Unusual Regioselectivity in the Aldehyde Addition Reactions of Allenyl/Propargyl Zirconium Complexes Derived from γ‑(2-Pyridyl)propargyl Ethers: Synthesis of Multisubstituted α‑Hydroxyallenes

Zirconium-mediated
reactions of γ-(2-pyridyl)­propargyl ethers with aldehydes afford
α-hydroxyallenes selectively via the formation of allenyl/propargyl
zirconium species. The reaction outcome is quite different to that
of the reactions using propargylic ethers without a pyridyl group
reported so far, in which homopropargyl alcohols were formed predominantly.
The structure of allenylzirconium intermediate has been confirmed
by X-ray crystal analysis which reveals an intramolecular Zr–N
coordination. DFT calculations suggest that the smaller steric effect
of the pyridine ring compared with the phenyl ring and its capability
to form hydrogen bondings with hydrogen atoms of the Cp ligand and
the aldehyde may account for the observed regioselectivity.