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 SE2‘ 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.