Synthesis, characterization and reactivity of some permethyltantalocene alkylidenes and unusually stable metallaoxetanes

Citation

Whinnery, Leroy L. (1991) Synthesis, characterization and reactivity of some permethyltantalocene alkylidenes and unusually stable metallaoxetanes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s4x8-zb56. https://resolver.caltech.edu/CaltechTHESIS:04112011-130949006

Abstract

Several tantalaoxetanes have been prepared and the X-ray crystal structure of O-exo-Cp_2*Ta(OCHPhCH_2)CH_3 is reported (Cp* = η^5-C_5Me_5). The kinetic products of the reactions of Cp_2*Ta(=CH_2)CH_3 with paraformaldehyde or benzaldehyde are O-endo-Cp_2*Ta(CH_2CH_20)CH_3 and O-endo-Cp_2*Ta(CH_2CHPhO)CH_3, respectively. These tantalaoxetanes undergo an acid and base-catalyzed isomerization to O-exo-Cp_2*Ta(OCH_2CH_2)CH_3 and O-exo-CP_2*Ta(OCHPhCH_2)CH_3 followed by thermal decomposition to Cp_2*Ta(=O)CH_3 and the appropriate olefin. Cp_2*Ta(=CH_2)H deoxygenates epoxides to form Cp_2*Ta(=O)CH_3 and olefin. No intermediates are observed in this deoxygenation under conditions where the appropriate tantalaoxetanes are stable and would have been spectroscopically observed. Stereolabeled epoxides were deoxygenated to probe the mechanism for the possible intermediacy of a 1,4-biradical. Retention of stereochemistry of the resulting olefin was observed and is indicative of a concerted mechanism. These results and their implications for the mechanism of olefin epoxidation are discussed. Treatment of Cp*_2 TaCI_2 with a variety of substituted benzyl potassium reagents affords an equilibrium mixture of CP*_2 Ta(=CHC_6H_5)H, 1, and Cp*_2 Ta(o-CH_2C_6H_4)H, 2, which interconvert presumably via the unstable 16 electron intermediate [Cp*_2Ta(CH_2C_6H_5)]. Several derivatives substituted at the phenyl ring have been prepared to explore the effect of both sterics and electronics on the a-hydrogen migration equilibrium. Trapping of the benzyl intermediate by methylenetrialkylphosphoranes results in methylene transfer to give Cp*_2 Ta(=CH_2)CH_2C_6H_5. The substituted benzyl derivatives have provided a system to determine the influence of phenyl sUbstituents on migratory aptitude of the benzyl group.

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