one equivalent of hydrogen peroxide in water can be efficiently catalyzed by the dinuclear peroxotungstate, K2[[W(=O)(O2)2(H2O)]2(mu-O)].2H2O (I). The catalyst is easily recycled while maintaining its catalytic performance. The catalytic reaction mechanism including the exchange of the water ligand to form the tungsten-alcoholate species followed by the insertion of oxygen to the carbon-carbon double bond, and the regeneration of the dinuclear peroxotungstate with hydrogen peroxide is proposed. The reaction rate shows first-order dependence on the concentrations of allylic alcohol and dinuclear peroxotungstate and zero-order dependence on the concentration of hydrogen peroxide. These results, the kinetic data, the comparison of the catalytic rates with those for the stoichiometric reactions, and kinetic isotope effects indicate that the oxygen transfer from a dinuclear peroxotungstate to the double bond is the rate-limiting step for terminal allylic alcohols such as 2-propen-1-ol (1a).     

Selective oxidations of allylic alcohols using vanadyl and cobalt(II) alkyl phosphonate modified silicas

A range of allylic alcohols can be selectively oxidised to either the corresponding epoxide or the enone in good yields using catalytic quantities of vanadyl or cobalt(II) alkyl phosphonate modified silicas, CoEPS3 and VOEPS3 and tert-butyl hydroperoxide.     

Synthesis and catalytic properties in olefin epoxidation of chiral oxazoline dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes with the general formula [MoO₂X₂(N,N)] (X = Cl, OSiPh₃) containing a chiral bidentate oxazoline ligand (N,N = 2,2′-bis[(4S)-4-benzyl-2-oxazoline]) have been prepared and characterised by ¹H NMR, IR spectroscopy and thermogravimetric analysis. The bis(chloro) complex was heterogenised in the ordered mesoporous silica MCM-41 by direct grafting in dichloromethane. Elemental analysis and ²⁹Si MAS NMR spectroscopy of the derivatised material indicated the presence of monopodally anchored species of the type MoO₂[(–O)₃SiO]Cl(N,N). The complex [MoO₂Cl₂(N,N)] and the derivatised material exhibited initial activities of 147 and , respectively, in the catalytic epoxidation of cyclooctene using tert-butylhydroperoxide (tBuOOH) as the oxidant, both yielding 1,2-epoxycyclooctane quantitatively within 24 h at 55 °C. The MCM-41 grafted catalyst could be recycled with no loss in performance with respect to the epoxide yields obtained for reaction times above 2 h. With trans-β-methylstyrene as the substrate, the bis(chloro) complex and the derivatised material gave epoxides as the only products with yields in the range of 56–64% after 24 h, but no catalytic asymmetric induction was observed. The triphenylsiloxy complex was more active than the bis(chloro) complex for the epoxidation of trans-β-methylstyrene, but the enantiomeric excess was negligible and the corresponding diols were also formed. For the reaction catalysed by the supported material, changing the oxidant from tBuOOH to cumene hydroperoxide greatly improved the catalytic activity but the enantiomeric excess continued very low and the corresponding diol was the main product.     

Scope, mechanism and diene inhibition of isomerization of allylic alcohols to saturated ketones catalyzed by ruthenium(II)-cyclopentadienyl complexes

