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|Title: ||Tandem radical reactions involving cyclisations onto nitriles|
|Authors: ||Brookes, Phillip|
|Issue Date: ||2000|
|Publisher: ||© Phillip Brookes|
|Abstract: ||Chapter 1 is the introduction to the thesis. The general principles of radical cyclisation
reactions are highlighted with a focus upon the reactivity of iminyl radicals. A more detailed
discussion follows on radical cyclisations onto nitriles including examples of tandem
cyclisations. The final section is concerned with cyano migration reactions, and provides
evidence for the reversibility of these translocations.
The investigation into tandem radical cyclisations of nitriles is discussed in Chapter 2.
The aim of the project was to form bicyclic nitrogen heterocycles from acyclic precursors by
utilising the nitrile function as a radical acceptor which could then undergo further cyclisation
onto a suitably placed alkene. We found a surprising chemoselectivity for 1,5-exo-cyclisation of
alkyl, aryl and vinyl radicals onto the cyano group over 1,6-exo-cyclisation onto suitably placed
alkenes. The presence of an electron-withdrawing group on the carbon a- to the nitrile group
resulted in an overalll,4-cyano migration reaction. The intermediate cyclic iminyl radical does
not undergo further cyclisation, nor does it abstract hydrogen from tributy ltin hydride. Instead,
fragmentation by β-scission yields a stabilised radical, e.g. by an ester or nitrile group.
In order to investigate the effect of substituents on the cyclisation of aryl radicals onto
nitriles, and the β-scission reactions of the iminyl radical intermediates, a series of aryl radical
precursors were prepared. a-Nitrile, amide, sulfone and phenyl groups favoured β-scission and
a-alkyl groups favoured cyclisation or reduction of the aryl radicals. The study indicated the
existence of a Thorpe-Ingold effect on the cyclisation of aryl radicals onto nitriles.
In Chapter 3 the largely unsuccessful studiesof the reversibility of radical cyclisations
onto nitriles is reported.
Finally, in Chapter 4, the results obtained from our research into vinyl radicals as
precursors for tandem radical cyclisations or" nitriles are presented. A vinyl iodide underwent
complete conversion when standard radical cyclisation conditions were applied; the cyclic ketone
resulting from a single 5-exo cyclisation was isolated in the absence of any other products.
The experimental relevant to the discussion is detailed in Chapter 5.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Chemistry)|
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