Addition of Allenylzinc Reagents, Prepared in Situ from Nonracemic Propargylic Mesylates, to Aldehydes. A New Synthesis of Highly Enantioenriched Homopropargylic Alcohols

Enantioenriched propargylic mesylates are converted to chiral allenylzinc reagents via transient allenylpalladium species by treatment with a Pd(0)-phosphine catalyst in the presence of excess Et₂Zn. These zinc reagents undergo S_E2‘ additions to various aldehydes to yield mainly the anti homopropargylic alcohol adducts of high ee. The reaction is most efficient in THF with Pd(OAc)₂·PPh₃ as the catalyst precursor. As little as 2.5 mol % of this precursor is effective.