Ketones as catalysts

Among many other methods for epoxidation of disubstituted E-alkenes, chiral dioxiranes generated in situ from potassium peroxomonosulfate and chiral ketones have appeared to be one of the most efficient. Recently, Wang et /. 2J reported a highly enantioselective epoxidation for disubstituted E-alkenes and trisubstituted alkenes using a d- or L-fructose derived ketone as catalyst and oxone as oxidant (Figure 6.3). 

SCHEME 7. Sugar-derived ketones as catalysts for asymmetric alkene epoxidation... 

The idea of using chiral ketones as catalysts for asymmetric epoxidation of olefins was first addressed by Curci et al. in the middle of the 1980s [7]. In this initial ex-... 

Carbohydrates as Chiral Catalysts. - A review on asymmetric epoxidation using chiral ketones as catalysts includes a section discussing the work of, in particular, Shi on the generation in situ of dioxiranes from sugar ketones and oxone, and their use in the asymmetric epoxidation of traws-alkenes. ... 

Another FBC approach was developed by Knochel and co-workers to oxidize olefins, sulfides, and aldehydes to epoxides, sulfoxides, sulfones, and carboxylic acids (79). They used the Ru and Ni complexes of a perfluorinated 1,3-diketone, obtained in one step in 80 % yield by the condensation of a perfluoromethyl ester and a perfluoromethyl ketone, as catalysts for these oxidation reactions. More importantly, these Ru and Ni complexes were found to be highly soluble in perfluorocarbons (Figure 3). 

Shing TKM, Leung GYC, Luk T. Arabinose-derived ketones as catalysts for asymmetric epoxidation of alkenes. J. Org. Chem. 2005 70(18) 7279-7289. 

Initially several research groups (Curd and Marples) studied chiral ketones as catalysts for asymmetric epoxidation, but until recently enantioselectivities with these systems were only up to ca. 20% ee (Fig. 1.2) [19-21]. 

Fluorenone/methanol/sodium hydroxide Ketones as catalysts... 

Although stoichiometric ethynylation of carbonyl compounds with metal acetyUdes was known as early as 1899 (9), Reppe s contribution was the development of catalytic ethynylation. Heavy metal acetyUdes, particularly cuprous acetyUde, were found to cataly2e the addition of acetylene to aldehydes. Although ethynylation of many aldehydes has been described (10), only formaldehyde has been catalyticaHy ethynylated on a commercial scale. Copper acetjlide is not effective as catalyst for ethynylation of ketones. For these, and for higher aldehydes, alkaline promoters have been used. 

Glycols are obtained by treatment with a ketone using alkali as catalyst or with an aldehyde using alkaU or copper acetyUde as catalyst (201,202). 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

Secondary alcohols ate oxidi2ed to ketones in good yields under mild conditions in a triphasic system using an inert solvent, soHd Ca(OCl)2, and a hypochlorite resin as catalyst (175). 

N O Y O R I Chiral homogeneous hydrogenation Homogeneous chiral hydrogenation ol unsaluraled alcohols, or cartMxyNc acids, enamides, ketones in the presence ol BINAP Ru or Rh complex 8 as catalyst. 

PhSH, BF3-Et20, CHCI3, 0°, 10 min, 86% yield. ZnCl2 and MgBr2 have also been used as catalysts. With MgBr2 acetals can be converted to thioketals in the presence of ketones. ... 

More frequently either methyl ethyl ketone peroxide or cyclohexanone peroxide is used for room temperature curing in conjunction with a cobalt compound such as a naphthenate, octoate or other organic solvent-soluble soap. The peroxides (strictly speaking polymerisation initiators) are referred to as catalysts and the cobalt compound as an accelerator . Other curing systems have been devised but are seldom used. 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

By a suitable choice of conditions (metal hydrides or metal/ammonia) ketones at the 1-, 2-, 4-, 6-, 7-, 11-, 12- and 20-positions in 5a-H steroids can be reduced to give each of the possible epimeric alcohols in reasonable yield. Hov/ever, the 3- and 17-ketones are normally reduced to give predominantly their -(equatorial) alcohols. Use of an iridium complex as catalyst leads to a high yield of 3a-alcohol, but the 17a-ol still remains elusive by direct reduction. 

Chlorination and bromination of perfluoroketimines take place only in the presence of alkali fluorides as catalyst [267] Chlorine fluoride adds readily across the CO or CN bond of polyfluonnated ketones and immes [268, 269]... 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

The general rule has been formulated (P) that the less substituted enamine is formed from unsymmetrical ketones such as the 2-alkylcyclohexanones. In enamine 21 the R, group and the N-alkyl groups would interfere with one another if overlap is to be maintained between the nitrogen unshared electrons and the double bond. There would be less repulsion if the isomeric enamine (22) were formed. 2-Phenylcyclohexanone and pyrrolidine with p-toluenesulfonic acid as catalyst in refluxing benzene gave enamine... 

Me3Si)2NH, Me3SiCl, Pyr, 20°, 5 min, 100% yield. ROH is a carbohydrate. Hexamethyldisilazane (HMDS) is one of the most common silylat-ing agents and readily silylates alcohols, acids, amines, thiols, phenols, hydroxamic acids, amides, thioamides, sulfonamides, phosphoric amides, phosphites, hydrazines, and enolizable ketones. It works best in the presence of a catalyst such as X-NH-Y, where at least one of the groups X or Y is electron withdrawing." Yttrium-based Lewis acids also serve as catalysts. ... 

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