Design and synthesis of ruthenium indenylidene-based catalysts for olefin metathesis
Abstract
As part of a European wide effort to develop metathesis catalysts for use in
fine chemical and pharmaceutical compound synthesis, this study focuses on the
design and synthesis of ruthenium based catalysts for olefin metathesis.
The aim, of this work was simple: to develop new, more active, more stable,
easy to synthesise and commercially viable Ruthenium based catalysts, as well
trying to rationalize the effect of structural changes on reactivity.
Two different approaches were explored in order to develop more active
catalysts bearing N-heterocyclic carbene (NHC) ligands: changing the leaving
group and the effect of the NHC moiety in indenylidene type complexes. Over 12
new catalysts were developed and their activity compared to that of commercially
available catalysts. Overall, the new complexes exhibited superior reactivity
compared to previously reported catalysts in several benchmark transformations.
However, olefin metathesis is a very substrate specific reaction, and rather than
finding one catalyst that is superior to all, a catalogue of catalysts suitable for
specific transformations was developed.
In addition, the effect of structural changes on substrate activity was
investigated in the ring closing metathesis of 1,8-nonadienes. The reaction
profiling showcased the presence of a gem-difluoro group as an accelerating group
in this incarnation of the olefin metathesis reaction and leads to ring formation
over polymerization.
In order to rationalize the effect of structural changes on catalyst activity,
kinetic studies dealing with the initiation mechanism of ruthenium-indenylidene
complexes were examined and compared with that of benzylidene counterparts. It
was discovered that not all indenylidene complexes followed the same mechanism,
highlighting the importance of steric and electronic properties of so-called
spectator ligands, and that there is no single mechanism for the ruthenium-based
olefin metathesis reaction. These results highlight the importance of systematic
development of catalysts and that as scientists we should not take for granted.
Type
Thesis, PhD Doctor of Philosophy
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