alpha. agostic' assistance in Ziegler-Natta polymerization of olefins. Deuterium isotopic perturbation of stereochemistry indicating coordination of an. alpha. C-H bond in chain propagation

Journal Article · Wed Dec 05 00:00:00 EST 1990 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00181a060· OSTI ID:5890153

Piers, W E; Bercaw, J E ^[1]

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1. California Inst. of Tech., Pasadena (United States)

The well-defined, homogeneous Ziegler-Natta olefin polymerization systems that have been reported recently provide an unprecedented opportunity to investigate the mechanism of this important process. While a consensus appears to be developing that in all these systems the active catalysts are the 14-electron, d{sup 0} (or d{sup 0}f{sup n}) metallocene alkyls, Cp{sub 2}MR (M = lanthanide or group 3 transition metal) or (Cp{sub 2}MR){sup +} (M = group 4 transition metal), the mechanism for chain propagation and the geometry of the transition state for olefin insertion into the metal-carbon bond have not yet been unequivocally established. In a cleverly conceived experiment, Grubbs et al. probed for an {alpha} agostic interaction in the transition state for olefin insertion. Racemic 1-d{sub 1}-5-hexenylchlorotitanocene was prepared and found to undergo AlCl{sub 2}(CH{sub 2}CH{sub 3})-induced cyclization to a mixture of cis- and trans-2-d{sub 1}-cyclopentylmethyl stereoisomers. Any {alpha} agostic assistance in the insertion step is expected to favor the trans product (vide infra). Hydrolysis and {sup 2}H NMR analysis of the resultant mixture of deuteriomethylcyclopentanes revealed a 1.00 {plus minus} 0.05 ratio of trans:cis products, arguing against an {alpha} agostic assisted insertion in their system, however. The scandium hydride, {l brace}({eta}{sup 5}-C{sub 5}Me{sub 4}){sub 2}SiMe{sub 2}{r brace}Sc(PMe{sub 3})H ( OpSc(PMe{sub 3})H'), cleanly catalyzes the hydrocyclization of 1,5-hexadiene to methylcyclopentane. The authors have adapted this catalytic hydrocyclization reaction along the lines of the Grubbs experiment to probe for {alpha} agostic assistance with the scandium system.

• OSTI ID: 5890153
• Journal Information: Journal of the American Chemical Society; (United States), Vol. 112:25; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ALKENES
CHEMICAL REACTION KINETICS
POLYMERIZATION
DEUTERIUM
ISOTOPE EFFECTS
CHIRALITY
CYCLOPENTADIENE
HYDROGENATION
NMR SPECTRA
POLYENES
SCANDIUM COMPLEXES
ZIEGLER CATALYST
CATALYSTS
CHEMICAL REACTIONS
COMPLEXES
CYCLOALKENES
DIENES
HYDROCARBONS
HYDROGEN ISOTOPES
ISOTOPES
KINETICS
LIGHT NUCLEI
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
PARTICLE PROPERTIES
REACTION KINETICS
SPECTRA
STABLE ISOTOPES
TRANSITION ELEMENT COMPLEXES
400202* - Isotope Effects
Isotope Exchange
& Isotope Separation