Palladium-Catalyzed Propargylic vs, Allylic Alkylation


The relative reactivities of allylic and propargylic acetates toward palladium(O)-catalyzed substitution by various nucleophiles were studied by using three types of model substrates: (a) monofunctional allylic and propargylic acetates with similar structural properties; (b) a bifunctional substrate containing both allyl and propargyl functionalities with no apparent interaction between them; (c) conjugated bifunctional systems, in which the two functionalities may interact with one another. Palladium(0)-catalyzed substitution of propargylic acetates by various carbon nucleophiles was found to be less general than the analogous substitution of allylic acetates. Three modes of reactivity were observed, corresponding to three groups of nucleophiles: (a) stabilized carbanions such as sodium dimethyl malonate, which do not react with propargylic acetates but react readily with allylic acetates; (b) nonstabilized organometallics such as phenylzinc chloride, which react with propargylic and allylic acetates at comparable rates (reaction with the former yielding the allenic product exclusively); and (c) allyl-and allenylstannanes, which react with allylic acetates but do not react with isolated propargylic acetates (except for special cases where the propargylic acetate is also an allylic one). Certain similarities between regioselectivity phenomena in organopalladium and organocopper chemistry are discussed.

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Journal of Organic Chemistry