MECHANISM AND STEREOCHEMISTRY OF THE ZIRCONOCENE-CATALYZED CYCLOMAGNESIATION OF DIENES

Kinetics, stoichiometric model reactions, and stereochemical studies h ave been employed to investigate the mechanism of the zirconocene-cata lyzed cyclization of nonconjugated dienes with organomagnesium reagent s. Transmetalation of a zirconacyclopentane intermediate with magnesiu m alkyls has been implicated as a key step in the catalytic reaction. A zirconacyclopentane derived from 9,9-diallylfluorene was prepared an d structurally characterized. Zirconium metallacyclopentanes transmeta late with dialkylmagnesium reagents having 8-hydrogens to form bis(mag nesiomethyl)-substituted carbocycles and products resulting from the e limination of alkane from dialkylzirconocene. Metallacycles of this ty pe also act as catalyst precursors for the catalytic cyclomagnesiation of 1,7-octadiene. The kinetics of the catalytic cyclomagnesiation of 9,9-diallylfluorene have been examined. The reaction obeys the followi ng rate law: rate = k(obs)[Zr][Bu(2)Mg] when [Bu(2)Mg] <0.35 M, and ra te = k(obs)[Zr] when [Bu(2)Mg] >0.35 M. The stereochemistry of the pro ducts depends upon the substrate, the magnesium reagent, the magnesium reagent concentration, and the temperature of the cyclization. The di astereoselectivity of the reaction depends on the relative rate of met allacycle formation, isomerization, and transmetalation. An understand ing of these relative rates allows for the rational control of the dia stereoselectivity to reflect either the kinetic selectivity of metalla cycle formation or the thermodynamic stabilites of the diastereomeric metallacycles.