Catalytic processes alkene isomerization

The selective hydrogenation of a triple bond to give an alkene without concomitant positional or geometric isomerization is particularly important in synthetic procedures and many industrial processes. In the absence of any isomerization, selective partial hydrogenation of a disubstituted alkyne produces the cis alkene. Small amounts of the trans alkene are sometimes formed in these reactions, but catalytic processes do not lead to the production of the irans olefin as the primary product. The Irons alkenes can be produced as a primary product by metal-ammonia reduction of disubstituted alkynes.2... 

Alkylrhodiums form from the reaction of Rh hydrides and alkenes. This reaction is important in hydrogenation, asymmetric hydrogenation, alkene isomerization and hydroformylation and other catalytic processes. The regiochemistry seen in this reaction is the subject of theoretical study that rationalizes the formation of the less substituted (T-alkyrhodium intermediate on electronic grounds Several Rh complexes form stable ff-alkylrhodiums on reaction of Rh hydrides with fluorinated alkenes (see Table 8) . 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

An important step in the catalytic reactions of alkenes is the complexation of the substrate at the transition metal center. Differences in ability of olefins to coordinate can influence the selectivity of a catalytic process to such an extent that, for example, in a positionally isomeric olefins, the terminal olefins react preferentially to give the desired product. 

In weaker superacids such as neat CF3SO3H, alkanes that have no tertiary hydrogen are isomerized only very slowly, as the acid is not strong enough to abstract hydride to form the initial carbocation. This lack of reactivity can be overcome by introducing initiator carbenium ions in the medium to start the catalytic process. For this purpose, alkenes may be added, which are directly converted into their corresponding carbenium ions by protonation, or alternatively the alkane may be electrochemically oxidized (anodic oxidation) (equation 22). Both methods are useful to initiate isomerization and cracking reactions. The latter method has been studied by Commeyras and coworkers. ... 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

The more highly phosphine substituted rhodium species RhH(CO) (PlCeHslsls is an even more active catalyst, 1 atm pressure and 25°C being sufficient, and it is even more selective for the n product (21). Rh4(CO)i2 is also very active but has very poor selectivity, so once again, the presence of phosphine improves the selectivity. The mechanism is broadly similar to the Co-catalyzed process. In practice, excess PlCeHsls is added to the reaction mixture to prevent the formation of the less selective HRh(CO)4 and HRhL(CO)3 species by phosphine dissociation. The system is also an active isomerization catalyst, because much of the same mixture of aldehydes is formed from any of the possible isomers of the starting alkene. This is a very useful property of the catalyst, because internal isomers of an alkene are easier to obtain than the terminal one. The commercially valuable terminal aldehydes can still be obtained from these internal alkenes. The catalyst first converts the internal alkene, for example, 2-butene, to a mixture of isomers including the terminal one. The latter is hydroformy-lated much more rapidly than the internal ones, accounting for the predominant n aldehyde product. As the terminal alkene can only ever be a minor constituent of the alkene mixture (because it is thermodynamically less stable than the other isomers), this reaction provides another example of a catalytic process in which the major product is formed from a minor intermediate (eq. 21). 

Many other catalytic reactions are discussed in detail in other articles in this encyclopedia, but in general, they can be divided in various useful ways. Some, like alkene isomerization, involve a single substrate, others like hydrogenation, involve two some, like hydroformylation or the Wacker oxidation have three rarer are the cases with more substrates. In each case, the catalyst must be recycled, so atom and redox balance is required, the catalyst cannot be a net source or sink of atoms or of redox equivalents. Many catalytic reactions are oxidations or reductions of one substrate, but the primary oxidant or reductant has to be included in the reaction mixture. Organometallic complexes are more stable toward reduc-tants than oxidants and so oxidation reactions tend to be somewhat rarer than reductions. The Wacker process is an exception, of course. 

One common class of catalytic reactions involves 1,2-addition of a CH bond across a multiple bond (eq. 23). Such is the case for alkene hydrogenation (XY = H2), hydrosilation (XY = RsSi—H), hydroboration (XY = R2B—H), and disilylation (XY = RsSi—SiRs). An alkene isomerization like equation 10 can be considered as intramolecular C—H addition across the 2,3—C=C bond. The Heck reaction (eq. 24) is an example of an addition of a C—Hal bond combined with an elimination of a H—Hal group. Similarly, the Wacker process is effectively an addition of H—OH across the ethylene C—C bond, followed by an elimination of H2. 

The advantages of the partial chlorination of alcohols (60-95% conversion) with HCl and completion of the chlorination by a catalytic phosgenation and subsequent decarboxylation of the resulting chloroformates have been combined in a two-stage process [974, 982]. Only small amounts of dialkyl ethers, alkenes, isomeric chloroalkanes, or dialkyl carbonates are claimed to be formed as side products. 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

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