University, Diss., 2000--Tübingen.

Universiẗat, Diss., 2002--München.

Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliographical references.

Thesis (Ph.D.) - University of St Andrews, May 2008. / Restricted until 29th May 2010.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references. Also available in electronic version.

No description available.

No description available.

The design of catalysts for transition metal-mediated reactions is an active field in chemistry in the 21st century. It has been well recognised that ligands employed in these processes have a significant impact on the outcome of the reactions. The design and application of ligands in homogeneous catalysis reactions were the overarching aims of this project. Two areas were explored in this project, on the ligand synthesis side: 1. The use of directed ortho metallation (DoM) technology to synthesise unsymmetrical phosphines. 2. The synthesis of sterically hindered phosphines and their application to transition metal-mediated reactions. For the first sub-project, a range of N,N-dialkyl-diphenylphosphinic amides was synthesised in near quantitative yields. Deprotonation with s-BuLi and quenching the anion formed in the reaction with a range of electrophiles resulted in the desired ortho-substituted products in good yields. Differentiation of the second aromatic ring was possible, if the isolated mono-substituted products were used in further DoM reactions as starting materials. This methodology provided a range of highly substituted unsymmetrical phosphinic amides. Incorporation of other directing groups can change the selectivity of subsequent DoM reactions providing efficient routes to 2,2’ and 2,6-disubstituted products. Hydrolysis of the phosphinic amide moiety to the phosphinic acid with aqueous HCl, chlorination (SOCl2) and reaction of the acid chloride with a range of Grignard reagents provided an efficient method for the conversion of the phosphinic amide into a P-chirogenic but racemic phosphine oxide. Known methodology can convert this phosphine oxide into a phosphine that can be applied to transition metal catalysed reactions. This methodology provides an effective method for the synthesis of highly functionalised unsymmetrical phosphines. The approach facilitated substantial modifications to the ligand, which allows for efficient tailoring thereof for the metal-mediated reaction in which it is to be used. In the second sub-project, the DoM reaction was again employed but with (diaryl or dialkyl) phosphines as electrophilic quenches, resulting in the isolation of sterically hindered phosphines. Comparison between the ligands was made using the palladium-catalysed Suzuki reactions (catalytic approaches), Vaska type complexes (electronic effects) and phosphine selenium coupling constants (stereo-electronic effects). It was concluded that steric bulk and electronic characteristics affect the activity of catalysts formed from the ligands of this study, in line with the literature. The ligands were successfully applied to the palladium-catalysed Suzuki reaction using strongly deactivated aryl bromides and also some activated aryl chlorides as substrates at low (0.1% Pd) catalyst loadings. Significant improvements in catalytic activity were observed as the project progressed, using a structure-activity study as a guide. / Prof. D.B.G. Williams

In this study polar ligands that would be suitable for use in homogenous catalysis in aqueous-organic and/or ionic liquid systems were designed and synthesised. Initially an attempt was made to design a synthetic route that would incorporate an imidazolium into a phosphorus compound which would then allow the variation of the alkyl group on the imidazole or of the anion, thereby producing a series of ionic ligands with wide range of anticipated chemical and physical properties. This approach was partially successful: phosphines containing imidazolium moieties were synthesised by incorporating N-methyl imidazolium into bi- and tri-aryl phosphine compounds. As these ligands contain imidazolium residues, the retention of the catalyst in the ionic liquid to be used in the catalytic transformation would be enhanced. Water-soluble ligands that are suitable to aqueous-organic systems were also synthesised. The ligands were synthesised by first performing a Michael type reaction with diphenyl phosphine and a Michael acceptor, in this case, malonate esters. These were reduced and subsequently reacted with 1,4-butanesultone. The addition of butane sultone to these phosphorus compounds made them sufficiently water-soluble and therefore ideal for catalysis in aqueous biphasic or ionic solutions. Several water-soluble/ionic ligands were synthesised by incorporating dimethylphosphoryl chloride into di- and tri-aryl phosphine ligands in high yields followed by subsequent hydrolysis of the phosphonate groups. All the ligands produce were tested in transition metal-catalysed reactions, namely the Heck reaction, the Suzuki cross coupling and the Hydroformylation reaction. Test reactions were carried out under conditions selected to show the effectiveness of the ligands in the reaction, and they were performed in organic and aqueous-organic systems, as well as in ionic liquids for the Heck reaction. The results of these reactions were compared to the benchmark triphenylphosphine and it was found that the new ligands produced results that were similar to the triphenylphosphine but there were numerous instances where the new ligands produced better results. This study, therefore allowed for the synthesis for a range of polar phosphine ligands that would be suitable for homogenous catalysis using aqueous and/or ionic liquid systems, and these ligands were shown to influence transition metal-catalysed reactions. / Prof. D.B.G. Williams

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