The isomerization of 3-buten-2-ol to butanone catalyzed by Ru(II)Cp-complexes (Cp = η⁵-cyclopentadienyl) with phosphine and amine ligands is described. The reaction catalyzed by [RuCp(MeCN)₃](PF₆) and two equivalents of triphenylphospine is first order in substrate with a k_ini of 0.43 h⁻¹ and an initial TOF of 13,000 h⁻¹. The catalyst precursor complex [RuClCp(dppb)] (dppb = bis(diphenylphosphino)butane) has been characterized by X-ray diffraction. This compound features a seven-membered ring incorporating the ruthenium centre and the dppb ligand.Combination of two equivalents of primary, secondary or tertiary amines and [RuCp(MeCN)₃](PF₆) results in active catalyst precursors. Within each group, increasing the bulk of the ligand gives lower isomerization rates. The combined effects of optimal pK_a, nucleophilicity and steric bulk make RuCp-complexes with secondary amines the most active precursors. With di-n-butylamine, 745 turnovers can be reached after 1 h. ³¹P NMR spectra indicate that the resting state in the catalytic cycle is a complex in which 3-buten-2-ol is η²-coordinated through the alkene moiety. This implies that coordination of the oxygen moiety and concomitant β-hydrogen abstraction is the rate-limiting step. A counterintuitive result is that allylic alcohols bind stronger to RuCp complexes with phosphine ligands than dienes. Inhibition of the catalyst appears to be a result of interaction of the diene with a ruthenium-allyl alcohol complex, which is sufficiently strong to prevent coordination of the oxygen moiety of the allylic alcohol. This hinders orientation of the allylic alcohol substrate in a suitable way to undergo β-hydrogen abstraction, thereby blocking isomerization catalysis.     

Selective aerobic oxidation of allylic and benzylic alcohols catalyzed by N-hydroxyindole and copper(I) chloride

In the presence of copper(I) chloride, tert-butyl 1-hydroxy-2-methyl-6-trifluoromethyl-1H-indole-3-carboxylate acted as a catalyst for the chemoselective aerobic oxidation of allylic and benzylic alcohols. A variety of primary and secondary allylic and benzylic alcohols were oxidized into the corresponding α,β-unsaturated carbonyl compounds in good yields without affecting non-allylic alcohols.     

Syntheses,structures and electrochemistry of manganese(III) complexes derived from N,N′-o-phenylenebis(3-ethoxysalicylaldimine): Efficient catalyst for styrene epoxidation

Syntheses, characterizations, electrochemistry and catalytic properties for styrene epoxidation of three manganese(III) compounds [Mn^IIIL¹(H₂O)(MeOH)](ClO₄) (1) [Mn^IIIL¹(N₃)(H₂O)]·dmf (2) [Mn^IIIL¹(Cl)(H₂O)] (3) derived from the Schiff base compartmental ligand N,N′-o-phenylenebis(3-ethoxysalicylaldimine) (H₂L¹) are reported. The three compounds are characterized by elemental analyses, IR, mass and UV–Vis spectra and conductance values. Single crystal X-ray structures of 1 and 2 have been determined. The structures of 1 and 2 show that these are mononuclear compounds having a salen type structure. In both structures, a dinuclear species is formed by bifurcated hydrogen bonding involving coordinated water molecule. The coordination of chloride in 3 is shown by conductance measurements. The compounds have also been characterized by UV–Vis and mass spectroscopic studies. Cyclic voltammetric and square wave voltammetric studies of the three compounds reveal that these undergo Mn(III)/Mn(II) reduction reversibly with the order of the ease of reduction as 3 > 2 > 1. This order has been explained proposing the composition of active species in solution. Catalytic properties for epoxidation of styrene by all the three complexes using PhIO and NaOCl as oxidant have been studied. The order of both the styrene conversion and styrene epoxidation using the three title compounds is 3 > 1 > 2. Again, it has been observed that more efficient conversion and epoxidation take place when PhIO is used as oxidant.     

Influence of the built-in pyridinium salt on asymmetric epoxidation of substituted chromenes catalysed by chiral (pyrrolidine salen)Mn(III) complexes

Chiral (pyrrolidine salen)Mn(III) complexes 1 with an N-benzoyl group and 2 with an N-isonicotinoyl group as well as the corresponding N-methyl (3) and N-benzyl (4) pyridinium salts of 2 were synthesized. The catalytic properties of 1–4 and 2 with excess CH₃I were explored to figure out the influence of the internal pyridinium salt in the catalyst on asymmetric epoxidation of substituted chromenes with NaClO/PPNO as an oxidant system in the aqueous/organic biphasic medium. The (pyrrolidine salen)Mn(III) complexes with an internal pyridinium salt, either formed in situ or isolated, displayed higher activities than analogous complexes 1, 2 and Jacobsen's catalyst in the aforementioned reaction, with comparable high yields and ee values. The acceleration of the reaction rate is attributed to the phase transfer capability of the built-in pyridinium salt of the (salen)Mn(III) catalyst. The effect of the internal pyridinium salt on the epoxidation of substituted chromenes is similar to that of the external pyridinium salts and ammonium halides.     

