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Title: Divergent rhodium catalysed cycloisomerisation of substituted 1,6-dienes : determination of the factors controlling the reaction pathways
Author: Pin-Nó, M.
ISNI:       0000 0004 6425 1642
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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The cycloisomerisation of dienes is of high interest as it involves the formal rearrangement of hydrogen atoms and it presents very good atom economy. Unfortunately, the diastereoselectivity of this process has hitherto not been successfully controlled. Herein, the first example of highly diastereoselective rhodium-catalysed cycloisomerisation of substituted 1,6-dienes triggered by C-H activation is described and an overview of the metal-catalysed cycloisomerisation of 1,6-dienes is depicted. A cationic rhodium catalyst cycloisomerises a 1,6-octadiene bearing a vinylpyridine moiety to a mixture of isomersin a highly diastereoselective fashion. The mechanism of the reaction, which was elucidated by the results of the D-labelling experiments, is examined. It is observed that the product selectivity can be improved by controlling the competition between the second migratory insertion and the 1,2- insertion and the competition between the reductive elimination and the ß-hydride elimination respectively. The control of the reductive elimination versus the ß-hydride elimination has been achieved after an extensive phosphine ligand screening. The use of large phosphines as ligands favours the reductive elimination, while small phosphines promote the formation of the ß-hydride elimination product. A careful monitoring of the reaction shows the transient character of the ß-hydride elimination product which can get converted into the reductive elimination product under specific conditions. The enhancement of the second migratory insertion is examined by modifying the steric and the electronic characteristics of the substrate in different positions and, overall, the tendency observed previously in the cycloisomerisation of the benchmark substrate is maintained. The stepwise syntheses of the fourteen tested substrates, and the unambiguous identification of the products, are also reported.
Supervisor: Aissa, C. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral