Abstract
The large number of new heteronuclear bidentate ligands recently reported in the
literature has unveiled a new area of research, namely that the use of different
heteroatoms in bidentate ligands has led to new avenues towards more selective
processes especially in asymmetric transformations. Instead of employing the
traditionally used bidentate bisphosphine ligands, the dual objective of this study was
therefore to design and synthesise P-N based bidentate ligands, and to apply these in
catalytic transformations.
A general route towards the synthesis of different series of P-N based ligands was
followed. This path involved, as initial step, the condensation of o-diphenylphosphinobenzaldehyde
with a series of primary amines to afford a series of
iminophosphine ligands. Upon subsequent reduction of the iminophosphine ligands, a
series of secondary aminophosphine analogues was obtained, and upon alkylation of
the latter in the presence of a base, their tertiary aminophosphine derivatives were
isolated. In these three series of ligands, the oxidation state of the nitrogen atom, as
well as the degree of alkylation on this atom, was varied.
These ligands were further elaborated by the incorporation of a second phosphorus
atom to the already bidentate P-N ligands. This was achieved by reacting the
secondary aminophosphine ligands with a base and chloro-diphenylphosphine. In
order to vary the electronic nature of the newly introduced second phosphorus atom, a
method for the incorporation of a dicyclohexyl phosphine moiety was developed. By
introducing this group into the P-N backbone of these ligands, it was thus possible to
electronically distinguish between two different phosphorus atoms in one ligand.
Apart from the nature of the nitrogen and phosphorus atoms in these ligands, the use
of different R-groups derived from the primary amines, enabled further variance in the
inherent characteristics of these ligands: the variation in R-groups included aliphatic
moieties, aromatic groups, and groups that contained additional heteroatoms. In these
cases, the variation in R-groups thus also influenced the denticity of these ligands,
which could render them more versatile in catalytic applications.
In addition, a series of S-N based ligands, as well as some chiral P-N based ligands,
was synthesised by using the previously-developed protocol.
The oxygen sensitive nature of phosphines sometimes requires protection of these
phosphines during different synthetic steps, and borane is often used for this purpose.
However, deprotection of these phosphine ligands is required before they can be used
in metal-catalysed reactions. As an adjunct to the present synthetic strategy, the
concept of deprotecting phosphine-borane complexes such that new, extractable
borane complexes are formed, was investigated, and formed a small part of this
overall study. The deprotection of phosphine-boranes was performed using various
“new” deprotecting reagents, and the borane species were separated from the free
phosphines by using water / organic phase separations. The various extraction
coefficients of the amine-borane complexes were determined.
All the newly prepared heteronuclear ligands were used in a variety of catalytic
transformations, including reactions catalysed by palladium, chromium and rhodium.
The results obtained from these catalytic transformations indicated that most of these
ligands afforded highly active catalysts in the different applications, and results were,
in many cases, better than those obtained with commercially available ligands such as
triphenylphosphine and dppp.
To end off this study, an investigation towards the ó-donating properties of a selection
of phosphine ligands was performed by converting the phosphines into their
corresponding phosphine selenides, and calculating the 1J(77Se-31P) coupling
constants. From this study, it was found that the electronic nature of different
phosphine atoms in the same ligand could be characterised, and such information, in
turn, can be used for the design of new ligands for specific catalytic systems.
Prof. D.B.G. Williams