Organic photochemistry: synthetic and computational studies
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I: In recent years a large number of synthetic methods have been developed which involve carbon radicals. Although many of these methods are synthetically very useful, they suffer from the significant disadvantage that the generation of the carbon radicals often involves the use of reagents which are based on toxic heavy metals such as tin, or reagents such as peroxides which are hazardous due to their thermal instability. An alternative method involves the use of a photomediator such as benzophenone, which on irradiation with UV light becomes electronically excited and abstracts a hydrogen atom from a carbon-hydrogen bond in a suitable substrate molecule, thus producing a carbon radical. These radicals are nucleophilic in character and react with electron deficient alkenes or alkynes producing a carbon-carbon bond. This thesis describes the intermolecular addition of photochemically generated carbon radicals to electron deficient C=N bonds, a process which provides a direct route to alpha-amino acid derivatives. Thus a series of oximes and oxime ethers were synthesized and irradiated in the presence of different hydrogen donor molecules, leading to a range of new alpha-monosubstituted and alpha,alpha-disubstituted alpha-amino acid derivatives. II: Cyclobutane rings are important as they are a common structural feature in a number of naturally occurring molecules. Due to the strain in the ring they can also be used as a starting point for many other transformations. The simplest method for cyclobutane formation remains the photochemical [2+2] cycloaddition reaction which involves 1,4-biradical intermediates in these reactions. For intramolecular reactions an empirical 'rule of five' has been used to account for the regiochemistry of these reactions. This rule proves accurate for a range of different systems but there are many exceptions. The biradical conformation control concept also provides a basis for predicting the regiochemical outcome of these reactions and has a more convincing theoretical basis which involves a consideration of the energy and structure of the possible 1,4-biradical intermediates. Triplet biradicals are sufficiently long-lived to allow conformational relaxation to occur. Once ISC occurs, closure or cleavage is very rapid and so further conformational change is unlikely. Low energy biradicals which have a high level of interaction between the singly occupied orbitals, as indicated in the spin density plot, are likely to be product forming; if however the biradical does not have a favorable orientation, a reversion to starting material is likely. Thus predicting the outcome of a cycloaddition reaction involves identifying which biradicals have these structural features. The possible 1,4-biradicals for a series of 6-allyl-cyclohex-2-en-1-ones and 1-allyl-naphthalen-2(H)-ones were investigated in an attempt to account for the regio- and stereochemical outcome of their intramolecular [2+2] cycloaddition reactions.
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