N-heterocyclic carbene ligands in palladium and iridium organometallic chemistry
Abstract
The use of ligand in transition-metal catalysed reactions has had a considerable impact. The
present manuscript aims at showing the influence of ligands in the palladium catalysed Suzuki-Miyaura cross-coupling reaction. In chapter one, the mechanism of this reaction will be described
based on the numerous contribution published in the literature. It will be shown that the electronic
and steric properties of the ligands have a huge repercussion on the catalytic activity of the metal.
In the second chapter, the electronic and steric properties of the widely used Buchwald-phosphine ligand will be investigated. For this purpose, bis-carbonyl iridium(I) complexes were
synthesized and their characterization allowed determining their TEP (Tolman electronic parameter)
and their buried volume %V[subscript(bur)].
Then three next chapters of this thesis will focus on the syntheses and characterizations of
new palladium complexes bearing N-heterocyclic carbenes (NHC). Their design was made in a
view to obtain high activity in cross coupling reaction, particularly in the Suzuki-Miyaura cross
coupling.
Their activation under the catalytic conditions leads to the formation of palladium(0) species
that can be mono- or bis-ligated. The influence of the ligand on the catalyst activity will be
discussed. A palladium(II) precatalyst leading to mono-ligated active species will be described. Its
activity in cross-coupling is very good, since activated and non-activated aryl chlorides could be
coupled with aryl boronic acids at room temperature using low catalyst loadings. Unfortunately, the
catalyst activity decreased with temperature. This result showed the fragility of the mono-ligated
active species. In a view to obtain more robust catalysts that can perform high turnover numbers,
new palladium(II) precatalysts bearing two ancillary ligands were developed. Mixed systems NHC-
phosphites and NHC-phosphines are described. NHC-phosphites precatalysts displayed very good
activity, but the phosphites are unfortunately sensitive to reaction conditions, limiting their role of
active species shield. NHC-phosphine bearing complexes were highly active and could perform up
to 10,000 turnovers with unactivated aryl chlorides. Very interestingly, these catalysts were also
able to couple benzylchlorides and allyl chlorides with a wide range of boronic acids at very low
catalyst loadings. These reactions had also the great advantage to proceed in aqueous solvents at
very high substrate concentration.
The activation mechanism of these complexes was investigated. Dichloropalladium(II)
complexes were reduced under the catalytic conditions to lead palladium(0) species. Therein, it is
shown that this reduction step was rate-determining in catalysis. Some palladium(0) intermediates
xxiv
were synthesized and showed good to excellent activities at low temperature under the exact same
conditions. They displayed high reactivity towards oxygen and moisture and have to be handled
under inert atmosphere. A particular complex had the ability to react with molecular dioxygen to
form a stable peroxo-complex. Interestingly, this complex displayed excellent activity in water
under aerobic conditions. This system was of great advantage since the reaction could be set up
under air using cheap and user-friendly reagents displaying low toxicity. Moreover, the readily
available distilled water used as solvent did not require prior degassing.
Symmetrical and unsymmetrical bis-NHC palladium(0) complexes were successfully
synthesized. They display excellent activity in the Suzuki-Miyaura cross coupling and turnover
frequencies as high as 300 could be obtained at room temperature with unactivated arylchlorides
and arylboronic acids. These complexes were also found excellent catalysts for the coupling of
benzylchlorides with arylboronic acids. Mechanistic studies showed that no ligand dissociation
occurred during the coupling suggesting as bis-ligated active species.
Type
Thesis, PhD Doctor of Philosophy
Rights
Embargo Date: Print and electronic copy restricted until 16th November 2015. (Restriction now expired. Awaiting final permissions to release or further restrict full text.)
Embargo Reason: Thesis restricted in accordance with University regulations
Collections
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.