Double bond migration inhibition

Double bond migration can be inhibited by amines and other nucleophilic agents and lead to higher selectivities in the reduction of dienes such as 1,4-cyclooctadiene or 1,3-cyclopentene to their respective monoenes (ref. 33). 

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). 

The Heck reaction with the simple representatives of this substracture shown in Scheme 1 generally involves a double bond migration to give aromatized products in moderate to good yields. " The highest yield was achieved for 3-methylindole, because in this case the reaction was driven to completion by the addition of several portions of Pd catalyst. The presence of silver salts inhibits the migration of the double bond as a result, products with an exocyclic methylene group can be isolated (Scheme 1). ... 

Steric hindrance at the y-position of the alkene inhibits smooth double bond migration. For example, in case of 2-phenyl-3-methyl-1-butene, consumption of the corresponding sulfonium salt was incomplete even after prolonged reaction time or heating, and yielded only 45% of the desired product. Trisubstituted alkenes also generate the desired products, however yields are low. 

Double bond migration generally competes with ILZ-isomerization and the formation of carbene-derived products, unless it is structurally inhibited as in 2-norbomene (14) and homobrexene (38). It presumably occurs in cyclohexene (27) and cyclopentene (32) but is not observable. In an alkene such as 43, in which the double bond is exocycKc to a five-membered ring, migration is the dominant photoprocess and occurs... 

After half-hydrogenation, migration of the remaining double bond (sometimes mistaken for 1,4-addition) can be inhibited by poisoning with CO.53... 

The photoisomerization of / -ionone (114) and / -ionylidene (115) derivatives in aqueous solution was inhibited in the presence of P-CD because of the restrictions imposed on the rotation of the double bond by the cavity. The only photoreaction observed was 1,5-H migration, which was probably promoted by a change in the nature of the lowest excited state from n,it to K,7t as induced by lowering the environment polarity [300]. 

The reaction is not a chain reaction there is no induction period and the reaction is unaffected by anti-oxidants. It is, however, inhibited by singlet oxygen quenchers such as carotene. Instead, oxidation occurs by the ene mechanism in which oxygen becomes attached to either of the olefinic carbon atoms with attendant migration and stereomutation of the double bond. 

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