Direct and Stereospecific Synthesis of Allenes via Reduction of Propargylic Alcohols with Cp₂Zr(H)Cl

Allenes can be synthesized via the direct S(N)2' addition of hydride to propargylic alcohols. Previous examples of this approach, however, have involved harsh reaction conditions and have suffered from incomplete transfer of central chirality to axial chirality. Here we show that Cp2Zr(H)Cl can react with the zinc or magnesium alkoxides of propargylic alcohols to generate allenes in good yield and in high optical purity. Dialkyl-, alkyl-aryl-, and diaryl-allenes are accessible by this method. Furthermore, the reaction can provide silyl-substituted allenes, trisubstituted allenes, and terminal allenes.