Insertion of H2C=CHX (X = F, Cl, Br, OiPr) into (tBU3SiO)3TaH2 and β-X-elimination from (tBu3SiO)3HTaCH2CH2X (X = OR):: Relevance to Ziegler-Natta copolymerizations

The insertion of H2C=CHX (X = OR; R = Me, Et, Pr-n,Pr-1, CH=CH2, Ph) into ((Bu3SiO)-Bu-t)(3)TaH2 (1) afforded ((Bu3SiO)-Bu-t)(3)HTaCH2CH2X (2-CH2CH2X), which beta -X-eliminated to give ethylene and ((Bu3SiO)-Bu-t)(3)- HTaX (3-X). beta -X-elimination rates were inversely proportional to the size of R. An X-ray crystallographic study of ((Bu3SiO)-Bu-t)(3)(HTaCH2CH2OBu)-Bu-t (2-(CH2CH2OBu)-Bu-t) revealed a distorted trigonal bipyramidal structure with an equatorial plane containing be hydride and a -(CH2CH2OBu)-Bu-t ligand with a staggered disposition. erythro and threo-((Bu3SiO)-Bu-t)(3)HTaCHDCHDOEt (2-CHDCHDOEt) are staggered in solution, according to H-1 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.