Acid-catalyzed alkene isomerizations

Few kinetic studies of acid-catalyzed alkene isomerizations have been reported. This is probably due in part to the complexity of these reactions and the number of competing processes in which the carbonium ion intermediates are 

When the catalyst is a proton acid, the reaction is complete after the second step. 

Cram and Sahyun studied the reversible p-toluenesulfonic acid-catalyzed isomerization of cis- and /raAZ5-2-phenyl-2-butenes and 2-phenyl-1-butene in acetic acid solutions, viz- 

Initial rates of disappearance of each of the isomeric phenylbutenes are first order in alkene and first order in p-toluenesulfonic acid. The kinetic and equilibrium data are accounted for by assuming that all three alkenes form the same carbonium ion, which is deprotonated to form 9% cw-2-phenyl-2-butene, 1% /ra/i.s-2-phenyl-2-butene and 90% 2-phenyl-I-butene in glacial acetic acid —0.01 M p-toluenesulfonic acid at 50°C. 

In aqueous 98.7% dioxane, perchloric acid catalyzes the isomerization of 3-methylenecholestane to 3-methyl-2-cholestene , 

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The protonation of alkenes, the essential first step in acid-catalyzed alkene isomerization, alkylation, hydration, and so on, is considered to involve the initial overlap of the carbon p-orbital forming the tc bond with the vacant s-orbital of the proton, forming a three-center bonded or tc complex" "" (Scheme 6.40). Taft seems to be the first to have suggested the initial formation, in acid-catalyzed hydration of isobutylene, of a proton tc complex of the type described originally by Dewar, in which the proton is embedded in the TC orbitals which extend above (or below) the plane of the C-C double bond. ... 

In hydrocarboxylations, as in the 0x0 process, selectivity of linear versus branched products is an important issue, because (in general) mixtures of isomeric carboxylic acids are obtained, owing not only the occurrence of both Markovni-kov and anti-Markovnikov addition of the alkene to the metal hydride, but also to metal-catalyzed alkene isomerization (eq. (2)). In the case of higher olefins, Co2(CO)g as catalyst leads to a number of different carboxylic acid isomers due to the isomerization activity of the catalyst. 

Addition of carbon monoxide and water to an alkene, i.e. hydrocarboxylation, is catalyzed by a variety of transition metal complexes, including [Ni(CO)4], [Co2(CO)s] and [HaPtClg]. Unfortunately this reaction usually leads to mixtures of products due to both metal-catalyzed alkene isomerization and the occurrence of Irath Markownikov and anti-Markownikov addition of the metal hydride intermediate to the alkene. The commercially available zirconium hydride [(C5Hs)2Zr(H)Cl] can be used as a stoichiometric reagent for conversion of alkenes to carboxylic acids under mild conditions (equation 23). In this case the reaction with linear alkenes gives exclusively terminal alkyl complexes even if the alkene double bond is internal. Insertion of CO followed by oxidative hydrolysis then leads to linear carboxylic acids in very good yield. 

Acid catalyzed dehydration of 2 2 dimethyl 1 hexanol gave a number of isomeric alkenes including 2 methyl 2 heptene as shown in the following formula... 

Although 2 methylpropene undergoes acid catalyzed hydration m dilute sulfuric acid to form tert butyl alcohol (Section 6 10) a different reaction occurs m more concentrated solutions of sulfuric acid Rather than form the expected alkyl hydrogen sulfate (see Sec tion 6 9) 2 methylpropene is converted to a mixture of two isomeric C Hig alkenes... 

Addition of phenylmagnesium bromide to 4 tert butylcyclohexanone gives two isomeric ter tiary alcohols as products Both alcohols yield the same alkene when subjected to acid catalyzed dehydration Suggest reasonable structures for these two alcohols... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Pyrazine 1,4-dioxides are available by the direct self-condensation of 1,2-hydroxyaminooximes (70JOC2790). 1,2-Nitrooximes are obtained by the isomerization of alkene initrogen trioxide adducts, which are reduced with palladium on charcoal to the hydroxyaminooximes which undergo acid-catalyzed auto-condensation to the pyrazine 1,4-dioxides (Scheme 19). 

As a method for the preparation of alkenes, a weakness in the acid-catalyzed dehydration of alcohols is that the initially formed alkene (or mixture of alkenes) sometimes isomerizes under the conditions of its formation. Write a stepwise mechanism showing how 2-methyl-1-butene might isomerize to 2-methyl-2-butene in the presence of sulfuric acid. 

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. 

Ru(H20)6], which is a precursor of ROM polymerization of cyclic dienes has also been found to possess good alkene isomerization activity [1], Among others it catalyzed the isomerization of allylphenyl ether to a vinylphenyl ether (Scheme 9.1) at room temperature. Allyl ethers are stable to acids and bases, while vinyl ethers are easily cleaved in acidic solutions. Therefore this isomerization gives a mild method for removal of protecting allyl groups under exceedingly mild conditions. 

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>. 

The acid-catalyzed isomerization of cycloalkenes usually involves skeletal rearrangement if strong acids are used. The conditions and the catalysts are very similar to those for the isomerization of acyclic alkenes. Many alkylcyclohexenes undergo reversible isomerization to alkylcyclopentenes. In some cases the isomerization consists of shift of the double bond without ring contraction. Side reactions, in this case, involve hydrogen transfer (disproportionation) to yield cycloalkanes and aromatics. In the presence of activated alumina cyclohexene is converted to a mixture of 1-methyl- and 3-methyl-1-cyclopentene 103... 

These observations demonstrate that multiplicity of products is a characteristic feature of acid-catalyzed alkylation with alkenes indicating the involvement of isomerizations. Some of the compounds isolated cannot be formed by the reaction of one molecule of the alkane with one or more of the alkenes. This points to secondary reactions accompanying alkylation. 

Fast side reactions under the conditions of acid-catalyzed alkylation are oligomerization (polymerization) and conjunct polymerization. The latter involves polymerization, isomerization, cyclization, and hydrogen transfer to yield cyclic polyalkapolyenes. To suppress these side reactions, relatively high (10 1) alkane alkene ratios are usually applied in commercial alkylations. 

When there is a hydrogen or perfluoroalkyl substituent on the alk-l-ene chain, Lewis acid catalyzed migration of the C = C bond to the 3-position can proceed to minimize the number of alkenic fluorine atoms,3 as illustrated by the isomerizations of 4//-perfluorobut-l-ene (3),8 4//-perfluoro(5-methylhex-l -enc) (5),18 perfluoro(4-methylpent-2-ene) (7),14 and perfluoro(3-methylcyclopentene) (9).2 3... 

The enzyme aconitase catalyzes the isomerization of citric acid to isocitric acid via the intermediate cis-aconitic acid (Scheme 46),530 and various attempts have been made to model this reaction.21 The cobalt Ill) complexes derived from methyl maleate (171) and methyl fumarate (172) have been prepared531 to study intramolecular attack by coordinated hydroxide on the alkene. Generation of the hydroxo species of the maleic acid complex leads to rapid cyclization to give the... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Step 2 Mild acid catalyzes the enol ether hydrolysis without subsequent 3, y to a, (3 alkene isomerization. 

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