Allenylpalladium methoxides

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. 

The alka-l,2,4-trienes (allenylalkenes) 12 are prepared by the reaction of methyl propargyl carbonates with alkenes. Alkene insertion takes place into the Pd—C bond of the allenylpalladium methoxide 4 as an intermediate and subsequent elimination of /3-hydrogen affords the 1,2,4-triene 12. The reaction proceeds rapidly under mild conditions in the presence of KBr. No reaction takes place in the absence of an alkali metal salt[4]. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-I,2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5]. The reaction can be explained by the transmetallation of the c-allenylpalladium methoxide 4 with copper acetylides to form the allenyl(alk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCN) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7],... 

Scheme 11.8 Formation of allenylpalladium methoxides from propargylic carbonates.

The key intermediate in the mechanism of the reaction is the formation of a cT-allenylpalladium complex 48 together with methoxide which serves as an in situ base to deprotonate the nucleophile. 

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