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With pyrrole (pKa17.5) and indole (pKa16.2) (entries 8–9), we mainly observe the formation of a mixture of allene oligomeric products. N-allyl indole has been obtained in only 16% yield after chromatography. A possible explanation means the intermediate ylide 6 (see Scheme 3) is not basic enough to deprotonate the azole. As a consequence, the acid– base reaction competes unfavorably with the polymerization of allene.An excess of triphenylphosphine does not improve the yields. Mechanism In an attempt to rationalize all these results, the following mechanism can be proposed (Scheme 3). The catalytic cycle might be initiated by a nucleophilic addition of triphenylphosphine to the electron-deficient allene 2. The enolate 6 then deprotonates the azole 1 generating the nucleophilic species 5 and the vinylphosphonium 4. Consecutively, nucleophilic addition of 5 to vinylphos- phonium 4 leads to the ylide 7. Finally, enolate 8 is obtained by prototropy and then undergoes a b-elimination affording the final g-addition allylazole 3 and regenerating the nucleophilic catalyst. |
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