Synthetic scope of Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions: an application to reduction of allylic alcohols by a sequential process of isomerization/Meerwein-Ponndorf-Verley-type reduction

Reduction of allylic alcohols can be promoted efficiently by the supported ruthenium catalyst Ru(OH)x/Al2O3. Various allylic alcohols were converted to saturated alcohols in excellent yields by using 2-propanol without any additives. This Ru(OH)x/Al2O3-catalyzed reduction of a dienol proceeds only at the allylic double bond to afford the corresponding enol, and chemoselective isomerization and reduction can be realized under similar conditions. The catalysis is truly heterogeneous and the high catalytic performance can be maintained during at least three recycles of the Ru(OH)x/Al2O3 catalyst. The transformation of allylic alcohols to saturated alcohols consists of three sequential reactions: oxidation of allylic alcohols to alpha,beta-unsaturated carbonyl compounds; reduction of alpha,beta-unsaturated carbonyl compounds to saturated carbonyl compounds; and reduction of saturated carbonyl compounds to saturated alcohols.     

A carboxylate-bridged Mn(II) compound with 6-methylanthranilate/bipy: oxidation of alcohols/alkenes and catalase-like activity

Abstract

A novel manganese compound, [Mn₂(μ_1,3-6-CH₃-2-NH₂C₆H₄COO)₂(bipy)₄](ClO₄)₂ (bipy = 2,2′-bipyridine), was synthesized and used as a catalyst precursor in the oxidation of alkenes and primary alcohols to corresponding aldehydes, ketones, and acids. The six-coordinate compound has a binuclear structure in which two Mn(II) ions adopt a syn-anti μ_1,3-bridging mode with two carboxylate groups and two chelated bipy ligands. The compound exhibits good activity in the oxidation of cyclohexene to 2-cyclohexene-1-one as the major product (93% conv. in 3 h, 79.3% selectivity) and of cinnamyl alcohol to cinnamaldehyde as the major product with 46% selectivity (100% conv. in 1.5 h) with tert-butyl hydroperoxide (TBHP) in acetonitrile at 70 °C. Furthermore, the catalase-like activity of the compound was studied in different solvents (acetonitrile, methanol, Tris-HCl buffer; TOF = 29,910 h^?1 in Tris-HCl buffer).     

Enantioselective catalytic epoxidation of nonfunctionalized prochiral olefins by dissymmetric chiral Schiff base complexes of Mn(III) and Ru(III) metal ions II

A series of new dissymmetric chiral Schiff base complexes has been obtained by a systematic condensation of (1S,2S)(+)-diaminocyclohexane and 3-acetyl-4-hydroxy-6-methyl-2-pyrone with salicylaldehyde, 5-chloro-, 5-methoxy-and 5-nitrosalicylaldehyde and by subsequent metallation with manganese and ruthenium. The characterization of the complexes 1–8 was accomplished by physico chemical studies viz. microanalysis, IR-, UV/VIS-, and CD spectral studies, optical rotation, molar conductance measurements and cyclic voltammetry. Enantioselective epoxidation of non functionalised olefins, viz. cis-stilbene, trans-3-nonene and trans-4-octene with iodosyl benzene as oxidant was demonstrated in the presence of catalytic amounts of chiral Mn(III) and Ru(III) dissymmetric Schiff base complexes. Good optical yields of epoxides were obtained for the catalyst 4 with the substrates trans-3-nonene and cis-stilbene.     

