CHEMOSELECTIVE FUNCTIONALIZATION OF ZIRCONACYCLOPENTENES

Zirconacyclopentenes, which were readily prepared by the reaction of Cp2ZrEt2 with alkynes or by the reaction of vinylsilane with alkynes in the presence of Cp2ZrBu2 (Negishi reagent), reacted with iodine to give either stereodefined alkenyl iodides or homoallylic iodides selectively after hydrolysis. The chemoselectivity of this reaction was strongly dependent on the substituent R group of the C2 carbon attached to zirconium. When R was a phenyl group, homoallylic iodides were selectively formed. On the other hand, alkyl substituted zirconacyclopentenes reacted with iodine to afford alkenyl iodides selectively. A small amount of diiodides were produced as by-products. Reactions of zirconacyclopentenes with an excess of MeOH and iodine in this order gave only alkenyl iodides with excellent selectivities. The formation of diiodides was not detected. This monohalogenation procedure using an excess of MeOH/I2 was not substituent dependent in the system used here. Treatment of alkylsubstituted zirconacyclopentenes with CBr4 or CCl3Br yielded only homoallylic bromides, after hydrolysis, with >99% chemoselectivity. It is in sharp contrast to the reaction with usual bromination reagents such as Br2 and NBS which led to the selective formation of alkenyl bromides. A sequential treatment of zirconacyclopentenes with CBr4 and I2 in this order, afforded a mixed dihalogenation product selectively. Reaction with Me3SnCl was not substituent dependent. The sp3 carbon attached to Zr selectively reacted with Me3SnCl to give homoallyltin compounds. Insertion reaction of isonitrile in the Zr-carbon bond of zirconacyclopentenes were chemoselective but neither substituent dependent nor reagent dependent in the system used here.