Metal salen catalysts for reactions of epoxides and heterocumulene

Abstract

This thesis details the research carried out into the reaction of epoxides with various heterocumulenes, forming five-membered heterocyclic products. The catalyst systems used are numerous forms of metal salen compounds. It begins with the introduction, which summarises the body of literature outlining the reactions of epoxides with heterocumulenes. Previously developed catalytic systems are discussed along with any mechanistic studies that have taken place. A conclusion regarding the most active systems for each reaction is given, which will provide an understanding of the requirements needed to be a successful catalyst. The results and discussion section is divided into four main chapters. It begins with a study into forming novel one-component catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Four new catalysts are conceived and tested on this reaction, but with very limited success due to the level of steric hindrance present in the backbone of the salen ligands. The catalysts that are successfully synthesised are then examined for other reactions for which they may be suited, namely the synthesis of thiocarbonates from epoxides and carbon disulphide and asymmetric cyanohydrin synthesis. Both reactions gave mixed results. The activity exhibited for thiocarbonate synthesis ranged from 0-100 % conversion, while the asymmetric induction of the cyanohydrin products was disappointing (18-79 % ee). Both proving to be less active than previously devised salen-based systems. The next chapter describes a mechanistic exploration of previously developed monometallic and bimetallic catalyst systems for the reaction of epoxides with isocyanates. The experiments result in a proposed mechanism for this reaction which is ii distinct from those proposed for similar reactions of other heterocumulenes. This work is followed by variable temperature kinetic studies which aimed to give a comparison of the activation parameters of the reactions of epoxides with carbon dioxide, carbon disulphide and isocyanates catalysed by aluminium salen complexes to explain the relative reactivity of each heterocumulene. The results found that the reactions of isocyanates and carbon disulphide had relatively high enthalpies of activation and therefore the catalyst systems could potentially be improved by using metals with more Lewis acidic character. The last two chapters of the results and discussion cover the development of new metal salen catalyst systems for both the synthesis of oxazolidinones and thiocarbonates from epoxides by reaction with isocyanates and carbon disulphide respectively. The development of these systems was based on the results obtained from the variable temperature kinetics studies carried out in the previous chapter. The results show that new, highly active catalysts for these transformations were developed by varying the metal at the centre of the salen ligand. Use of these systems allowed catalyst loading to be decreased, in some cases by up to a factor of ten (from 5 mol% to 0.5 mol%). Kinetic studies are also included which provided the information required to propose mechanisms for the reactions. The thesis is completed with the experimental section, which provides details of all procedures (both synthetic and kinetic), along with all physical and spectral data from isolated compounds. This is then followed by an appendix of all kinetic data collected.