Generation of Allenic/Propargylic Zirconium Complexes and Subsequent Cross-Coupling Reactions: A Facile Synthesis of Multisubstituted Allenes

The β-alkoxide elimination reaction of propargylic ether with Negishi reagent leads to allenes and/or alkynes after hydrolysis. The product distribution is highly dependent on the substitution pattern of starting propargylic ethers; that is, aryl- or alkyl-substituted propargylic ethers favor the allene products, whereas TMS-substituted propargylic ethers afford alkynes. DFT calculations revealed that both the large steric effect and the β-effect of the TMS group favor the alkyne products, reversing the selectivity. Subsequent coupling reactions of the allenic/propargylic zirconium intermediates with aryl iodides in the presence of Pd(PPh3)4/CuCl provide a straightforward route for the synthesis of multisubstituted allenes.