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|Title: ||Pd catalysed synthesis of phosphines for homogeneous catalysis|
|Authors: ||Damian, Karen Serena|
|Supervisors: ||Clarke, Matt|
|Issue Date: ||2009|
|Abstract: ||The synthesis of ligands has been identified as the limitation for wider application of catalytic
asymmetric synthesis. Indeed, synthesis of phosphorus-based ligands, has been often shown to be
challenging and not always efficient. It has also been observed that subtle changes in the ligand
structure can lead to big differences in the catalytic activity of the ligand when coordinated to a
metal. Therefore, it was considered useful to develop a methodology in order to obtain a library of
The first chapter of the thesis is a review of recent development in catalytic phosphine synthesis.
In the second chapter of this thesis, the microwave mediated Suzuki cross coupling reaction has
been investigated. In particular, attention has been focussed on the coupling of different arylboronic
acids to chloroarylphosphine oxides, which are, in general, considered challenging coupling
partners for this type of reaction. The reaction conditions have been optimised starting from the
coupling of phenylboronic acid to tris(4-chlorophenyl)phosphine oxide. Different solvents, bases,
and catalysts have been then tested and the better conditions have been developed for this substrate.
Indeed, it was shown that the coupling occurs in only 30 minutes at 140°C, leading to reasonably
high yields. These conditions were then applied to two other different chloroarylphosphine oxides
with a range of boronic acids, with the aim to verify if the optimised reaction conditions could be
applied to other substrates and it was noticed that good yields could be attained. This methodology
led us to obtain a library of phosphine oxides. It was then decided to investigate the reduction of
phosphine oxides under microwave irradiation. This reaction occurs under conventional heating but
it can take several hours. It was observed that reaction times could be importantly reduced when
reducing some phosphine oxides under microwave heating. It was found that some phosphine
oxides are reduced rapidly under conventional conditions but for more difficult substrates to reduce
there are significant advantage to microwave method.
We decided to investigate the microwave mediated P-C bond forming reaction, with the aim to
rapidly synthesise a library of phosphines cleanly. The conditions were optimised at first using o-
trifluoromethylbromobenzene as the substrate. Once the appropriate reaction conditions and catalyst
were identified, the reaction was run on other substrates to verify that this could be a general
methodology for the synthesis of phosphines. It was found that it is indeed a general method for the
synthesis of monophosphines. However, the synthesis of diphosphines with the microwave assisted
P-C bond forming reaction on dibromo- and diiodo- aryl compounds proved to be very challenging.
The fourth chapter presents different attempts for the synthesis of the new ligand Ph-DuPHOS. The
synthesis of this ligand was considered interesting because of the previous results of other phospholane-based ligands, such as Ph-BPE and Me-DuPHOS. However, the synthesis of this
ligand has proven to be elusive. A monodentate P-N phospholane-based ligand was prepared and its
catalytic activity was tested in the rhodium catalysed hydrogenation of alkenes. Moreover, a
bidentate (1,2-bisphospholano)xylene ligand was also prepared and its catalytic activity was also
tested in the rhodium catalysed hydrogenation of alkenes.
This latter ligand was also used in the hydroxycarbonylation of styrene, since for this reaction bulky
diphosphines are required to give branched selectivity. In hydroxycarbonylation it is very rare to
give good branched selectivity and there were no examples of substantial enantioselectivity prior to
this work. The high regioselectivity and moderate e.e.’s observed suggest promise for future
studies. Finally, mechanistic studies on the hydroxycarbonylation of styrene have been carried out
in order to investigate the intermediates involved in the reaction as well as the role of the promoters.
The possibility of (1-chloroethyl)benzene was proposed as the active intermediate of the reaction.
Our results have disproved this possibility, suggesting that the reaction is likely to proceed through
the hydride mechanism.|
|Publisher: ||University of St Andrews|
|Appears in Collections:||Chemistry Theses|
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