Isomerization and Migration of Double Bonds

Migration of double bonds. Rhodium trichloride trihydrate is useful for certain exocyclic-endocyclic migrations of a double bond that are usually difficult to achieve. Some examples are formulated. The isomerization of ergosterol is interesting in that it yields a previously unknown coprostatrienol together with crgosterols Bi and B2. 

Isomerizations of Unsaturated Compounds. r-BuOK is an effective base for bringing about migrations of double bonds in alkenes and alkynes via carbanion intermediates, but since the base promotes these reactions most effectively in DMSO, they will be described in more detail under Potassium tert-Butoxide-Dimethyl Sulfoxide. Important exanples of enone deconjugations with i-BuOK/f-BuOH which proceed via di- and trieno-late intermediates are shown in eqs 30 and 31. Potassium i-pentoxide is effective in promoting the latter reaction, but various lithium amide bases are not, apparently because they de-protonate the enone at the a -position regioselectively. The isomerization of Q ,/8-unsaturated imines to alkenyl imines (eq 32) is an important step in an alternative method for reduction-alkylation of a ,/8-unsaturated ketones. ... 

Solvents can have a large influence on the extent of double-bond migration (6). The factors involved are complex as shown in the hydrogenation of methylenecyclohexane, 3-methylcyclohexene, and 4-methylcyclohexene to methylcyclohexane in benzene-ethanol, in peniane, and in ethanol over 5% Pd, 5% Pt, and 5% Rh-on-carbon. The amount of isomerized 2-methylcy-clohexene was measured ai 25% completion and, depending on the system,... 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

Another possible mechanism interprets cis—trans isomerization in combination with double-bond migration assuming the participation of jr-allyl intermediates54 55 (Scheme 11.2). Such dissociatively adsorbed species were originally suggested by Farkas and Farkas.56... 

Reactions of isoxazolotropone 90 with 1,2-diaminoethane or -propane afford, for instance, mixtures of isomeric dihydropyrazines 517a and isomeric pyrazines 517b in low yields [83H(20)1117], The latter structure is the result of double-bond migration from the seven-membered ring into the heterocyclic one. 

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... 

Example 11.2. Streamlined network for hydroformylation of n-heptene catalyzed by phosphine-substituted cobalt hydrocarbonyl. In hydroformylation of straight-chain olefins with a phosphine-substituted cobalt hydrocarbonyl catalyst, the model must account for three complications that are absent with cyclohexene isomerization by migration of the double bond along the hydrocarbon chain, formation of isomeric aldehydes and alcohols, and condensation of the straight-chain aldehyde to "heavy ends (chiefly an alcohol of twice the carbon number, such as 2-ethylhexanol from propene via n-butanal and a C8 aldol). A streamlined network for n-heptene is ... 

The occurrence of double bond isomerization under the conditions of hydropyrolysis is clearly indicated by the decrease in concentration of 1-olefins and a parallel increase in the concentration of the thermodynamically more stable internal olefins with increase in temperature (Figure 3) or contact time. Such a reaction could proceed by abstraction of an allylic H atom from the 1-olefin, followed by double bond migration ... 

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