Olefin Insertion and β-Elimination Reactions of Permethylniobocene Olefin Hydride and Permethylscandocene Alkyl Complexes

Citation

Burger, Barbara J. (1987) Olefin Insertion and β-Elimination Reactions of Permethylniobocene Olefin Hydride and Permethylscandocene Alkyl Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BJEC-TH82. https://resolver.caltech.edu/CaltechETD:etd-01162008-111854

Abstract

Permethylniobocene styrene hydride complexes, Cp^*₂Nb(CH₂=CHC₆H₄-m-X)H (Cp^* = C₅Me₅, X = CH₃, NMe₂, CF₃) have been prepared and their rates of olefin insertion measured by coalescence techniques. The rates compare favorably with those measured for the analogous para-substituted complexes, suggesting that electronic effects in the transition state are largely inductive in nature. The crystal structure of Cp^*₂Nb(CH₂=CHC₆H₅)H 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 Cp^*₂ScR (R = CH₃, CH₂CH₃, CH₂CH₂CH₃ have been measured at low temperature by ¹³C NMR; the second order rate constants (M^-1 sec^-1, -80°C. The slow rate for the ScCH₂CH₃ complex is attributed to a ground state stabilization by a β-C-H interaction.

The β-hydrogen elimination rates for a series of permethylscandocene alkyl complexes Cp^*₂ScCH₂CH₂R (R = H, CH₃, CH₂CH₃, SiMe₃, C₆H₅, C₆H₄-p-CH₃, C₆H₄-p-CF₃, and C₆H₄-p-NMe₂) were measured by trapping kinetics. A transition state for the β-hydrogen elimination was proposed where there is a partial positive charge on the β-carbon and the hydrogen is transferred to the scandium center as H^-.

[Cp^*₂ScH] catalyses the ring opening of methylenecyclopropane and methylenecyclobutane. The ring opening (β-alkyl elimination) is reversible in the case of methylenecyclobutane. Addition of one equivalent of methylenecyclopentane to [Cp^*₂ScH] results in the formation of the scandium cyclopentylmethyl complex. This complex undergoes preferential β-alkyl (reversibly) over β-hydrogen elimination due to the unfavorable steric congestion encountered in the transition state of the latter.

[Cp^*₂ScH] reacts rapidly at -80°C with ethyl vinyl ether and vinylfluoride to generate an equimolar mixture of the Cp^*₂ScX(X = OEt, F, respectively) and Cp^*₂ScCH₂CH₃. No intermediates are seen in these reactions and two mechanisms, one involving a β-X ethyl intermediate and one invoking direct σ-bond metathesis are proposed to account for the products. With vinyldiphenylphosphine, the initial product of insertion, Cp^*₂ScCH₂CH₂PPh₂ is stable at low temperature; upon warming, this complex undergoes β-PPh₂ elimination.

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