Two-phase oxidations

The oxidizing properties of positive halogen precursors have been reviewed by Filler. Compounds which have been used in steroid hydroxyl oxidations are Wbromosuccinimide Wbromoacetamide (NBA), A-bro- 

Oxidations usually proceed in the dark at or below room temperature in a variety of solvents ranging from aqueous bicarbonate to anhydrous benzene-pyridine. Base is quite commonly used to consume the hydrogen halide produced in the reaction, as this prevents the formation of high concentrations of bromine (or chlorine) by a secondary process. The reaction time varies from a few minutes to 24 hours or more depending on the nature of the reagent and the substrate. Thus one finds that NBS or NBA when used in aqueous acetone or dioxane are very mild, selective reagents. The rate of these oxidations is noticeably enhanced when Fbutyl alcohol is used as a solvent. In general, saturated, primary alcohols are inert and methanol is often used as a solvent. 

The mechanism of oxidation has not been studied in detail. In aqueous media it may be assumed that hypobromous acid is formed and that this is the true oxidizing agent. Indeed, Deno has shown that the rate of oxidation of alcohols by aqueous bromine closely parallels the HOBr concentration and that the alkyl hypohalide is a most likely intermediate. Rate studies and 

Generally, isolated olefinic bonds will not escape attack by these reagents. However, in certain cases where the rate of hydroxyl oxidation is relatively fast, as with allylic alcohols, an isolated double bond will survive. Thepresence of other nucleophilic centers in the molecule, such as primary and secondary amines, sulfides, enol ethers and activated aromatic systems, will generate undesirable side reactions, but aldehydes, esters, ethers, ketals and acetals are generally stable under neutral or basic conditions. Halogenation of the product ketone can become but is not always a problem when base is not included in the reaction mixture. The generated acid can promote formation of an enol which in turn may compete favorably with the alcohol for the oxidant. 

17a-Dihydroxy-5j -pregnane-ll,20-dione. To a solution of 0.3 g (0.853 mmole) of 3a,l lj9,17a-trihydroxy-5jS-pregnan-20-one in 24 ml of -butyl alcohol and 0.5 ml of pyridine is added 0.13 g (0.94 mmole) of A-bromoace-tamide. lodometric titration of an aliquot shows the reaction to be essentially complete after 20 hr. Water (50 ml) is added and most of the t-butyl alcohol is removed under reduced pressure. The crystalline product which separates is filtered and washed with water. Drying affords 0.19 g (65 %) of 3a,17a-di-hydroxy-5j -pregnane-ll,20-dione, mp 202-204°. 

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Much higher asymmetric induction was observed in the two-phase oxidation of simple alkyl aryl and diaryl sulphides296, substituted alkyl aryl sulphides297 and dithioacetals of formaldehyde298 by sodium metaperiodate in the presence of proteins such as bovine serum y-globulin and egg albumin. Optical purities of the sulphoxides so formed ranged between 20 and 85%. 

III. Chromium VI Reagents / 227 Chromium Vi-acetic acid / 227 Chromium Vl-acetone / 228 Chromium trioxide-pyridine complex / 229 Chromic acid-dimethylformamide / 231 Two-phase oxidations / 232... 

When l,l-dimethoxy-4-propyl-2,6-diphenyl-A5-phosphorin (53) is transformed into its 4-carbenium tetrafluoroborate (54) and treated with a base the 4-(l-propenyl) derivative (55) is formed. This, by a two phase oxidation with KMn04, gives the aldehyde (56) which can be further transformed, e.g. into the nitrile (57 equation 34) (75AG(E)lll). An Arbusov type dealkylation of the cation salts (49) (or 54) with LiBr gives the phosphinic ester (58 equation (35)). 

Several biogenetic-type syntheses of 5,10b-ethanophenanthridine derivatives have been realized. The model compound 273 (R2 = OMe) in a two-phase oxidation in CHC13/1 N FeCl3 (1 1) yielded 12% of the dienone 271 which, under basic conditions, gave the product 272 (56). 

A.T. Fromhold, N. Sato. Quasy-steady-state growth of layered two-phase oxides on pure metals // Oxid.Metals.-1981.- V.16, No.3/4.- P.203-220. 

