Phase-transfer catalytic oxidation

Phase Transfer Catalytic Oxidation in an Aqueous/Supercritical CO2 Medium... 

Antibody catalysts, role in environmentally benign synthesis of chemicals, 125-126 Aqueous-supercritical carbon dioxide medium, phase-transfer catalytic oxidation, 144-145 Arene cw-dihydrodiols, biocatalytic conversion of aromatics to optically pure synthons for pharmaceutical industry, 180-195... 

Figure 12 Phase transfer catalytic oxidation of cyclohexene to adipic acid in an aque-0US/SCCO2 medium. (From Ref. 24.)...

Primary and secondary alcohols are oxidized slowly at low temperatures by benzyltriethylammonium permanganate in dichloromethane primary alcohols produce methylene esters (60-70%), resulting from reaction of the initially formed carboxylate anion with the solvent, with minor amounts of the chloromethyl esters and the carboxylic acids. Secondary alcohols are oxidized (75-95%) to ketones [34] the yields compare favourably with those obtained using potassium permanganate on a solid support. 1,5-Diols are oxidized by potassium permanganate under phase-transfer catalytic conditions to yield 8,8-disubstituted-8-valerolactones [35] (Scheme 10.1). 

Methylbenzenes are oxidized to the corresponding benzoic acids in very high yield under phase-transfer catalytic conditions by sodium hypochlorite in the presence of ruthenium trichloride, which is initially oxidized to ruthenium tetroxide [5]. Absence of either the ruthenium or the quaternary ammonium salt totally inhibits the reaction. 

The Hoffman rearrangement of amides by quaternary ammonium hypochlorites is not particularly efficient under phase-transfer catalytic conditions and only low yields of nitrile, aldehydes, or ketones, which result from oxidation of the amines, are... 

Ferrate salts have been used under phase-transfer catalytic conditions for the oxidation of alcohols. Selective oxidation of allylic and benzylic alcohols to the corresponding aldehydes occurs under mild conditions [4],... 

Under traditional conditions with the hexacyanoferrate ion, the major product from (1) is the quininoid ether (3), whereas it is the minor product under phase-transfer catalytic conditions. Similarly, the carbinol (5) is oxidized to the ketone (6) by the quaternary ammonium salt, whereas the quinone (7) is obtained in the absence of the quaternary ammonium ion [5]. 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

The oxidations take place without affecting the sulfur atom when it is in one of its highest oxidation states (i.e., sulfur has an oxidation number of the value + 4 or +6, see Table 18). Thus, epoxidation of (pcntafluorosulfanyl)alkenes, e. g. 1, is achieved by treatment with sodium hypochlorite under phase transfer catalytic conditions.281... 

Tetrahydrophosphinine oxide 166 reacts with dichlorocarbene under phase-transfer catalytic conditions giving 7,7-dichloro-1-methyl-3-phenyl-3-phosphabicyclo[4.1.0]heptane (167) in 71% yield. Heating the phosphabi-cyclo[4.1.0]heptane 167 at 220 °C makes the bicyclic ring opening and the simultaneous elimination of hydrogen chloride gives 4-chloro-3-methyl-l-phenyl-2,7-dihydrophosphepin 1-oxide (168), a seven-membered phosphorus heterocycle, in 67% yield (Scheme 54) [66]. 

Abramovid, S., Neumann, R. and Sasson, Y. (1985) Sodium hypochlorite as oxidant in phase transfer catalytic systems. Part 11. Oxidation of aromatic alcohols. /. Mol. Catal., 29, 299. 

Halogenated heterocycles (2-, 3-, 4-bromopyridines, 2-chloro-3-bromoquinolines, 2-chloropyrimidine) react with anion 163a. The substitution products then undergo oxidative decyanation under phase-transfer catalytic conditions to afford phenyl hetaryl ketones in good yield (33-77%) (equation 109)202. 

Table 1 TVpical Temperatures for the Transfer of Reactive Intermediates from the Catalyst Surface to the Gas Phase in Catalytic Oxidations...

Cyanopyrazine reacts with cyclopropyllithium to give pyrazinyl cyclopropyl ketone <91JHC1147>. Benzoyl pyrazine is formed by oxidative decyanation of a-pyrazinyl-a-phenylacetonitrile under phase-transfer catalytic conditions <87JHC1061 >. (2-Hydroxy-2-phenylethyl)pyrazines undergo retro-ene reaction on pyrolysis to yield methylpyrazines and benzaldehyde quantitatively <87JOC397i>. 

