Polyacenaphthylene supported t-butyl chromates: a new class of solid phase oxidizing reagents

Copolymers of acenaphthylene with divinylbenzene were functionalized by incorporating t-butyl chromate groups, and the resulting polymeric reagents were used to oxidize alcohols to carbonyl compounds. Primary and secondary alcohols were oxidized to the corresponding carbonyl compounds in quantitative yields. The extent of oxidation was found to depend on the various reaction parameters including the temperature, nature of the solvent, concentration of the reagent functions, duration of the reaction and the presence of catalyst. © 1998 SCI.     

Design and applications of a solid-phase oxidizing reagent consisting of a crosslinked polystyrene matrix and a t-butyl hypochlorite function separated by a trimethylene spacer

A crosslinked polystyrene–supported solid-phase analogue of t-butyl hypochlorite containing a trimethylene spacer group between the polymer matrix and the t-butyl hypochlorite function was prepared and used as a recyclable oxidizing reagent for alcohols. The synthetic route to this new polymeric reagent involved a seven-step polymer-analogous reaction starting from styrene-divinyl benzene 2%-crosslinked polymer. A β-ketopropionic acid function was introduced into the polystyrene matrix by Friedel–Crafts reaction with succinic anhydride. The keto function in the resulting polymer (2) (capacity, 3.57 meq of COCH₂CH₂COOH/g) was converted to the methylene group by Clemmensen reduction using zinc amalgam and HCl. The carboxyl function in the product polymer (3) was converted to the acylmalonic ester function by malonic ester synthesis through the reaction of the polymeric acid chloride (4) with ethoxymagnesium diethylmalonate. The polymeric acyl malonic ester (5) was decarboxylated to yield the 2-oxopentyl polystyrene resin (6). This on Grignard reaction with methyl magnesium iodide followed by hydrolysis afforded the polystyrene derivative with the t-butyl alcohol function separated by three methylene groups (7) . The t-butyl alcohol resin (7) was converted to the corresponding hypochlorite resin (8) by reaction with sodium hypochlorite. The resin was found to have a capacity of 2.84 mmol Cl/g by iodometric analysis. The capacities of the resins 2–8 were determined from the weight changes in the corresponding conversions and verified by quantitative determination of the functional groups. This new hypochlorite was found to oxidize alcohols to carbonyl compounds in 85–98% yield. The oxidizing efficiency of this new reagent was found to be significantly greater than those of the reagents containing only one spacer and no spacer between the reagent function and the polymer support. The presence of a 3-methylene spacer also facilitated the hypochlorite formation step significantly.     

Polystyrene supported pyrazolinium Cr(VI) reagents: preparation and use in synthetic organic chemistry

Divinylbenzene (DVB) and ethyleneglycol dimethacrylate (EGDMA) crosslinked polystyrenes (2%) were functionalized to generate pyrazolinium chromate, chlorochromate and pyrazole–CrO₃ complex functionalities. These were found to oxidize primary and secondary alcohols to the corresponding carbonyl compounds in high yields. The influence of solvent, temperature, catalyst and molar excess of the reagent in these oxidation reactions was investigated to find out the optimum conditions for effective oxidation reactions. EGDMA crosslinked polystyrene supported reagents showed higher reactivity in terms of functional group capacity and percentage yield. Also, chlorochromate reagent was found to be very efficient in oxidizing alcohols to carbonyl compounds. The spent polymeric reagent after the oxidation step can be easily removed by filtration and can be regenerated many times. © 1998 SCI.     

Epoxidation of methallyl chloride with t-butyl hydroperoxide

By applying a statistical method of experiment planning the optimum conditions of methallyl chloride epoxidation with t-butyl hydroperoxide have been determined. The influences of the temperature, molar ratio of the methallyl chloride to t-butyl hydroperoxide, catalyst concentration and reaction time on the selectivity of the synthesis of methylglycerol epichlorohydrine in relation to reacted t-butyl hydroperoxide have been examined.     