Synthesis of the new chiral tridentate ligand bis(pyrid-2-ylethyl) menthylphosphine and its use in the palladium-catalyzed allylic alkylations

We describe the synthesis of a new asymmetric P,N,N′-tridentate ligand (bis(pyrid-2-ylethyl) menthylphosphine, BPEMP), containing two pyridyl rings and (1S,2R,5S)-menthylphosphino group. The ligand is obtained in five steps from natural abundant l-menthol. The coordination behavior of the ligand toward cationic (allylic)Pd(II) moiety and its first application in palladium-catalyzed asymmetric allylic alkylation are presented. Crystallographic and spectroscopic analyses reveal that [(η³-allylic)Pd(BPEMP)]⁺ complex forms only one isomer in the solid state as well as in solution.     

A mechanistic study on oxidation of benzylic alcohols with PPh4HSO5 catalysed by manganese(III) porphyrins in homogeneous solution

The oxidation of variously ring-substituted 1-phenylethanols with Ph₄PHSO₅ catalysed by Mn(TMP)Cl and Mn(TDCPP)Cl in the presence of 4-tert-butylpyridine was studied in 1,2-dichloroethane homogeneous solution. The process leads only to C–H bond cleavage products, namely acetophenones. The oxidation rates are independent of the substrate concentration and, when Mn(TMP)Cl is the catalyst, even of the substrate nature. By increasing the concentration of 4-tert-butylpyridine, which acts as an axial ligand of the catalyst, a bell-shaped curve for the rate constants trend is observed. Hammett plots obtained by changing the substituents on the phenyl ring of the benzylic alcohol give different ρ values depending on the technique employed for rate constants determination, i.e., individual or competitive experiment. The observations reported above, together with a KIE of 2.5 in 1- -1-phenylethanol oxidation measured by competitive experiment, are rationalised on the basis of a mechanistic scheme in which the oxo-manganese derivative is formed in the rate determining step of the catalytic process. Furthermore, it is suggested that alcohol dehydrogenation proceeds through a hydride abstraction involving an alcohol-oxo-porphyrinato complex.     

Enantioselective epoxidation reaction of non-functionalised prochiral alkenes using optically resolved [brucine](R)-[Ru(PDTA-H)Cl] and [brucine](S)-[Ru(PDTA-H)Cl] complexes

The racemic metal complex K[Ru(PDTA-H)Cl]¹ has been resolved into its optical isomers using brucine as the resolving agent counter ion, [brucine](S)-[Ru(PDTA-H)Cl] (1) and [brucine](R)-[Ru(PDTA-H)Cl] (2) and their structures are determined by single crystal X-ray methods. Longer Ru–Cl bonds in both the complexes (2.3974(13)A in 1 and 2.415(6) in 2 along with one relatively weaker and strained chelation ring could be responsible for their catalytic activity. The CD pattern of the complex 1 shows the presence of the two isomers λ and δ with more contribution of λ form while the complex 2 acquire only λ conformation. Catalytic activity of 1 and 2 for enantioselective epoxidation of non-functionalised alkenes viz. styrene, 4-chloro-, 4-methyl-, 4-nitrostyrene, 1,2-dihydronaphthalene and indene was accomplished by using molecular oxygen and iodosyl benzene as terminal oxidant. Excellent conversions (85–89%) were obtained in case of 1,2-dihydronaphthalene with both the catalysts while catalyst 2 gave good conversion with styrene and 4-methylstyrene. The enantiomeric excess of the epoxide was determined by ¹H NMR using chiral shift reagent Eu(hfc)₃/ by chiral capillary column. The extent of enantioselectivity with respect to the substituents on substrate is shown on Hammet plot. A possible mechanism at the oxo transfer stage is also envisaged.     

On the key role of water in the allylic activation catalysed by Pd (II) bis phosphinite complexes

Palladium and platinum complexes of bisphosphinites and bisphosphines derived from mandelic acid have been prepared and characterized. Their ability to catalyze allylation of imines with allyltributylstannane has been studied. Bisphophinite complexes of Pd (II) are shown to be ideal and they work best in the presence of one equivalent of water. The near neutral conditions employed make the catalysts suitable for a wide variety of substrates.