COMPARATIVE PHOTO-INDUCED OXIDATIVE ADDITION OF B-H, C-H, Si-H AND B-B BONDS

A knowledge of reaction mechanisms is what enables chemists to predict the outcome of new reactions and work out how to make new compounds most effectively. Transition-metal compounds containing metals such as rhodium and ruthenium are used extensively as catalysts for organic and industrial processes. Oxidative addition reactions form a well-known class of reactions of such catalysts. There are two major developments in oxidative addition reactions that this project addresses: (1) the development of catalytic reactions of the simplest hydrocarbons (e.g. benzene or pentane) with boron compounds that yield very useful products for further reactions (2) the realisation that oxidative addition reactions sometimes proceed via non-classical intermediates called sigma complexes ; for instance, a silane may bind to the metal without breaking the silicon-hydrogen bond in the intermediate sigma-complex, although it is completely broken in the product. Firstly, in proof of principle experiments, we have demonstrated that we can make rhodium compounds that contain right-handed ligands. We can then obtain oxidative addition products that have metal centres that are either right-handed or left-handed and can monitor exchange of these two isomers. This exchange process tells us about the existence of intermediates (or transition states) with a mirror plane. With the help of theory and experiment we can infer the existence of sigma complexes. Secondly, this proposal addresses the question of what is special about boron by measuring the rates of reaction of carefully selected transition metal compounds with the boron compounds using a short light pulse to initiate the reactions. The results will be interpreted with the aid of theoretical calculations carried out in collaboration with Eisenstein, a well-known theorist. Thirdly, we will use NMR spectroscopy to determine the distribution of products when different combinations of reagents are in competition, for instance allowing silicon and boron compounds to compete for the same transition metal species. Finally, we will investigate transition metal complexes that are claimed to be suitable for two oxidative addition reactions in succession. These precursor complexes react on irradiation of light and it would be especially interesting for applications if they can be initiated with visible light rather than ultra-violet light. Overall, this study will give us a detailed picture of the relationships between different types of oxidative addition reactions that can be fed into catalyst design.