Oxide catalysts alkene oxidation

Catalyst Alkene Oxidizing agent Stage 1 alkene to epoxide Stage 2 epoxide to cyclic carbonate ... 

Although the mechanism shown in Fig. 7.3 broadly applies to a variety of alkenes, the details and the sequence of reactions could vary significantly. We will see that for the hydrogenation of a-acetamidocinamic acid, the catalyst-alkene complex is formed first, which then oxidatively adds to dihydrogen. More precisely, out of the two equilibria, one with the alkene and the other with dihydrogen, which one would dominate is determined by the nature of the alkene. Even within the class of unfunctionalized alkenes there may be significant differences between the types of catalytic intermediates that are involved. 

Ethylene oxide is manufactured by oxidizing ethylene with air or oxygen in the presence of a silver catalyst Alkenes furnish hydroperoxides when oxidized by oxygen in the presence of catalysts like salts of cobalt and manganese the hydroperoxides are transformed to a number of products, including epoxides. Only in a few cases, such as oxidation of 1-phenylcyclooctene, have moderate yields of epoxides been obtained during autoxidation. ... 

With 1 as catalyst, alkene bonds which have oxidation potentials less than 1.6 V (vs standard... 

Catalyst Alkene Oxidant Time (h) rrc) Yield (%) TON° ee (%) Runs Refei... 

With 1 as catalyst, alkene bonds which have oxidation potentials less than 1.6 V (vs standard calomel electrode) are considered potentially susceptible to this transformation. With the stronger oxidant 2, the scope of the reaction can be extended to include, for example, tetraalkyl-substituted double bonds, but obviously not disubstituted alkenes such as cyclohexene. On the other hand, electron-rich alkenes such as enol ethers and vinyl sulfides cannot be cyclo-propanated by this method. In order to suppress cyclodimer formation from the alkene and its radical cation, the diazo ester is sometimes applied in a four- to fivefold amount with respect to the alkene. 

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

Adams catalyst, platinum oxide, Pt02 H20. Produced by fusion of H2PtCl6 with sodium nitrate at 500-550 C and leaching of the cooled melt with water. Stable in air, activated by hydrogen. Used as a hydrogenation catalyst for converting alkenes to alkanes at low pressure and temperature. Often used on Si02... 

Cobalt has an odd number of electrons, and does not form a simple carbonyl in oxidation state 0. However, carbonyls of formulae Co2(CO)g, Co4(CO)i2 and CoJCO),6 are known reduction of these by an alkali metal dissolved in liquid ammonia (p. 126) gives the ion [Co(CO)4] ". Both Co2(CO)g and [Co(CO)4]" are important as catalysts for organic syntheses. In the so-called oxo reaction, where an alkene reacts with carbon monoxide and hydrogen, under pressure, to give an aldehyde, dicobalt octacarbonyl is used as catalyst ... 

The most useful reaction of Pd is a catalytic reaction, which can be carried out with only a small amount of expensive Pd compounds. The catalytic cycle for the Pd(0) catalyst, which is understood by the combination of the aforementioned reactions, is possible by reductive elimination to generate Pd(0), The Pd(0) thus generated undergoes oxidative addition and starts another catalytic cycle. A Pd(0) catalytic species is also regenerated by /3-elimination to form Pd—H which is followed by the insertion of the alkene to start the new catalytic cycle. These relationships can be expressed as shown. 

The oxidation of simple internal alkenes is very slow. The clean selectiv oxidation of a terminal double bond in 40, even in the presence of an internt double bond, is possible under normal conditions[89,90]. The oxidation c cyclic alkenes is difficult, but can be carried out under selected condition Addition of strong mineral acids such as HCIO4, H2S04 and HBF4 accelerate the oxidation of cyclohexene and cyclopentene[48,91], A catalyst system 0 PdSO4-H3PM06W6Oii(j [92] or PdCF-CuCF m EtOH is used for the oxidatioi of cyclopentene and cyclohexene[93]. 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

The decarbonylation-dehydration of the fatty acid 887 catalyzed by PdCl2(Ph3P)2 fO.Ol mol%) was carried out by heating its mixture with acetic-anhydride at 250 C to afford the terminal alkene 888 with high selectivity and high catalyst turnover number (12 370). The reaction may proceed by the oxidative addition of Pd to the mixed anhydride[755]. 

The best procedures for 3-vinylation or 3-arylation of the indole ring involve palladium intermediates. Vinylations can be done by Heck reactions starting with 3-halo or 3-sulfonyloxyindoles. Under the standard conditions the active catalyst is a Pd(0) species which reacts with the indole by oxidative addition. A major con.sideration is the stability of the 3-halo or 3-sulfonyloxyindoles and usually an EW substituent is required on nitrogen. The range of alkenes which have been used successfully is quite broad and includes examples with both ER and EW substituents. Examples are given in Table 11.3. An alkene which has received special attention is methyl a-acetamidoacrylate which is useful for introduction of the tryptophan side-chain. This reaction will be discussed further in Chapter 13. 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

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