TY - JOUR
T1 - Mechanism and Stereochemistry of the Zirconocene-Catalyzed Cyclomagnesiation of Dienes
AU - Knight, Kyle S.
AU - Wang, David
AU - Waymouth, Robert M.
AU - Ziller, Joseph
PY - 1994/9/1
Y1 - 1994/9/1
N2 - Kinetics, stoichiometric model reactions, and stereochemical studies have been employed to investigate the mechanism of the zirconocene-catalyzed cyclization of nonconjugated dienes with organomagnesium reagents. Transmetalation of a zirconacyclopentane intermediate with magnesium alkyls has been implicated as a key step in the catalytic reaction. A zirconacyclopentane derived from 9, 9-diallylfluorene was prepared and structurally characterized. Zirconium metallacyclopentanes transmetalate with dialkylmagnesium reagents having β-hydrogens to form bis-(magnesiomethyl)-substituted carbocycles and products resulting from the elimination of alkane from dialkylzirconocene. Metallacycles of this type 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 following rate law: rate = Kobs[Zr][Bu2Mg] when [Bu2Mg] < 0.35 M, and rate = Kobs[Zr] when [Bu2Mg] > 0.35 M. The stereochemistry of the products depends upon the substrate, the magnesium reagent, the magnesium reagent concentration, and the temperature of the cyclization. The diastereoselectivity of the reaction depends on the relative rate of metallacycle formation, isomerization, and transmetalation. An understanding of these relative rates allows for the rational control of the diastereoselectivity to reflect either the kinetic selectivity of metallacycle formation or the thermodynamic stabilites of the diastereomeric metallacycles.
AB - Kinetics, stoichiometric model reactions, and stereochemical studies have been employed to investigate the mechanism of the zirconocene-catalyzed cyclization of nonconjugated dienes with organomagnesium reagents. Transmetalation of a zirconacyclopentane intermediate with magnesium alkyls has been implicated as a key step in the catalytic reaction. A zirconacyclopentane derived from 9, 9-diallylfluorene was prepared and structurally characterized. Zirconium metallacyclopentanes transmetalate with dialkylmagnesium reagents having β-hydrogens to form bis-(magnesiomethyl)-substituted carbocycles and products resulting from the elimination of alkane from dialkylzirconocene. Metallacycles of this type 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 following rate law: rate = Kobs[Zr][Bu2Mg] when [Bu2Mg] < 0.35 M, and rate = Kobs[Zr] when [Bu2Mg] > 0.35 M. The stereochemistry of the products depends upon the substrate, the magnesium reagent, the magnesium reagent concentration, and the temperature of the cyclization. The diastereoselectivity of the reaction depends on the relative rate of metallacycle formation, isomerization, and transmetalation. An understanding of these relative rates allows for the rational control of the diastereoselectivity to reflect either the kinetic selectivity of metallacycle formation or the thermodynamic stabilites of the diastereomeric metallacycles.
UR - http://www.scopus.com/inward/record.url?scp=0001195578&partnerID=8YFLogxK
U2 - 10.1021/ja00084a028
DO - 10.1021/ja00084a028
M3 - Article
AN - SCOPUS:0001195578
VL - 116
SP - 1845
EP - 1854
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 5
ER -