Synthetic, structural, and mechanistic studies of homogeneous Ziegler-Natta catalysis

Citation

Hajela, Sharad (1995) Synthetic, structural, and mechanistic studies of homogeneous Ziegler-Natta catalysis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9nxq-m253. https://resolver.caltech.edu/CaltechETD:etd-10052007-131102

Abstract

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The reaction of [...] with isobutene produces [...] along with isobutane, 2-methylpentane, isobutene, 2-methyl-l-pentene, propane and n-pentane. These products arise from a series of reactions involving olefin insertion, [...] and (faster) [...] elimination which proceeds until only the 2-methyl-l-alkenes [...] and the predominant organoscandium product [...] remain. A transient observed in the reaction sequence has been characterized as [...]. Slower [...] bond metathesis involving the methyl C-H bonds of [...] and the Sc-C bonds of the scandium alkyls accounts for the observation of saturated alkanes (2-methylalkanes [...], etc. ), normal alkanes [...] as well as a minor organoscandium product [...] in the product mixture. [...]-Ethyl migration is not observed for the closely related 2-ethyl-butyl derivative, [...], obtained from reaction of 2-ethyl-1-butene with [...].

The cyclopentylmethyl derivative, [...], slowly and cleanly decomposes in cyclohexane to give methylcyclopentane and a yellow crystalline precipitate. The [...] NMR spectrum of the yellow product is consistent with a compound containing one [...] and one [...], the latter arising via metallation of a [...] ligand (i.e. a "tuck-in" complex). The structure of the complex which crystallizes as the dimer, [...], when allowed to form slowly at room temperature is reported.

Reaction of the neutral ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Cn) with [...] in acetonitrile affords the novel trihalide complexes [...]. Subsequent alkylation with [...] in THF cleanly gives the corresponding trimethyl species [...]. Reactivity studies reveal that the metal carbon bonds of these 12 [...] complexes are remarkably unreactive toward insertion chemistry with typical unsaturated substrates such as olefins and acetylene. 2-butyne does, however, react with [...] by C-H activation to give a compound that is characterized as an equilibrium mixture of a major allenyl form with a minor propargyl component. In general, [...] is significantly more stable, but less reactive, than [...]. Activation of [...] with [...] or [...] results in partially characterized complexes which exhibit olefin polymerization chemistry. The crystal structure of [...] has been determined.

We have recently described the highly iso-specific Ziegler-Natta polymerization of [alpha]-olefins using the single component, [...] symmetric metallocene [...]. The steric bias exerted by the cyclopentadienyl substitution pattern allows for formation of only the desired racemic product upon metallation, with no detectable amount of the undesired meso isomer. A natural extension is the further development of methods for obtaining enantiomerically pure catalysts based on the [Bp] ligand system. Such single-component systems, in addition to serving as powerful mechanistic probes of Ziegler-Natta catalysis, would also be promising candidates for use in a variety of other catalytic asymmetric transformations. The most desirable ligand design would produce only one enantiomer upon coordination to the metal. Thus, not only would the wasteful and tedious separation of the meso isomer be avoided, but the subsequent resolution of the racemate would also be obviated. We have found that employment of chiral silyl-linkers is an effective design strategy for the synthesis of such enantiomerically pure complexes. Described herein are our efforts which have led to the synthesis of the ligand [...], designed to coordinate to give only one enantiomer, and BnBpY-H, the first example, as far as we are aware, of an enantiopure [...]-symmetric, group III metallocene which is competent for coordination polymerization of [alpha]-olefins.

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