Carbon-carbon bond formation of alkenylphosphonates by aldehyde insertion into zirconacycle phosphonates

Ethyl 1-butynylphosphonate reacts with Cp2ZrCl2/2n-BuLi to give a three-membered zirconacycle that readily inserts aldehydes. Hydrolysis of the intermediate five-membered zirconacycles leads to two products, 4 and 5. In the major product, 5, the aldehyde inserts into C2 of the zirconacycle, while in the minor product, 4, the aldehyde inserts into C1. Products 5 are obtained in 38-75% isolated yields. Products 4 are obtained in approximately 1-12%. Essentially, only compounds 5 are produced with ortho-substituted aldehydes. The regio- and stereochemistry of 4 and 5 were determined by (3)J(PH), (2)J(PC2), and (3)J(PC3) coupling constants.