Hydrozirconation for unsaturated fatty acid derivatives

In the hydrozirconation reaction, developed by Schwartz and coworkers,bis(π-cyclopentadienyl) zirconium hydridochloride Cp₂-Zr(H)Cl, is added to the double bond of an olefin. The organozirconium intermediate can be functionalized by reaction with a variety of electrophiles such as oxygen, halogens, acetyl chloride and carbon monoxide. Furthermore, the double bond can be reformed by treatment with a hydride acceptor such as triphenylmethyl tetrafluoroborate. When a short-chain internal olefin is hydrozirconated, the initially formed alkylzirconium intermediate is rapidly isomerized to a compound in which the zirconium moiety is bound to the sterically least hindered position, which most often is the terminal position. The isomerization occurs rapidly at room temperature in contrast to the corresponding organoboron or aluminum compounds, which slowly positionally rearrange only at elevated temperatures. Because of the facile isomerization of internal alkylzirconium compounds to the terminal ones, we investigated application of the reaction to unsaturated fatty acids such as oleic and erucic acids. However, reactions on long-chain alkenes (such as oleic acid) are frequently much slower than those conducted on shorter-chain alkenes, and attention must be given to optimizing the reaction conditions if good yields are to be obtained. It would also be necessary to find an easily removable protecting group for the carboxylic function, as Cp₂Zr(H)Cl reduces carboxylic acids to alcohols. We found that the 4,4-dimethyl-2oxazoline function is a suitable protecting. group, and therefore synthesized the oxazolines from oleic acid and erucic acid. Hydrozirconation of the 4,4-dimethyl-2-oxazoline of oleic acid followed by oxidation witht-butyl hydroperoxide and conversion to methyl esters, gave methyl 3-hydroxy and methyl 18-hydroxy stearate in 13% and 17% yield, respectively. The relatively low yield is due to competing hydrogenation, the mechanism of which is discussed. Recent results indicate that the carboxyl group can be protected ast-butyl esters in the hydrozirconation and that oleyl alcohol derivatives can also be used. To understand the isomerization pattern in hydrozirconation, the reaction with α,β- and β,γ-unsaturated fatty acid oxazolines is discussed. Possibilities of making the hydrozirconation reaction catalytic by binding of the hydrozirconation reagent to a solid support as well as the synthetic potential in combining hydrozirconation with the olefin metathesis reaction are briefly reviewed.     

A mechanistic investigation of di-tert-butyl nitroxide using scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used to evaluate the electron-transfer mechanism of di-tert-butyl nitroxide (DTBN) in acetonitrile. The oxidation of DTBN is coupled to a rapid, irreversible chemical follow-up step that is difficult to characterize quantitatively with CV due to distortion of the voltammograms by solution resistance and mixed radial–linear diffusion within the scan rate region of interest. Collection efficiencies from the tip generation substrate collection (TGSC) mode of SECM were used to determine a rate constant of 21 s⁻¹ for the follow-up reaction. Collection efficiency versus distance plots obtained at 5 and 50 mM DTBN concentration are identical, confirming the first-order nature of the chemical reaction. Numerical simulation of linear scan voltammograms obtained at different tip/substrate distances provides a heterogeneous electron-transfer rate constant of 0.85 cm s⁻¹.     

Optimizing reaction parameters for the enzymatic synthesis of epoxidized oleic acid with oat seed peroxygenase

Peroxygenase is a plant enzyme that catalyzes the oxidation of a double bond to an epoxide in a stereospecific and enantiofacially selective manner. A microsomal fraction containing peroxygenase was prepared from oat (Avena sativa) seeds and the enzyme immobilized onto a hydrophobic membrane. The enzymatic activity of the immobilized preparation was assayed in 1 h by measuring epoxidation of sodium oleate (5 mg) in buffer-surfactant mixtures. The pH optimum of the reaction was 7.5 when t-butyl hydroperoxide was the oxidant and 5.5 when hydrogen peroxide was the oxidant. With t-butyl hydroperoxide as oxidant the immobilized enzyme showed increasing activity to 65°C. The temperature profile with hydrogen peroxide was flatter, although activity was also retained to 65°C. In 1 h reactions at 25°C at their respective optimal pH values, t-butyl hydroperoxide and hydrogen peroxide promoted epoxide formation at the same rate. Larger-scale reactions were conducted using a 20-fold increase in sodium oleate (to 100 mg). Reaction time was lengthened to 24 h. At optimized levels of t-butyl hydroperoxide 80% conversion to epoxide was achieved. With hydrogen peroxide only a 33% yield of epoxide was obtained, which indicates that hydrogen peroxide may deactivate peroxygenase.     

