Allenylpalladium hard carbon nucleophiles

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Hard carbon nucleophiles M—R (M = Mg, Zn, B, Cu, etc.) undergo facile transmelalla-tion with the allenylpalladium intermediate A followed by reductive elimination to give aUene derivatives (Schemes 4 and... 

TT-Allylpalladium complexes have long been recognized as an allyl cation equivalent and widely utilized for the allylation of a variety of hard and stabilized carbon nucleophiles. Both allenylpalladium and vinylpalladium complexes are chemically neutral, or very feebly nucleophilic, if any. Under conditions described below, however, these rr-allyl-, vinyl-, and allenylpalladium complexes alter their reactivity in mode and behave as nucleophilic species. "... 

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