Preparation of 5-Hydroxy-1-alkenyboronates from 1-Alkynylboronates, Cp₂ZrCl₂/2EtMgBr, and Aldehydes

1-Alkynylboronates form five-membered zirconacycles with Cp₂ZrCl₂/2-EtMgBr as indicated by deuterium labeling. The zirconacycles add aldehydes to form seven-membered zirconacycles. Hydrolysis of the latter provides 5-hydroxy-1-alkenylboronates in fair to good isolated yields. Both aliphatic and aromatic aldehydes undergo insertion.