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 04:00:00 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), Journal Name: Journal of the American Chemical Society; (United States) Vol. 112:25; ISSN 0002-7863; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

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