Alcohols, secondary, oxidation molecular sieves

The use of sofid supports in conjunction with permanganate reactions leads to modification of the reactivity and selectivity of the oxidant. The use of an inert support, such as bentonite (see Clays), copper sulfate pentahydrate, molecular sieves (qv) (151), or sifica, results in an oxidant that does not react with alkenes, but can be used, for example, to convert alcohols to ketones (152). A sofid supported permanganate reagent, composed of copper sulfate pentahydrate and potassium permanganate (153), has been shown to readily convert secondary alcohols into ketones under mild conditions, and in contrast to traditional permanganate reactivity, the reagent does not react with double bonds (154). 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

Aluminophosphates (A1P04) were discovered in 198248 and a large amount of research has been directed towards the incorporation of various elements into the framework of these molecular sieves 49 A particular area of study is the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, which are useful synthetic intermediates. Traditionally, alcohol transformations are performed with stoichiometric chromium(VI) reagents.50 However, due to environmental problems associated with chromium-containing effluent, attention has focused on the use of chromium in conjunction with oxidizing agents such as tert-butyl hydroperoxide.51 Sheldon and co-workers... 

A catalytic method which promises to find wide application in view of its mildness and ease of execution uses a catalytic amount of tetra-n-propylammonium perruthenate (TPAP) with A7-methylmorpholine -oxide (NMO) as the cooxidant. ° l4imary (and secondary) alcohols which contain a range of ftinc-tional groups (alkenes, tetrahydropyran ethers, epoxides, lactones, silyl ethers and indoles inter alia) can be oxidized without interference by the other functional group (equations 21-23). The performance of the reagent is improved further by including molecular sieves in the reaction mixture. ... 

Maruoka has successfully developed a highly accelerated Oppenauer oxidation [31,32] system using a bidentate aluminum catalyst [29]. This modified, catalytic system effectively oxidizes a variety of secondary alcohols to the corresponding ketones as shown in Sch. 9. For example, reaction of (2,7-dimethyl-l,8-biphenylene-dioxy)bis(dimethylaluminum) (8, 5 moI%) with carveol (14) at room temperature in the presence of 4-A molecular sieves, and subsequent treatment with pivalaldehyde (3 equiv.) at room temperature for 5 h yielded carvone (15) in 91 % yield. Under these oxidation conditions, cholesterol (16) was converted to 4-cholesten-3-one (17) in 75 % yield (91 % yield with 5 equiv. t-BuCHO). 

Oxidations. Various primary and secondary alcohols are oxidized to give aldehydes and ketones by PdfOAcl -O -pyridine in the presence of 3A molecular sieves. Modified Wacker processes which obviate copper and chloride employ the Pd(OAcl -02 system and a water-soluble 1, 10-phenanthroline ligand or polypyrrole as redox-active ligand.- ... 

Alternatively, pyridinium chlorochromate in dichloromethane has successfully been used to obtain both aldehydes and ketones, 516,517,540,547,607,785 - 788,790-802 most cases from the corresponding alcohols, although the silyl ether of a secondary alcohol has been oxidized to the corresponding ketone, In order to facilitate the reaction, sodium acetate, molecular sieves, " alumina, and Celite have been added (Table 2), The closely related reagent pyridinium dichromate, however, has been far less frequently utilized. ... 

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

As part of an ongoing programme on redox molecular sieves we are investigating the use of metal substituted alumino-phosphates (MeAPOs) in liquid phase oxidations. We have found that CrAPO-5 is an active and selective catalyst for the liquid phase oxidation of secondary alcohols with TBHP or O2. 

Primary and secondary alcohols such as benzyl alcohol (483) and 1-phenylethanol (484) are oxidized with O2 efficiently to benzaldehyde and acetophenone using Pd(OAc)2, complexed with pyridine, as a catalyst in the presence of a molecular sieve [199] or supported on hydrotalcite [200]. Also, the palladacycle of 4,5-dihydro-l,3-oxazole 485 is a good catalyst for oxidation of alcohols in DMSO under oxygen [201]. 

The traditional method for alcohol purification is distillation. When the alcohol concentrations in the mixture and in the steam reach 95.57 and 97.6%, respectively, the mixture cannot be further purified, so a second distillation must be carried out. At present, the domestic secondary distillation methods include calcium oxide dehydration, azeotropic distillation, extraction distillation, absorption, molecular sieves, and vacuum dehydration. In addition, chemical film separation is a very promising alcohol purification technology, but it is still in the experimental stage. 

However, these methods suffer from low activities and/or narrow scope. Uemura and coworkers [74,7 5] reported an improved procedure involving the use of Pd(OAc) 2 (5 mol%) in combination with pyridine (20 mol%) and 3 A molecular sieves (500 mg per mmol of substrate) in toluene at 80 °C. This system smoothly catalyzed the aerobic oxidation of primary and secondary aliphatic alcohols to the corresponding aldehydes and ketones, respectively, in addition to benzylic and allylic alcohols. Representative examples are summarized in Table 5.7. The corresponding lactones were afforded by 1,4- and 1,5-diols. This approach could also be employed under fluorous biphasic conditions [76]. 

Chromium(VI) catalyzes the oxidation of alcohols with alkyl hydroperoxides . Chromium-incorporated molecular sieves, in particular chromium-substituted aluminophosphate-5 (Cr-APO-5) were shown to be effective for the aerobic oxidation of secondary alcohols to the corresponding ketones (Reaction 19). This, and related catalysts, were first believed to be heterogeneous but more detailed investigations revealed that the observed catalysis is due to small amounts of soluble chromium that are leached from the framework by reaction with hydroperoxides. Reaction 19 may involve initial chromium-catalyzed free radical autoxidation of the alcohol to the a-hydroxyalkyl hydroperoxide followed by chromium-catalyzed oxygen transfer with the latter and/or H202 (formed by its dissociation) via an oxochromium(VI)-chromium(IV) cycle. 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

Starting from l-(2-ethynylquinolin-3-yl)ethanone 140, available by subsequent Sonogashira reaction from 2-chloroquinoline, the Grignard reaction with MeMgBr to give a secondary alcohol, and Dess—Martin periodinane oxidation, a method was developed which provided 1-aminoacridines 141 (Scheme 53). On addition of secondary amines, enamines E formed under TiCl4 catalysis (method A), in the presence of molecular sieves... 

Palombi et al. (1997) reported the oxidation of secondary (linear and cyclic) and benzylic alcohols to the corresponding carbonyl compounds using t-butyl hydroperoxide, 3A molecular sieves (MS) and microwave irradiation under solvent-free conditions. Under the same conditions, a,P-unsaturated alcohols are converted into a,P-epoxyalcohols in regio- and diastereoselective way. The reactions provided the corresponding carbonyl compounds in good yields. 

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