Acid catalysis alkene isomerization

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. 

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

Another approach to alkene isomerization would be to use a catalyst. Base catalysis is of no use as there are no acidic protons in the alkene. Acid catalysis can work (Chapter 19) if a carbocation is formed by protonation of the alkene. 

Branched alkenes from zooplankton Phytol-derived olefinic hydrocarbons have been isolated in low concentration from marine zooplankton (1) isomeric mono-olefins from mixed zooplankton from the Gulf of Maine by Blumer and Thomas (1965b) (2) isomeric phytadienes derived by dehydration of phytol, presumably by acid catalysis (Blumer and Thomas, 1965a) (3) C19 di- and tri-olefinic hydrocarbons (Blumer et al., 1969). These olefins are not present in ancient sediments and in petroleum and are therefore valuable markers in the distinction between marine hydrocarbons from organisms and from oil pollution. 

The isomeric epoxy triflates 153 and 157 undergo triflate displacement-epoxide opening with the dianion of methyl propanoyl acetate. Reaction of 153 generates the epimeric bicyclic tetrahydrofurans 154 and 155, subsequent treatment with triflic acid leading to isomerization about the alkene bond to a mixture of 154-156. Isomeric 157 under similar conditions gives 158 and 159, with triflic acid catalysis leading to some of the alkene isomers 160 along with 158 (Scheme 32). ... 

Molecular heats of adsorption play a role in many catalytic reactions. Figure 6.23 illustrates this for an isomerization reaction catalyzed by a solid acid. As explained in Chapter 3, the hydroisomerization of alkanes on a zeolite-supported metal proceeds through a bifunctional reaction mechanism, in which the metal has the function of activating C-H bonds and H2 at a low reaction temperature. The alkane-alkene equilibrium is established by metal catalysis, and the alkene is protonated and isomerized by the acidic protons of the zeolite... 

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