Insertion of H2C=CHX (X = F, Cl, Br, O(i)Pr) into (tBu3SiO)3TaH2 and beta-X-Elimination from (tBu3SiO)3HTaCH2CH2X (X = OR): relevance to Ziegler-Natta copolymerizations.
The insertion of H2C=CHX (X = OR; R = Me, Et, nPr, (i)Pr, CH=CH2, Ph) into (tBu3SiO)3TaH2 (1) afforded (tBu3SiO)3HTaCH2CH2X (2-CH2CH2X), which beta-X-eliminated to give ethylene and (tBu3SiO)3HTaX (3-X). beta-X-elimination rates were inversely proportional to the size of R. An X-ray crystallographic study of (tBu3SiO)3HTaCH2CH2O(t)Bu (2-CH2CH2O(t)Bu) revealed a distorted trigonal bipyramidal structure with an equatorial plane containing the hydride and a -CH2CH2O(t)Bu ligand with a staggered disposition. erythro- and threo-(tBu3SiO)3HTaCHDCHDOEt (2-CHDCHDOEt) are staggered in solution, according to (1)H NMR spectroscopic studies, and eliminated cis- and trans-HDC=CHD, respectively, helping verify the four-centered transition state for beta-OEt-elimination. When X = F, Cl, or Br, 2-CH2CH2X was not observed en route to 3-X, signifying that olefin insertion was rate-determining. Insertion rates suggested that substantial positive charge on the substituted carbon was incurred. The reactivity of other H2C=CHX with 1, and a discussion of the observations and their ramifications on the incorporation of functionalized monomers in Ziegler-Natta copolymerizations, are presented.
Strazisar
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Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
Wolczanski
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