Alkenes catalyst

In the presence of certain metal catalysts, alkenes and alkynes can be carbonylated or carboxylated. An intramolecular carbonylation converted an alkene-alkyne to a... 

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

Although most of the reports that have appeared since 1980 on hydroformylation of alkenes focus on rhodium catalysts, alkene hydroformylation catalyzed by Ptn complexes in the presence of Sn11 halides has been the object of great interest and platinum can be considered as the second metal in hydroformylation.77-79... 

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

Alkene Catalyst Alkene conversion (%) Selectivity towards alkylbenzene (%) ... 

When the diphosphine is chiral, binding of a prochiral alkene creates diastereomeric catalyst-alkene adducts. (Diastereomers result because binding of a prochiral alkene to a metal center generates a stereogenic center at the site of unsaturation.) Through a powerful combination of3lP and l3C NMR methods, Brown and Chaloner first demonstrated the presence of two diastereomeric catalyst-enamide adducts with bidentate coordination of the substrate to the metal (Figure 1) [19]. 

The dependence of selectivity for dehydrogenation on the conversion of alkane shows that for the more selective catalysts known, the reaction proceeds with a sequential mechanism. The first step of the reaction is the breaking of a C—H bond of the alkane molecule, which is also the rate-limiting step. For these more selective catalysts, alkene is the primary product. Therefore, high selectivities can be obtained at low conversions. However, as the conversion increases, the selectivity decreases because of the secondary reaction of the alkene. The rate constant for the reaction of the alkene on the most selective catalyst is still about the same in magnitude as the rate constant for the activation of alkane. It is larger for the less selective catalysts. Thus the maximum yield of alkene among the catalysts known to date is still less than about 35%. To improve this yield, catalysts that react with alkene less rapidly than with alkane need to be found. 

Catalyst Alkene N2CHCOOR R = trans. cis % ee/de trans % ee/de cis1... 

Experimental studies by the groups of Halpern and Brown [69, 70] on the hydrogenation of prochiral alkenes by a chelating-diphosphine catalyst provided a detailed, and unexpected, picture of the overall mechanism of hydrogenation. The reaction of the catalyst with a prochiral alkene can produce two diastereomeric catalyst-alkene complexes. The most stable (and most abundant) diastereomeric complex is relatively unreactive, whereas the small amount of the complex in the less populated diasteromeric state gives the productive catalytic cycle. Landis has coined the phrase anti-lock-and-key motif to describe this mechanistic model. 

Their initial research was a QM study at the B3LYP level on a model system, where they chose [Rh(PH3)2(a-formamidoacrylonitrile)]+ as a model catalyst-alkene complex [75]. Since this model system is achiral, they do not need to consider different diastereomeric manifolds to study the reaction mechanism. Nevertheless, they need to consider four different reaction pathways corresponding to the four possible czs-dihydride isomers that can be formed on the addition of H2 to the metal complex (Fig. 9). 

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. 

Two hydrogen molecules are added to one triple bond (— C = C —) using a nickel, platinum or palladium catalyst. Alkenes are the intermediate products. 

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 this catalyst alkenes such as 1-hexene, 2-pentene, 3-hexene, cyclohexene, 2,3-dimethyl-2-butene, styrene, allyl alcohol, allyl cyanide and acrylamide, are reduced under mild conditions. Rates of... 

By far the most important of these methods is the hydrogenation of alkencs. When shaken under a slight pressure of hydrogen gas in the presence of a small amount of catalyst, alkenes are converted smoothly and quantitatively into alkanes of the same carbon skeleton. The method is limited only by the availability of the proper alkene. This is not a very serious limitation as we shall see (Sec. 5.11), alkenes are readily prepared, chiefly from alcohols, which in turn can be readily synthesized (Sec. 15.7) in a wide variety of sizes and shapes. 

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... 

When alcohols are dehydrated in the presence of a H2S04 catalyst, alkenes or ethers are produced. The temperature of the reaction medium determines whether the product is an alkene or ether. Formation of an ether takes place at a lower temperature than the formation of an alkene. 

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. 

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