Elimination reactions double bond migration

With some catalyst systems, selectivity to primary metathesis products is near 100%, but side reactions (double-bond migration, dimerization, cyclopropanation, polymerization) often reduce selectivity. Such side reactions, such as oligomerization and double-bond shift over oxide catalysts, may be eliminated by treatment with alkali and alkaline-earth metal ions.26... 

Heck reactions of allylalcohols, with the involvanent of neutral intermediates and more strongly coordinating leaving groups, alternatively provided aldehydes or ketones, as a result of H /3-elimination or double bond migration by hydridopalladium leaddition/elimination to give unstable enols (Sect. B.vii). 

Conversion of 4 to 6 consumes no hydrogen and appears to be a consequence of double-bond migration. In this case, however, the reaction proceeded in two stages, hydrogen addition (5) followed by hydrogen elimination and migration (2S). 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

A deuterium-labeling study of a reaction of this type demonstrated syn stereoselectivity in both the oxypalladation and P-elimination, which indicates that the cyclization occurs by internal migration, rather than by an anti nucleophilic capture.113 This particular system also gives products from double-bond migration that occurs by reversible Pd(II)-D addition-elimination. 

The regiochemistry of the Heck reaction is determined by the competitive removal of the (3-proton in the elimination step. Mixtures are usually obtained if more than one type of (3-hydrogen is present. Often there is also double-bond migration that occurs by reversible Pd-H elimination-addition sequences. For example, the reaction of cyclopentene with bromobenzene leads to all three possible double-bond isomers.146... 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

Desulfurization or sulfoxide elimination of the adducts effects addition of the nitrogen nucleophile to an olefin or a substitution reaction with double-bond migration. 

For the Heck reaction as discussed in Section III.2.1 the final position of the olefi-nic double bond of the products must not necessarily be the same as in the starting materials (for example Schemes 8, 9, and 10 of Section III.2.1) [1], The selectivity is often driven by stereochemical requirements, because the /1-hydrogen elimination step which forms the double bond proceeds exclusively in a syn manner (if a trans /3-hydrogen is eliminated, one should suspect major deviations from the general mechanism of the Heck reaction, for example electrophilic substitution instead of carbopalladation). An impressive example of a double bond migration is depicted in Scheme 1 - instead of olefins the coupling reaction of iodobenzene 1 with the olefmic alcohol 2 results in the isomeric aldehydes 3 and 4 as final products [2], Reactions of this type have emerged as valuable tools for the synthesis of carbonyl compounds and also as crucial steps in domino processes. 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

Concurrently with the Mannich reaction, structural modifications of the substrate, such as double bond migration, cyclization, rearrangement, and elimination accompanying the aminomethyl group attachment, may also occur. These are treated in Sec. C.5 and summarized in Fig. 8. 

Not all terpene synthases catalyse complex reactions. Isoprene synthase converts DMAPP to the hemiterpene (G5), isoprene (Fig. 5.1), a comparatively simple process involving the ionization of the diphosphate group, followed by double-bond migration and proton elimination (Silver and Fall, 1991). Present in chloroplasts in both stromal and thylakoid-bound forms, isoprene synthase is a homodimer that differs from other terpene synthases in many properties, such as subunit architecture, optimum pH and kinetic parameters... 

The method of choice for the hydrogenolysis of primary allylic alcohols involves treatment with a pyridine-S03 complex (to form a sulfoester) followed by reduction with LiAlH4. " This method eliminates complications encountered for the reactions in allylic systems, such as double bond migration or stereoisomerization ... 

Vinylation of heteroaryl triflates also possible. Vinyl halides can be coupled to alkenes to form dienes. " The reaction of dihydrofurans with vinyl triilates and a palladium catalyst leads to a nonconjugated diene, 33. This example illustrates that the product is formed by an elimination step, as with the Heck reaction (13-10), and double bond migration can occur resulting in allylic rearrangement. 

With chiral ligands the Heck reaction can be enantioselective. The amino-acid-derived phosphine ligand in the margin controls the Heck reaction of phenyl triflate with dihydrofuran. The ligand selects one enantiotopic face of the alkene (see Chapter 45 if you have forgotten this term) and the usual double bond migration and (5 elimination complete the reaction. 

Double bond migration such as that involved in 86 going to 87 is an important reaction, useful on both an industrial and a laboratory scale its mechanism has been well studied. Equations 9.43 and 9.44 describe two major pathways for migration, the former involving metal hydride insertion and (3-elimination steps... 

When Pt-olefin complexes such as PtCl3(olefin) are treated with a second olefin, replacement of the coordinated olefin by the incoming olefin does not result in either double bond migration or (Z), ( ) isomerization of the displaced olefin. However, the olefin complexes when treated at low temperature with a nucleophile such as pyridine (py) or a secondary amine undergo conversion to a or-complex by a stereospecific trans-process, i.e., frans-addition and tra s-elimination. Treatment of frans-LPtCljCZ)-ethylene-l,2-reversible formation of the carbon (7-bonded complex with slow release of pure (Z)-ethylene-l,2-[Pg.383]

The rate of olefin elimination is enhanced by catalytic amounts of Ni compounds, which suppress the growth reaction (in contrast to compounds of Ti, Zr or V) however, they also catalyze double-bond migration and often the catalytic activity rapidly decreases. The catalysis of the displacement reaction is of interest for the preparation of EtjAl or PrjAl from i-BujAl, because no isomerization of the olefin can occur and the conditions (50-100°C, Ni) exclude the growth reaction. ... 

Side reactions such as double-bond migration and others are observed, similar to hydroformylation. Mechanistically, hydrocarboxylation is related to hydroformylation up until the metal acyl formation stage13. The presence of an acidic compound shifts the reaction towards formation of carboxylic acid derivatives and suppresses reductive elimination which forms aldehydes. The mechanism of the final steps is unclear13. 

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