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Title: Alkynylboronate cycloadditions towards aromatic boronic esters
Author: Kirkham, James David
ISNI:       0000 0004 2722 9147
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2011
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The [4+2] cycloaddition of 2-pyrones with phenyl, trimethylsilyl and n-butyl substituted alkynylboronates has been studied. In general, the highest yields for the cycloadditions were obtained using trimethylsilyl alkynylboronate. The highest regioselectivities were obtained using phenyl alkynylboronate, which provided a single regioisomer irrespective of the 2-pyrone used. Mechanistic studies suggest that the high regioselectivity observed is due to stabilisation of a zwitterionic species in the transition state. 2-Pyrone cycloadditions have been shown to be a viable route for the formation of the benzyne precursors, ortho-silylaryltriflates. By oxidation of the aromatic boronic ester products formed from cycloadditions, followed by sulfonylation of the resulting phenol species, a mild route has been developed for the formation of these highly synthetically useful intermediates. Aromatic difluoroboranes can be formed from the cycloaddition of 2-pyrones with in situ generated alkynyl difluoroboranes, at mild temperatures and in short reaction times by use of a directing group. Pyridines and amides have been incorporated into the 2-pyrone ring for this purpose, and high yields of the respective cycloadducts have been obtained with only one regioisomer formed in each case. Direct functionalisation of the aromatic difluoroboranes has been achieved using palladium catalysed cross-coupling, oxidation and azidonation reactions.
Supervisor: Harrity, Joseph Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available