Selective oxidation of steroidal allylic alcohols using pyrazole and pyridinium chlorochoromate

Abastract This paper presents a modified method for the selective oxidation of allylic alchols. Pyrazole, when used with pyridinium chlorochromate, is a mild and useful reagent system for the rapid and selective oxidation of steroidal allylic alcohols to the corresponding α, β-unsaturated ketones. The reaction of each substrate was carried out by adding the oxidant to a dry methylene chloride solution containing pyrazole and an allylic alchol. This report is the first on the use of pyrazole to augment selective oxidation by a chronium (VI) reagent.     

The solvent-free side chain oxidation of 4-t-butyltoluene via a photolytically assisted HBr/H2O2 system

A process has been developed for the oxidation of 4-t-butyltoluene to 4-t-butylbenzaldehyde via an indirect route involving the formation of either 4-t-butylbenzyl bromide or 4-t-butylbenzal bromide. The organic bromides were formed using a photolytic HBr/H₂O₂ route in the absence of solvent. The bromination steps were found to be highly efficient in that all the substrate could be converted, consuming all the hydrogen peroxide at this stage of the reaction. Partial hydrolysis (up to 50%) of the benzyl bromide to the aldehyde was achieved employing the Sommelet route using hexamethylenetetramine. However, up to 58% aldehyde yield could be afforded from the benzal bromide using a suitable phase transfer agent and a small amount of co-solvent. In both cases, the extent of over-oxidation to 4-t-butylbenzoic acid was reduced by careful control of the bromination step and eliminating dioxygen from the reactor. © 1998 SCI.     

New oxidation products of 2,2,5,7,8-pentamethyl-6-chromanol

Oxidation of the α-tocopherol model compound 2,2,5,7,8-pentamethyl-6-chromanol (1) byt-butyl hydroperoxide in chloroform, to which an alcohol has been added, produces 5-alkoxymethyl-2,2,7,8-tetramethyl-6-chromanol as the major product. In the present study,1 was oxidized byt-butyl hydroperoxide in water-saturated chloroform to determine whether water would influence product formation in the same way as alcohols. In addition to the usual products of oxidation such as 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (9), 5-formyl-2,2,7,8-tetramethyl-6-chromanol (11), the spirodimer and spirotrimer of1, three new products have been identified−2,2,7,8-tetramethyl-5-(2,2,5,7,8-pentamethyl-6-chromanoxy)methyl-6-chromanol (4), 5-hydroxymethyl-2,2,7,8-tetramethyl-6-chromanol (5) and 3-hydroxymethyl-2-(3-hydroxy-3-methylbutyl)-5,6-dimethyl-1,4-benzoquinone (7).     

Biomimetic oxidation studies. 7. Alkane functionalization with a MMO structural model, [Fe2O(OAc)(tris((1-methylimidazol-2-yl) methyl) amine)2]3+, in the presence oft-butyl hydroperoxide and oxygen gas

A biomimetic structural model of the active site of methane monooxygenase enzyme, [Fe₂O(OAc)(tris((1-methylimidazol)-2-methyl)amine₂]³⁺, 1, has been shown to functionalize cyclohexane, toluene, adamantane, propane, and ethane in the presence oft-butyl hydroperoxide and oxygen gas. A mechanism is proposed to account for these results which implicates an alkyl hydroperoxide intermediate to the alcohol, ketone, and aldehyde products in an oxygen gas dependent reaction, while aldehyde and ketone products can also be formed from the further oxidation of the alcohols in an oxygen gas independent reaction.Partially presented at the 12th North American Catalysis Society Meeting, Lexington, KY, May 1991, Abstract D 26 and the 8th ISHC meeting in August 2–7, 1992, Amsterdam, The Netherlands, Abstract O–13. For previous biomimetic oxidation papers see ref. [1 ].     

