Cobalt(III)-Mediated Cycloalkenyl-Alkyne Cycloaddition and Cycloexpansion Reactions

• Author / Creator

  Chan, Bryan Chi Kit

• A comprehensive investigation of cycloalkenyl-alkyne coupling reactions mediated by cobalt(III) templates is presented. The in situ derived cationic η3-cyclohexenyl complexes of cobalt(III) react with some terminal alkynes to afford either η1,η4-bicyclo[4.3.1]decadienyl or η2,η3-vinylcyclohexenyl products, depending on the type and concentration of the alkyne. The mechanism for this cyclohexenyl-alkyne cycloaddition reaction is consistent with the previously reported cobalt-mediated [3 + 2 + 2] allyl-alkyne coupling reaction.
  The carbon-carbon bond activation/cyclopentenyl-alkyne ring expansion process was also studied using the 1,3-di-tert-butylcyclopentadienyl cobalt system. A modified synthetic strategy to the requisite half-sandwich cobalt(I) cyclopentadiene precursor was developed using cobalt(II) acetoacetonate, avoiding the use of simple cobalt(II) halides which are prone to ligand disproportionation. Furthermore, the preparation of the cobalt(III) cyclopentenyl precursor, (t-Bu2C5H3)Co(η4-C5H6), was accomplished via hydride addition to the easily prepared cobalticenium complex [(t-Bu2C5H3)Co(C5H5)]BF4 and avoids the use of the thermally sensitive (t-Bu2C5H3)Co(ethylene)2.
  Disubstituted alkynes such as 2-butyne or diphenylacetylene undergo cyclopentenyl coupling to afford the corresponding η5-cycloheptadienyl products, albeit in lower yields compared to the pentamethylcyclopentadienyl cobalt system. The 1,3-di-tert-butylcyclopentadienyl ancillary ligand shows unique and unusual reactivity, coupling with tert-butylacetylene to afford a novel spiro[4.5]decatrienyl complex. Ultimately, the poor isolated yields of seven-membered products demonstrate that the disubstituted cyclopentadienyl ligand system is a poor candidate for future studies in this area.
  A mechanistic investigation of the cobalt-mediated carbon-carbon bond activation process was performed. Cationic cobalt η2-vinyl complexes were proposed as viable intermediates in the activation process and synthetic routes to these compounds were examined. However, the resulting vinyl complexes were unstable and could not be directly isolated and characterized. Preparation of cobalt vinyl complexes in the presence of cycloalkadienes did not furnish the expected cycloexpanded products, suggesting alternative routes to the cobalt vinyl intermediates are necessary. During the course of the mechanistic investigation, a high-yielding alternative synthetic procedure for (C5Me5)Co(η4-butadiene) from the easily prepared precursor, [(C5Me5)CoI2]n, was found, circumventing the use of the thermally sensitive (C5Me5)Co(ethylene)2.

• Subjects / Keywords

  • Cycloaddition
  • Cycloalkenyl
  • Cycloexpansion

• Graduation date

  Spring 2010

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R30T6D

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.