ZIRCONOCENE-MEDIATED CYCLIZATION OF 2-BROMO ALPHA,OMEGA-DIENES

When it is first treated with n-butyllithium at -78 degrees C, zircono cene dichloride is an effective reagent for the cyclization of some 2- bromo alpha,omega-dienes at 25 degrees C. The products are carbo- and heterocyclic five-membered rings with adjacent methylene and methyl su bstituents. Reductive cyclization to form zirconacyclopentanes, follow ed by beta-bromo elimination, is indicated as the order of steps in th e reaction mechanism. Use of catalytic zirconocene dichloride with exc ess butylmagnesium chloride in diethyl ether at 25 degrees C is effect ive only for cyclization of 2-bromo-1,6-heptadiene.