Benzyl- and methyltriarylphosphonium salts bound to 2% and 8% cross-linked, and to 20% cross-linked macroporous, polystyrene undergo Wittig reactions in good yield with both aldehydes and ketones. The Wittig reaction has been carried out under gas-liquid phase-transfer catalytic conditions using solid potassium carbonate as the base. An advantage of the procedure is that the alkene product separates from the phosphine oxide. Unprotected phenolic aldehydes can be converted into alkenes by Wittig reactions provided two moles of base are used. ... 

In 2007, the Lygo group surveyed the optimal oxidant for the asymmetric Weitz-Scheffer epoxidation of chalcone 50 through use of various oxidants in the presence of catalyst 8p. Among the oxidants, they found that aqueous sodium hypochlorite was the most effective oxidant for the epoxidations under mild phase-transfer catalytic conditions (Scheme 16.38). ... 

Formamidinesulfmic acid (obtained by the oxidation of thiourea with hydrogen peroxide) has been used to reduce disulfides to sulfides (see Eqs. 12.6 and 13.22) and N-tosylsulfilimines to sulfides (see Eqs. 12.7 and 13.23), under phase transfer catalytic conditions in the presence of hexadecyltributylphosphonium bromide [15]. Diphenyl, dibenzyl and dibutyldisulfides were reduced by this method to the corresponding sulfides in 72%, 62% and 90% yields respectively. Examples of the reduction of N-tosylsulfilimines are recorded in Table 12.4. 

The Keglevich group has continued to deal with environmentally friendly and P-heterocyclic chemistry. The solid liquid phase alkylation of P=0-functionalised CH acidic compounds was accomplished under phase transfer catalytic and microwave (MW) conditions." a-Hydroxy-benzylphosphine oxides were synthesized by the addition of diphenylphosphine oxide to the carbonyl group of substituted benzaldehydes under MW conditions." The double Kabachnik-Fields (phospha-Mannich) reaction was utilized in the preparation of bis(phosphinoxidomethyl)amines. The Diels Alder cycloadditions of 1,2-dihydrophosphinine oxides and subsequent... 

Over the past few decades, the biosyntheses of tropane alkaloids such as (—)-hyoscyamine, (—)-scopolamine, and (—)-cocaine have been studied extensively [18]. The skeletons of all these alkaloids feature the tropinone moiety, produced by oxidation and subsequent cyclization of (-b)-hygrine (17) [19]. It was envisioned that RCM could be invoked in order to develop an enantioselective synthesis of 17 [20]. As depicted in Scheme 2.5, a phase-transfer catalytic allylation reaction with methaUyl bromide converted t-butyl ester 13 into product 14 with 97% ee. Metathesis precursor 15 was then prepared in six more steps, including transformation of the t-butyl ester of 14 into a terminal olefin. Dihydropyrrole 17 was formed in quantitative yield by RCM employing [Ru]-II (8 mol%). Subsequent hydrogenation and deprotection of 16 led to (-l-)-hygrine 17 with overall yield of 29% and an ee of 97%. 

The most widely accepted mechanism of reaction is shown in the catalytic cycle (Scheme 1.4.3). The overall reaction can be broken down into three elementary steps the oxidation step (Step A), the first C-O bond forming step (Step B), and the second C-O bond forming step (Step C). Step A is the rate-determining step kinetic studies show that the reaction is first order in both catalyst and oxidant, and zero order in olefin. The rate of reaction is directly affected by choice of oxidant, catalyst loadings, and the presence of additives such as A -oxides. Under certain conditions, A -oxides have been shown to increase the rate of reaction by acting as phase transfer catalysts. ... 

Epoxides such as ethylene oxide and higher olefin oxides may be produced by the catalytic oxidation of olefins in gas-liquid-particle operations of the slurry type (S7). The finely divided catalyst (for example, silver oxide on silica gel carrier) is suspended in a chemically inactive liquid, such as dibutyl-phthalate. The liquid functions as a heat sink and a heat-transfer medium, as in the three-phase Fischer-Tropsch processes. It is claimed that the process, because of the superior heat-transfer properties of the slurry reactor, may be operated at high olefin concentrations in the gaseous process stream without loss with respect to yield and selectivity, and that propylene oxide and higher... 

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

Bortoletto et al. (1997) have used neutral and cationic triphenylphosphine trisulphonate or triphenylphosphine mono-sulphate Rh complexes. Monosulphonate is less readily oxidized and can replace trisulphonate in industrial processes. The presence of a quaternary ammonium counter ion associated with the trisulphonate confers phase-transfer properties to the catalytic. species, which makes use of co-solvents unnecessary. 

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