Reactions of zirconacyclopentadienes with C=O, C=N, and N=N moieties with electron-withdrawing groups: Formation of six-membered heterocycles

Reactions of zirconacyclopentadienes with diethyl ketomalonate gave α-pyrans in excellent yields in the presence of BiCl₃. In the absence of BiCl₃, zirconacyclopentadienes did not react with diethyl ketomalonate. Tetraphenylzirconacyclopentadiene reacted with diethyl ketomalonate in the presence of BiCl₃ to give a ring-opening product, dienolic ether, in 53% yield. The structures of the α-pyran prepared from diethyldiphenylzirconacyclopentadiene and the ring-opening product were determined by X-ray analysis. When oximinosulfonate was added to tetraethylzirconacyclopentadiene in THF at -78 °C, 3,4,5,6-tetraethylpyridine-2-carbonitrile was obtained in 95% yield within 10 min. The structure of the product was confirmed by X-ray analysis. When tetraethylzirconacyclopentadiene was treated with azodicarboxylate in the presence of 2 equiv of CuCl at -78 °C, 1,2-dialkoxycarbonyl-3,4,5,6-tetraethyl-1,2-dihydropyridazine derivatives were obtained. The structure of one of dihydropyridazine was also confirmed by X-ray analysis.