Allylic oxidation molecular sieves

This method has proven to be an extremely useful means of synthesizing enantiomerically enriched compounds. Various improvements in the methods for carrying out the Sharpless oxidation have been developed.48 The reaction can be done with catalytic amounts of titanium isopropoxide and the tartrate ester.49 This procedure uses molecular sieves to sequester water, which has a deleterious effect on both the rate and enantioselectivity of the reaction. Scheme 12.9 gives some examples of enantioselective epoxidation of allylic alcohols. 

Sheldon and coworkers have developed chromium-substituted molecular sieves (CrAPO-5) as recyclable solid catalysts for several selective oxidations, among them also the allylic" and benzylic ° " ° " ° oxidations using TBHP or O2 as the terminal oxidants (equation 63), which yielded the corresponding benzylic ketones in moderate yield (conv. 13-70%) and moderate to good selectivity (41%, 65-97%). The benzylic alcohols were formed as side products. Allylic oxidation also proceeded with good conversions, while selectivities were lower and both possible products, the allylic ketone (31-77% selectivity) and the allylic alcohol (0-47% selectivity), were formed. Chromium sUicalite showed activity for selective benzylic oxidation in the presence of TBHP as well as giving mainly the allylic ketone (2-cyclohexen-l-one with 74% selectivity) and the allylic alcohol as minor product (2-cyclohexen-l-ol with 26% selectivity) -. ... 

In 1980, Katsuki and Sharpless described the first really efficient asymmetric epoxidation of allylic alcohols with very high enantioselectivities (ee 90-95%), employing a combination of Ti(OPr-/)4-diethyl tartrate (DET) as chiral catalyst and TBHP as oxidant Stoichiometric conditions were originally described for this system, however the addition of molecular sieves (which trap water traces) to the reaction allows the epoxidation to proceed under catalytic conditions. The stereochemical course of the reaction may be predicted by the empirical rule shown in equations 40 and 41. With (—)-DET, the oxidant approaches the allylic alcohol from the top side of the plane, whereas the bottom side is open for the (-l-)-DET based reagent, giving rise to the opposite optically active epoxide. Various aspects of this reaction including the mechanism, theoretical investigations and synthetic applications of the epoxy alcohol products have been reviewed and details may be found in the specific literature . 

Propylene carbonate [108-32-7] M 102.1, b 110°/0.5-lmm, 238-239°/760mm, d 1.204, n 1.423. Manufactured by reaction of 1,2-propylene oxide with CO2 in the presence of a catalyst (quaternary ammonium halide). Contaminants include propylene oxide, carbon dioxide, 1,2- and 1,3-propanediols, allyl alcohol and ethylene carbonate. It can be purified by percolation through molecular sieves (Linde 5A, dried at 350° for 14h under a stream of argon), followed by distn under vac. [Jasinski and Kirkland AC 39 163 1967], It can be stored over molecular sieves under an inert gas atmosphere. When purified in this way it contains less... 

Activated manganese dioxide, which is generally used for the selective oxidation of allylic alcohols, permitted the synthesis of an unsaturated (3-keto-arabino-hexopyranosyl)pyridine.13 New oxidative systems for alcohols, involving molecular sieve-assisted... 

Taylor and Blackburn proved64 that the in situ condensation of an aldehyde with an amine can be made to occur in an intermolecular fashion. Thus, treatment of primary allylic, propargylic and benzylic alcohols with active Mn02 in the presence of diverse primary amines and molecular sieves in boiling CH2C12 leads to the selective oxidation of the alcohols in the presence of the primary amines and to the formation of the corresponding imines by reaction of the alcohols with the intermediate aldehydes. 

The oxidation of an allylic alcohol with active M11O2 leads to an aldehyde that condenses in situ with MeONH2 HC1 in the presence of molecular sieves, producing an... 

For the first time, catalytic activity of lanthanide isopropoxides was detected in the TBHP (tert-butylhydroperoxide) assisted oxidation of allylic alcohols to epoxyalcohols (Eq. 26) [231], For example epoxy geraniol was obtained in up to 96% yield by using YbfOiPr in the presence of molecular sieves (4 A) [232]. 

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

Chiral epoxides are important intermediates in organic synthesis. A benchmark classic in the area of asymmetric catalytic oxidation is the Sharpless epoxidation of allylic alcohols in which a complex of titanium and tartrate salt is the active catalyst [273]. Its success is due to its ease of execution and the ready availability of reagents. A wide variety of primary allylic alcohols are epoxidized in >90% optical yield and 70-90% chemical yield using tert-butyl hydroperoxide as the oxygen donor and titanium-isopropoxide-diethyltartrate (DET) as the catalyst (Fig. 4.97). In order for this reaction to be catalytic, the exclusion of water is absolutely essential. This is achieved by adding 3 A or 4 A molecular sieves. The catalytic cycle is identical to that for titanium epoxidations discussed above (see Fig. 4.20) and the actual catalytic species is believed to be a 2 2 titanium(IV) tartrate dimer (see Fig. 4.98). The key step is the preferential transfer of oxygen from a coordinated alkylperoxo moiety to one enantioface of a coordinated allylic alcohol. For further information the reader is referred to the many reviews that have been written on this reaction [274, 275]. 

Ac, acetyl AIBN, azobis(isobutanonitrile) All, allyl AR, aryl Bn, benzyl f-BOC, ferf-butoxycarbonyl Bu, Butyl Bz, benzoyl CAN, ceric ammonium nitrate Cbz, benzyloxycarbonyl m-CPBA, m-chloroperoxybenzoic acid DAST, diethylaminosulfur trifluoride DBU, l,8-diazabicyclo[5.4.0]undec-7-ene DCC, /V. /V - d i eye I oh e x y I c ar bo -diimide DCM, dichloromethyl DCMME, dichloromethyl methyl ether DDQ, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone DEAD, diethyl azodicarboxylate l-(+)-DET, L-(+)-diethyl tartrate l-DIPT, L-diisopropyl tartrate d-DIPT, D-diisopropyl tartrate DMAP, 4-dimethylaminopyridine DME, 1,2-dimethoxyethane DMF, /V./V-dimethylformamide DMP, 2,2-dimethoxypropane Et, ethyl Im, imidazole KHMDS, potassium hexamethyldisilazane Me, methyl Me2SO, dimethyl sulfoxide MOM, methoxymethyl MOMC1, methoxymethyl chloride Ms, methylsulfonyl MS, molecular sieves NBS, N-bromosuccinimide NIS, /V-iodosuccinimide NMO, /V-methylmorpho-line N-oxide PCC, pyridinium chlorochromate Ph, phenyl PMB, / -methoxvbenzyl PPTs, pyridiniump-toluenesulfonate i-Pr, isopropyl Py, pyridine rt, room temperature TBAF, tetrabutylammonium fluoride TBS, ferf-butyl dimethylsilyl TBDMSC1, f-butylchlorodimethylsilane Tf, trifhioromethylsulfonyl Tf20, trifluoromethylsulfonic anhydride TFA, trifluoroacetic acid THF, tetrahydrofuran TMS, trimethylsilyl TPAP, tetra-n-propylammonium perruthenate / -TsOH. / -toluenesulfonic acid... 

A combination of acetic acid and PDC has also been used to effect selective allylic oxidations on unprotected, unsaturated carbohydrates.Interestingly, ethyl acetate was found to be the solvent of choice, and the reaction proceed better if molecular sieves were omitted, since they appeared to cause unselecdve oxidation in this case (equation 13). 

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