Tab. 15. Survey of iron(lll) sources used in the two-phase oxidative dimerization of 2-naphthol (6 a).

Oxidation of arenes (8, 80-81). This oxidation can be carried out in a two-phase system with sodium dodecyl sulfate as a micellar catalyst. CAS can be used in catalytic amounts if ammonium persulfate is used in excess to convert Ce(lll) as formed to Ce(lV). This oxidation is slow in the absence of a Ag(l) salt. This catalytic two-phase oxidation is very useful for preparation of polycyclic quinones from hydrocarbons, but is ineffective for other substrates. 

Two-phase oxidations have been developed to protect sensitive substrates from prolonged exposure to strongly acidic conditions, thus avoiding degradation of the product and/or epimerization of compounds containing a-chiral centres. Benzene and dichloromethane are commonly used for the organic phase, but diethyl ether is often found to be superior since it is less likely to form emulsions. 

Brown et al. initially reported the use of two-phase oxidation for the preparation of a range of ketones the method has also recently been reported for the oxidation of a selection of primary alcohols. ... 

Oxidation of aikenes. KMn04 can be solubilized in CH2CI2 by an equimolar amount of benzyltriethylammonium chloride. This solution can be used for homogeneous oxidation of aikenes to intermediates that can be decomposed either to dialdehydes or to s-l,2-diols. In two-phase oxidations with KMnOa and phase-transfer catalysts, diols or carboxylic acids are obtained. ... 

Two-phase oxidation (1. 143 2, 71). Brown ei a . have published the definitive paper on the two-phase oxidation of secondary alcohols in diethyl ether with aqueous chromic acid. A number of water-immiscible solvents were examined, but diethyl ether... 

Two-phase oxidation. A simple procedure for the oxidation of a secondary alcohol to a ketone consists in adding a cold solution of sodium dichromate and sulfuric acid containing a little acetic acid to a stirred solution or suspension of the alcohol in benzene while cooling to 1-6°. Since the ketone as formed is distributed into the benzene layer and protected against secondary reactions, sensitive ketones can be isolated in high yield without appreciable inversion at adjacent centers of asymmetry. Thus a cis-trans mixture of 1,5-dihydroxydecalins affords a cis-trans mixture of decalin-l,5-diones in 71-76% yield in the two-phase system the cis isomer present is not converted into the more stable trans diketone. 

Two-phase mixtures of oxides have also been used in mixed potential sensors. Such examples include Cr2O3 + NiO [297] for NO sensors, CuO + ZnO [298, 299] or SnO2 + CdO [300] for CO sensors, and In2O3 + MnO2 [301, 302] for hydrocarbon sensors. Some examples ofthe outputs of NO -and CO sensors with two-phase oxide mixtures as electrodes are shown in Figure 13.24 [270, 297, 298, 300]. 

Figure 13.24 Outputs of CO [298, 300] and NO, [270, 297] sensors with YSZ electrolytes and two-phase oxide mixtures electrodes.

Two-phase oxidation [1, 143, last paragraph]. Allylic alcohols (1) are oxidized to the corresponding vinyl ketones (2) in acceptable yields by the procedure of Brown and Garg.11 A 30-40% excess of oxidant is required because of concurrent oxidation at the double bond.113 The Jones reagent is less satisfactory. 

The pentose phosphate pathway is an alternative metabolic pathway for glucose oxidation in which no ATP is generated. Its principal products are NADPH, a reducing agent required in several anabolic processes, and ribose-5-phosphate, a structural component of nucleotides and nucleic acids. The pentose phosphate pathway occurs in the cytoplasm in two phases oxidative and nonoxidative. In the oxidative phase of the pathway, the conversion of glucose-6-phosphate to ribu-lose-5-phosphate is accompanied by the production of two molecules of NADPH. 

Vyskocil S, Smrcina M, Lorenc M, Hanus V, Polasek M, Kocovsky P (1998) On the Novel two-phase oxidative cross-coupling of the two-component molecular crystal of 2-naphthol and 2-naphthylamine . Chem Commun 585-586... 

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