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Permanent link (DOI): https://doi.org/10.7939/R3KS83

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Cobalt-mediated pentadienyl/alkyne [5+2] cycloaddition reactions Open Access

Descriptions

Other title
Subject/Keyword
hapticity
decomplexation
bicyclization
pentadienyl complex
cobalt-mediated
density functional theory
cycloaddition
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Ylijoki, Kai Erik Oskar
Supervisor and department
Stryker, Jeffrey M. (Chemistry)
Examining committee member and department
Green, James R. (Chemistry and Biochemistry)
Brown, Alexander (Chemistry)
West, Frederick G. (Chemistry)
Hall, Dennis G. (Chemistry)
Kuznicki, Steven M. (Chemical Engineering)
Department
Department of Chemistry
Specialization

Date accepted
2010-03-11T15:30:01Z
Graduation date
2010-06
Degree
Doctor of Philosophy
Degree level
Doctoral
Abstract
A new method for the preparation of seven-membered carbocycles via cobalt-mediated [5+2] cycloaddition methodology is presented. We have demonstrated that Cp*Co(η5-pentadienyl)+ systems undergo cycloaddition reactions with alkynes in a diastereocontrolled and high-yielding process. When acetylene is employed as the cycloaddition partner, unprecedented Cp*Co(η2,η3-cycloheptadienyl)+ complexes were isolated as the cycloaddition product under kinetic control. These allyl/olefin species were further transformed to the thermodynamic Cp*Co(η5-cycloheptadienyl)+ complexes. Also described are two methods for the preparation of high-valent Co(III) η5-pentadienyl complexes, a compound class that has been under-reported in the literature. This work fills this void and provides a valuable view of the structural properties of η5-pentadienyl complexes as a function of the substitution pattern. The incorporation of tethered pronucleophiles onto the pentadienyl ligand allowed the preparation of fused bicyclic structures of relevance to natural product synthesis. Both conjugated and unconjugated cycloheptadiene species were prepared, made possible via the differing cycloheptadienyl complex hapticity. The oxidative decomplexation of the organic products is also described. Initial steps towards a divergent pronucleophile-bearing pentadienyl synthesis were also undertaken. The mechanism and structure/reactivity relationships for the [5+2] cycloaddition process were studied via density functional theory calculations. These investigations revealed the existence of several convergent reaction pathways on the potential energy surface, and provided a new rationale for the η2,η3→η5 isomerization, thereby explaining the low activation barrier for the isomerization of 2-butyne cycloadducts. Of interest is the elucidation of a radical-type pathway, calculated to be of high energy for the Cp* ligand system, yet energetically competitive in the Cp complex reaction manifold. Further, computations on the Cp system demonstrate a potentially viable pathway on the triplet energy surface, suggesting spin-forbidden transitions may play a role in the mechanism. These observations provide an explanation for the differing cycloaddition efficiencies in these two pentadienyl systems. Calculations also suggest that reaction chemoselectivity is determined during the rate-limiting alkyne complexation step; the energetics of this process being dominated by steric interactions between the pentadienyl substituents and the ancillary ligand.
Language
English
DOI
doi:10.7939/R3KS83
Rights
License granted by Kai Ylijoki (kylijoki@ualberta.ca) on 2010-03-10T19:53:39Z (GMT): Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of the above terms. The author reserves all other publication and other rights in association with the copyright in the thesis, and except as herein provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
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