CHEMOSELECTIVE FUNCTIONALIZATION OF ZIRCONACYCLOPENTENES

Zirconacyclopentenes, which were readily prepared by the reaction of C p2ZrEt2 with alkynes or by the reaction of vinylsilane with alkynes in the presence of Cp2ZrBu2 (Negishi reagent), reacted with iodine to gi ve either stereodefined alkenyl iodides or homoallylic iodides selecti vely after hydrolysis. The chemoselectivity of this reaction was stron gly dependent on the substituent R group of the C2 carbon attached to zirconium. When R was a phenyl group, homoallylic iodides were selecti vely formed. On the other hand, alkyl substituted zirconacyclopentenes reacted with iodine to afford alkenyl iodides selectively. A small am ount of diiodides were produced as by-products. Reactions of zirconacy clopentenes with an excess of MeOH and iodine in this order gave only alkenyl iodides with excellent selectivities. The formation of diiodid es was not detected. This monohalogenation procedure using an excess o f MeOH/I2 was not substituent dependent in the system used here. Treat ment of alkylsubstituted zirconacyclopentenes with CBr4 or CCl3Br yiel ded only homoallylic bromides, after hydrolysis, with >99% chemoselect ivity. It is in sharp contrast to the reaction with usual bromination reagents such as Br2 and NBS which led to the selective formation of a lkenyl bromides. A sequential treatment of zirconacyclopentenes with C Br4 and I2 in this order, afforded a mixed dihalogenation product sele ctively. Reaction with Me3SnCl was not substituent dependent. The sp3 carbon attached to Zr selectively reacted with Me3SnCl to give homoall yltin compounds. Insertion reaction of isonitrile in the Zr-carbon bon d of zirconacyclopentenes were chemoselective but neither substituent dependent nor reagent dependent in the system used here.