Effect of alcohols on the oxidation of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol

The vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, has been oxidized witht-butyl hydroperoxide in chloroform in order to simulate in vivo oxidations due to lipid hydroperoxides. In the presence of a variety of alcohols, ranging from methanol to cholesterol, the corresponding 5-alkoxymethyl-2,2,7,8-tetramethyl-6-chromanols were formed in fair to good yield and were the major products in each reaction.     

The selective oxidation of glycerol over model Au/TiO2 catalysts — The influence of glycerol purity on conversion and product selectivity

The activity and selectivity of a model Au/TiO₂ catalyst was studied in the selective oxidation of glycerol as a function of the purity of the glycerol source. A reasonable conversion was noted when reagent grade starting materials were used. When crude glycerol from a FAME production facility was used, the activity of the catalyst was severely compromised and the selectivity of the reaction changed. Several low-cost approaches to purifying the crude glycerol were attempted but none resulted in the formation of a glycerol substrate whose conversion under reaction conditions matched that of the pure reagent grade substrate.     

Phenol Removal by Modified Peroxidases

Horseradish peroxidase (HRP) catalyses the oxidation of toxic aromatic compounds, especially phenols, in the presence of hydrogen peroxide. Reaction products polymerise to form insoluble precipitates which readily separate from aqueous solution, unlike their monomeric precursors. High-temperature phenol-containing gas liquors (produced from coal conversion processes) or effluent from bleach plants of kraft mills can substantially affect the stability of enzymes such as HRP and thus their oxidation capabilities. Apparent inactivation of peroxidase during high temperature polymerisation reactions is mainly due to unfolding of the protein backbone. The catalytic lifetime of HRP at high temperatures can be extended by chemical modification of lysine ε-amino groups using succinimides. The bifunctional, ethylene glycol bis-succinimidyl succinate (EG-NHS) and the monofunctional, acetic acid N-hydroxysuccinimide ester (AA-NHS) were used. The extent of stabilisation is dependent on the nature and concentration of the reagent used. The optimum pH for phenol removal is 9·0 (8·0 for 4-chlorophenol) for both native and modified forms of the enzyme; the optimum molar ratio of hydrogen peroxide and phenolic substrate is around 1·0. The effects of peroxide and enzyme concentration on the polymerisation reaction were investigated. HRP derivatives significantly reduced the oxidation reaction time at 70°C.     

Selective catalytic oxidation of aniline to azoxybenzene over titanium silicate molecular sieves

The oxidation of aniline using aqueous H₂O₂ and titanium silicates, TS-1 and TS-2 as catalysts was carried out in a batch reactor in the temperature range 333–353 K. TS-1 catalyzes aniline selectively to azoxybenzene and is superior to TS-2. The influence of different solvents, concentration of H₂O₂ and the catalyst in the reaction mixture on the conversion and product distribution has been studied. Acetonitrile is a suitable solvent in this reaction, while acetone is not. For the TS-1 catalyzed oxidation reaction,t-butyl hydroperoxide is not a suitable oxidant. At optimum conditions, a H₂O₂ efficiency of about 100% for aniline conversion is obtained with a selectivity of 97% to azoxybenzene in the product.     

Aldehyde‐Catalyzed Transition Metal‐Free Dehydrative β‐Alkylation of Methyl Carbinols with Alcohols

Different to the borrowing hydrogen strategy in which alcohols were activated by transition metal‐catalyzed anaerobic dehydrogenation, the direct addition of aldehydes was found to be an effective but simpler way of alcohol activation that can lead to efficient and green aldehyde‐catalyzed transition metal‐free dehydrative C‐alkylation of methyl carbinols with alcohols. Mechanistic studies revealed that the reaction proceeds via in situ formation of ketones by Oppenauer oxidation of the methyl carbinols by external aldehydes, aldol condensation, and Meerwein–Ponndorf–Verley (MPV)‐type reduction of α,β‐unsatutated ketones by substrate alcohols, affording the useful long chain alcohols and generating aldehydes and ketones as the by‐products that will be recovered in the next condensation to finish the catalytic cycle.     

