Insertion of H2CCHX (X = F, Cl, Br, OiPr) into (tBu3SiO)3TaH2and β-X-Elimination from (tBu3SiO)3HTaCH2CH2X (X = OR): Relevance to Ziegler−Natta Copolymerizations

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Abstract
The insertion of H2CCHX (X = OR; R = Me, Et, nPr, iPr, CHCH2, Ph) into (tBu3SiO)3TaH2 (1) afforded (tBu3SiO)3HTaCH2CH2X (2-CH2CH2X), which β-X-eliminated to give ethylene and (tBu3SiO)3HTaX (3-X). β-X-elimination rates were inversely proportional to the size of R. An X-ray crystallographic study of (tBu3SiO)3HTaCH2CH2OtBu (2-CH2CH2OtBu) revealed a distorted trigonal bipyramidal structure with an equatorial plane containing the hydride and a −CH2CH2OtBu ligand with a staggered disposition. erythro- and threo-(tBu3SiO)3HTaCHDCHDOEt (2-CHDCHDOEt) are staggered in solution, according to 1H NMR spectroscopic studies, and eliminated cis- and trans-HDCCHD, respectively, helping verify the four-centered transition state for β-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 H2CCHX with 1, and a discussion of the observations and their ramifications on the incorporation of functionalized monomers in Ziegler−Natta copolymerizations, are presented.