Synthesis of 2-boryl-1,3-butadienes from tributylphosphine stabilized zirconacyclopropenes and alkynes

Boryl zirconacycopropenes stabilized with tributylphosphine react with alkynes (terminal and internal) to give predominately 2-boryl-1,3-butadienes, 5, in 40-81% isolated yields. Products 5 are accompanied by 4-boryl-1,3-butadienes in 7-23% when terminal alkynes are inserted. However, the use of an internal alkyne (3-hexyne) gave predominantly 6c.