Fast chromogenic identification of phenolic pollutants via homogeneous oxidation with t-BuOOH in the presence of iron (III) octacarboxyphthalocyanine

An efficient method has been developed for fast chromogenic identification of some phenolic pollutants (phenol, 2-chlorophenol and 1-naphthol) via tert-butyl hydroperoxide (t-BuOOH) oxidation in the presence of water-soluble iron (III) octacarboxyphthalocyanine complexes (catalyst) and 4-aminoantipyrine (chromogenic reagent). Among them, the chromogenic reaction of 2-chlorophenol could be completed rapidly, just within 6 min. The catalytic mechanism and detailed dye formation process were proposed on the basis of control experiments under different conditions. Potentially, the catalytic system composing of iron (III) octacarboxyphthalocyanine and t-BuOOH is promising for the application in fast chromogenic assay of phenolic pollutants.     

Polyhalide Derivatives of Polystyrene-based Benzyltriethylammonium Resins as Oxidizing Reagents: Synthetic and Reactivity Correlations

The oxidation of alcohols using polystyrene-based benzyltriethylammonium dichloroiodate, tetrachloroiodate and dichlorobromate reagents is described. On reaction with different alcohols, the polymeric polyhalide reagents afforded the respective carbonyl compounds in high yields. The insoluble spent reagents could be regenerated and reused without significant loss in reactivity. The reactivities of the polymeric polyhalides were investigated as a function of the nature of solvents, effective molar concentration of the reagent, temperature and by varying the polarity of the macromolecular support, and the results are discussed. © of SCI.     

A Synergic Blend of Newly Isolated <Emphasis Type="Italic">Pseudomonas mandelii</Emphasis> KJLPB5 and [hmim]Br for Chemoselective 2° Aryl Alcohol Oxidation in H<Subscript>2</Subscript>O<Subscript>2</Subscript>: Synthesis of Aryl Ketone or Aldehydes via Sequential Dehydration-Oxidative C=C Cleavage

Abstract  

Pseudomonas mandelii KJLPB5 is reported for the oxidation of aryl alcohols in ionic liquid [hmim]Br (1-hexyl-3-methyl imidazolium bromide) with H₂O₂. With a slight alteration of reaction conditions, the developed protocol leads either to (i) chemoselective oxidation of 2° aryl alcohols over 1° and aliphatic counterparts or (ii) direct one pot-two step sequential conversion of 2° aryl alcohols into corresponding one or two carbons shorter aryl aldehydes through oxidative cleavage pathway, thus providing a new facet to metal-free oxidations. The key operational parameters such as substrate concentration, incubation temperature, incubation time, ionic liquid type and ionic liquid concentration are also optimized.     

Selective conjugation of soybean esters to increase hydrogenation selectivity

Polyunsaturated fatty acid methyl esters of soybean oil (MeSBO) were selectively conjugated as a means of increasing the linolenate selectivity of various homogeneous and heterogeneous hydrogenation catalysts. Kinetics of the conjugation reaction in various solvents indicated that linolenate conjugated 5–8 times faster than linoleate. Selective conjugation of MeSBO with potassiumt-butoxide in dipolar solvents resulted in an increase in linolenate hydrogenation selectivity to 7–8 with Ni and Pd heterogeneous catalysts, and to 7–10 with homogeneous and heterogeneous chromium carbonyl catalysts.Trans-unsaturation in the hydrogenated products was only 1–3% with the chromium carbonyl catalysts, in contrast to 30–39% with the heterogeneous metal catalysts. Triglycerides were readily converted to partial glycerides andt-butyl esters with the potassiumt-butoxide reagent. Presented at the AOCS North Central Section Symposium, March 1980.