Isomerization of double bond

Isomerization of double bonds in vitamin D analogs such as calciferol by oxidation and reduction has been carried out via the formation of the tt-allylpalladium complex 334 with PdCl2(PhCN)2 in 70% yield, followed by hydride reduction to afford 335[295],... 

Addition of Ag2C03 cleanly suppresses the double bond isomerization[20]. Thus, with procedure B, no isomerization of double bond is observed. Addition of TlO.Ac also suppresses the double bond isomerization[23]. 

When heating treatments are applied to obtain stand oils, the following chemical modifications are likely to occur cross-linking of triacylglycerols, isomerization of double bonds, and formation of dimers through Diels-Alder cyclization [50,51]. As a result of double bond isomerization, the amounts of suberic and sebacic acids increase with respect to azelaic acid. Consequently, the ratio of suberic acid to azelaic acid may help to indicate a pre-polymerized oil [52,53]. 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

CEJ1358> and the ruthenium mediated isomerization of double bonds (cf. Scheme 89, Section 8.11.7) <2007TL137> are recent examples of transition metal catalyzed manipulations at the side chain carbon atoms of 1,3-heterocycles. A novel side-chain addition reaction of aldehydes to 6-alkylidene-l,3-dioxin-4-ones was used for the construction of intermediates of lophotoxin <2006CJC1226>. An acid-catalyzed intramolecular cycloaddition of a hydroxy group to an alkene has been effected by the presence of an adjacent 1,3-dithiane moiety <2006TL4549>. 

Complex 34, which is prepared by treating 33 with carbon monoxide (1 atm) catalyzes the isomerization of double bonds. This catalyst also accelerates the reaction of bound substrates. Under identical conditions (reaction time 2 hours) allylcatechol and allylbenzene are isomerized by 34 to the corresponding methylstyrenes in yields amounting to 68% for the former substrate and only 12% for the latter one [37]. 

Hay and Morrison (1970) identified the monoenoic positional and geometric isomers in milk fat and determined the amounts of each total acid class and percentage of trans isomers. The geometric and positional isomers of the monoenes are primarily the result of biohydrogenation of polyunsaturated fatty acids in the rumen. Stearate is also produced, and cis-9-18 l accounts for most of the monoenes. The several positional isomers in trans 16 1 and 18 1 are due to the positional isomerization of double bonds which accompanies elaidinization. 

Pentacarbonyliron can catalyze the isomerization of double bonds under photochemical conditions. Using catalyst loadings as low as 1-5 mol%, this process proceeds smoothly for allyl alcohols, which isomerize to the corresponding saturated carbonyl compounds. 

Palladium, by far the most effective of platinum metals for isomerization, is also the most effective in this reaction. The isomerization involves not only tautomerization of an enamine to an imine, but additionally, isomerization of double bonds remote from the amine into a vinylic position. Platinum, ruthenium, and rhodium afford mixtures of less cyclo-hexanols and more dimethylcyclohexylamines (17). 

Migration or isomerizations of double bonds and epimerizations are frequently observed in PKRs. Several groups have studied these processes and believe they take place during the PKR or with participation of the metal... 

With regard to isomerizations of double bonds, sulfinyl enynes gave unexpected results when submitted to PKR. These chiral substrates were thought to give high asymmetric inductions due to the proximity of the chiral sulfur atoms to the reaction centers. Surprisingly, both cis and trans ferf-butyl vinyl sulfoxides (237-238) were transformed into the same PK diastereoisomer 239 with high ee and moderate yields (Scheme 67) [103,104]. 

The industrial Wacker process is carried out in aqueous hydrochloric acid using PdClj/CuCh as the catalyst under oxygen pressure. The oxidation of higher terminal alkenes under the same conditions is slow and sometimes accompanied by undesired by-products formed by the chlorination of carbonyl com-poimds by CuCh, and isomerization of double bonds. Earlier examples of oxidation of various alkenes, mainly in aqueous solutions, have been tabulated.The pseudo-first-order rate constants for oxidation of various alkenes, relative to the value for cycloheptene, with PdCb in the presence of benzoquinone in aqueous solution have been rqwrted. An accelerating effect of surfactants such as sodium lauryl sulfate on the stoichiometric oxidation of higher alkenes in an aqueous solution has been reported. 

The complexes derived from platinum complexes and SnCh exhibit catalytic activity for hydrogenation under mild conditions. These complexes have also been used for alkene hydroformylation. The above catalyst system and [PtCl2(PPh3)2] assist hydrogenation of polyenes to monoalkenes, but isomerization of double bonds has been observed. The anionic cluster complex [ Pt3(CO)6 5] has been used as a catalyst for hydrogenation of cyclohexene. 

Figure 7. Reaction scheme for positional isomerization of double bonds and formation of trans-, trans-hydroperoxides during oxidation of linoleic acid via reversible p-scission of oxygen. Adapted from (246, 248, 250).

Oxidations of steroidal keto alcohols to diketones are frequently accomplished biochemically. In addition, isomerization of double bonds, dehydrogenations, and hydroxylations often take place. 

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