Allylic acetoxylations, alkenes, palladium® acetate

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

While it is evident that aUyhc acetoxylation and related reactions proceed via two different mechanisms, mainly depending on the structure of the alkenes, it is less clear how to choose reaction conditions in order to favor one route or the other. There is some evidence from early smdies that the use of polar solvents such as DMF will promote aUyl acetate formation. It also appears that excess acetate promotes the formation of products compatible with the rr-allyl route. This is also suggested by recent factorial experiments with variation of carboxylate concentration. Since trimeric palladium acetate will induce rr-allyl formation from a series of monoolefins, it might be assumed that high concentration of palladium acetate could be used for creating conditions that favor a 77-aUyl route. " Another possibility is adding strong acids, which can increase the electrophilicity of the catalyst, but this can drive the reaction toward homoallylic acetates and other isomerized products. "... 

This one-step transformation of an alkene to an allylic acetate compares well with other methods of preparation such as hydride reduction of a, 8-unsaturated carbonyl compounds followed by esterification. The scope and limitations of the reaction have been investigated. The allylic acetoxylation proceeds via a TT-allylpalladium intermediate, and as a result, substituted and linear alkenes generally give several isomeric allylic acetates. With oxygen nucleophiles the reaction is quite general, and reactants and products are stable towards the reaction conditions. This is normally not yet the case with nitrogen nucleophiles, although one intramolecular palladium-catalyzed allylic amination mechanistically related to allylic acetoxylation has been reported. ... 

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