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The influence of heteroatom substitution on heterocyclic carbenes and their complexes

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Graham, David C.(David Carl) (2003) The influence of heteroatom substitution on heterocyclic carbenes and their complexes. PhD thesis, University of Tasmania.

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Abstract

This thesis describes the influence of heteroatom substitution on heterocyclic
carbenes and their corresponding complexes, using both density functional theory (DFT)
and synthetic methods.
The substitution of oxygen, sulfur or phosphorus for nitrogen in imidazol-2-
ylidenes gives rise to a set of ten heterocyclic carbenes that were investigated using DFT.
These replacements were found to have implications for both the structure and reactivity
of the free carbene, which extended to complexes bearing ligands of this type. The
balance between the σ-donation and π-withdrawal conferred by the heteroatoms onto the
carbene centre was found to be a key factor influencing the reactivity at the carbenic
carbon. Additional thermodynamic stability was observed for those carbenes that
exhibited extensive π-delocalisation (aromaticity). The self-dimerisation of heteroatom-substituted
carbenes is accelerated in the presence of protons. The activation barrier for
dimerisation shows little correlation with the enthalpy of reaction, indicating that the
estimation of carbene stability based purely on the enthalpy of reaction is not always
appropriate.
A number of novel palladium complexes bearing thiazol-2-ylidene ligands were
prepared in the laboratory and their catalytic activity for Heck and Suzuki coupling
assessed. While the catalysts were moderately active for both reactions, their performance was impeded by their insoluble nature and the lack of a second exocyclic
ring substituent.
DFT studies on the oxidative addition of imidazolium, thiazolium and oxazolium
salts to zerovalent group-10 metals indicated that activation of C-C bonds resulting in the
generation of hydrocarbyl-carbene complexes is likely to be achieved experimentally.
Thiazolium salts and azoliums bearing 2-phenyl substituents show increased barriers to
activation due to additional stabilizing interactions with the metal centre.

The engineering of hydrocarbyl palladium carbene complexes that show enhanced
stability to reductive elimination was undertaken using DFT. Complexes bearing
imidazole-based ligands showed enhanced stability over imidazoline, thiazole and
oxazole as a consequence of both their superior σ-donating ability and significant
occupation of the carbene px-orbital. Widening of the auxiliary ligand bite-angle
conferred a dramatic reduction in the barrier to reductive elimination, while the dihedral
twist of the carbene relative to the PdL2 plane had little effect. Incorporation of increased
steric bulk on nitrogen had little influence on the barrier to reductive elimination given its
location distant from the active-site. More important were the σ-donating and π-withdrawing
properties of the substituents, which conferred changes on the Pd-C bonding
interaction resulting in an influence on the reductive elimination behaviour.

Item Type: Thesis (PhD)
Keywords: Chemistry of heterocyclic compounds, Heterocyclic compounds, Heterocyclic chemistry
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Library has additional copy on CD-ROM. Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:08
Last Modified: 20 Dec 2016 00:07
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