Efficient and Selective Formation of Unsaturated Carboxylic and Phenylacetic Acids from Diketene

A nickel catalyst promotes the multicomponent coupling reaction of diketene, an alkyne, and Me₂Zn to provide 3‐methylene‐4‐hexenoic acids in excellent yields. Under similar conditions, the combination of the nickel catalyst and Et₂Al(OEt) promotes a cycloaddition reaction involving dimerization of an alkyne to furnish phenylacetic acids. In the presence of PPh₃, a formal [2+2+1+1] cycloaddition reaction proceeds to afford regioisomeric phenylacetic acids via cleavage of the C?C bond.