Lau, Raymond; (2000) Studies in enantioselective catalysis using a novel D-mannitol based titanium trimer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The work presented in this thesis is divided into three parts. This research entails the discovery and development of a novel type of chiral titanium alkoxide, which acts as a Lewis acid for the reduction of prochiral ketones using catecholborane as the stoichiometric reductant. In order to place this methodology in context, the first part is an overview highlighting the usefulness of such transition metal alkoxides in asymmetric organic synthesis. The review mainly revolves around the use of titanium and lanthanide alkoxides. The second part is a discussion of the results obtained. It was discovered that reversible exchange reactions of a series of 3,4-di-O-benzyl-D-mannitol derivatives with titanium(IV) isopropoxide lead to the isolation of a novel trimeric Lewis acid complex which acts as an efficient precursor catalyst for the reduction of a range of prochiral ketones using catecholborane as reducing agent. The behaviour of this Lewis acid as a catalyst in a range of reactions was studied and the mechanism of the borane reduction was shown to involve boron-titanium exchange. The trimeric titanium alkoxide Lewis acid have also been used in a series of other asymmetric reactions and the results were a catalogue of failures. Other ligands derived from D-mannitol have been prepared and used for complexation with both titanium and lanthanum, but the structures of these complexes have so far not been characterised. Testing these unknown complexes on a number of asymmetric reactions proved to be unsuccessful. The third part is an account of the experimental results and procedures employed throughout this work.

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