Olefin insertion and [Greek letter beta]-elimination reactions of permethylniobocene olefin hydride and permethylscandocene alkyl complexes

Citation

Burger, Barbara J. (1987) Olefin insertion and [Greek letter beta]-elimination reactions of permethylniobocene olefin hydride and permethylscandocene alkyl complexes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01162008-111854

Abstract

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Permethylniobocene styrene hydride complexes, [...] [...] have been prepared and their rates of olefin insertion measured by coalescence techniques. The rates compare favorably with those measured for the analogous pare-substituted complexes, suggesting that electronic effects in the transition state are largely inductive in nature. The crystal structure of [...] was determined. The phenyl ring of the styrene is twisted out of resonance with the olefin to avoid unfavorable steric interactions with the bulky Cp* rings.

The rates of ethylene insertion in the Sc-C bond for [...] have been measured at low temperature by [...] NMR; the second order rate constants [...]. The slow rate for the ScCH2CH3 complex is attributed to a ground state stabilization by a [...] interaction.

The [...]-hydrogen elimination rates for a series of permethylscandocene alkyl complexes [...] were measured by trapping kinetics. A transition state for the 8-hydrogen elimination was proposed where there is a partial positive charge on the [...]-carbon and the hydrogen is transferred to the scandium center as H.

[...] catalyses the ring opening of methylenecyclopropane and methylenecyclobutane. The ring opening [...] elimination) is reversible in the case of methylenecyclobutane. Addition of one equivalent of methylenecyclopentane to [...] results in the formation of the scandium cyclopentylmethyl complex. This complex undergoes preferential [...]-alkyl (reversibly) over [...]-hydrogen elimination due to the unfavorable steric congestion encountered in the transition state of the latter.

[...] reacts rapidly at -80°C with ethyl vinyl ether and vinylfluoride to generate an equimolar mixture of the [...](X = OEt, F, respectively) and [...]. No intermediates are seen in these reactions and two mechanisms, one involving a [...] ethyl intermediate and one invoking direct a-bond metathesis are proposed to account for the products. With vinyldiphenylphosphine, the initial product of insertion, [...] is stable at low temperature; upon warming, this complex undergoes [...] elimination.

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