Side reactions, olefin isomerization

Mono- and 1,1-disubstituted olefins are the most reactive, but even tetrasubsti-tuted alkenes have been used [17]. Because double-bond isomerization is a common side-reaction, olefins with a non-isomerizable bond are usually used. 

The competing side reactions, olefin hydrogenation, olefin isomerization (in case of higher olefins than propylene), and aldehyde hydrogenation to the corresponding alcohol, can also be explained based on this mechanism ... 

In order to avoid side reactions, particularly isomerization, alkaline ions can be added during the impregnation stage of catalyst preparation. By this method, 1-butene production can be greatly reduced during metathesis of propylene and thus avoid the synthesis of other olefins by cross disproportionation between the normal metathesis products and the isomerized products. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

With the renaissance in alkene chemistry engendered by the rising versatility of olefin metathesis in both fine chemical and commodity production, new methods for alkene isomerization are of increasing interest and importance. Alkene isomerization can be performed using Bronsted-Lowry acid or base catalysis (1). However, these reactions are limited to substrates which tolerate carbanionic or carbocation intermediates, and are susceptible to undesired side reactions. 

It has been shown that hydrosilylation may not perform as ideally as is required when preparing co-olefinic silicone compounds from organic a,co-dienes and hydrosil(ox)anes isomerization is a concern and the chemical equivalence of the double bonds requires a large excess of the diene compound to achieve essentially monohydrosilylation. Further side reactions are discussed by Torres et al [9],... 

Methanation and olefin hydrogenation or isomerization (secondary and side reactions)... 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptor-substituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3-dipolar cycloaddition to the olefin [651-654]. The resulting 3//-pyrazolines can either be stable or can isomerize to l//-pyrazolines. 3//-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions. 

Isomerization is a frequent side-reaction of catalytic transformations of olefins, however, it can be a very useful synthetic method, as well. One of the best-known examples is the enantioselective allylamine enamine isomerization catalyzed by [Rh (jR)-or(S)-BINAP (COD)] which is the crucial step in the industrial synthesis of L-menthol by Takasago [42]... 

Alkene isomerization/migration is a potential side-reaction in olefin metathesis processes that can significantly decrease the yield of a desired product, particularly in reactions that employ electron-rich allylic or homoallylic... 

Numerous side reactions take place in the reaction zone, some of which are beneficial, but many of which are harmful (4). Some catalysts act to promote olefin isomerization to some extent as well as alkylation. In aviation and motor alkylate production, where more highly branched products are desirable, this isomerization reaction is of considerable benefit. This is especially true when the isomerization of butylene-1 to butylene-2 precedes the alkylation reaction. Harmful side reactions include hydrogen transfer and polymerization. The production of propane from propylene and of normal butane from butylene occurs by hydrogen transfer. Polymerization is very harmful because it not only produces the tar which spends the catalyst, but also reduces the yield of valuable products. 

Homogeneous hydrogenation in the fluorous phase has been so far reported only for a limited set of simple olefins (Richter et al., 1999, Rutherford et al., 1998), as exemplified with the neutral rhodium phosphine complex 18 as catalyst precursor (eq. 5.7). Isomerization of the substrate 1-dodecene (17a) was observed as a competing side reaction under the reaction conditions. The catalyst formed from 18 could be recycled using a typical FBS protocol, but deactivation under formation of metal deposits limited the catalyst lifetime. 

Olefin isomerization was found to occur as a side reaction. This was presumed to occur via the alkylcobalt carbonyls formed by Eq. (71) as discussed in Section V and elsewhere. 

The isomerization of epoxides is discussed in Section II, D,l. The isomerizations of olefins and of alkyl- and acylcobalt carbonyls have been considered as side reactions to hydroformylation but studies dealing principally with these reactions will now receive attention. 

The 10-undecenoic acid motif has also been attached to isosorbide in the preparation of a fatty acid-/carbohydrate-based monomer [131]. ADMET polymerization in the presence of C3 and C4 produced fully renewable unsaturated polyesters (Scheme 18). Most importantly, the transesterification of these polyesters with MeOH, and subsequent analysis by GC-MS of the products, allowed for the quantification of double-bond isomerization during ADMET in a very simple manner. This strategy was then extended to fatty acid-based ADMET polyesters synthesized in the presence of indenylidene metathesis catalysts [132]. With these studies, the knowledge on the olefin isomerization in ADMET reactions was widened, and it is now possible to almost completely suppress this undesired side reaction. 

An interesting side reaction is the isomerization of the C=C-double bond in the molecule, which changes the regioselectivity of hydroformylation. Depending on the products, isomerization in this reaction is either suppressed or enhanced to use the isomerization. In the case of a terminal olefin and a desired linear product, isomerization is hindered for better selectivity for the n-aldehvde. In the case of internal double bonds and desired linear products, isomerization is used to bring the unsaturated site to the end of the molecule to conduct the reaction on the terminal end [10, 11]. 

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