| Achoimre: | <p>This thesis describes the development and mechanistic investigations into novel
photochemical transformations mediated by visible light. </p>
<p>Chapter one introduces key concepts pertaining to excited state chemistry, mainly the
interaction between matter and light, fundamental photophysical events, and the subsequent
changes in physical and chemical properties that ensue upon excitation.</p>
<p>Chapter two provides an overview of recent developments into visible light mediated [2+2]
photocyclizations, with the emphasis being placed on photocyclizations of indoles. Although
the synthesis of cyclobutanes via photochemical [2+2] processes is highly facile, deviations
from standard behaviour do occasionally occur, leading to the formation of novel chemical
scaffolds. Here we describe the development of an interrupted [2+2]-photochemical process
that leads to the formation of highly functionalized heterocycles with the aid of visible-light
mediated energy transfer catalysis.</p>
<p>Chapter 3 describes mechanistic investigations into the regioselectivity of the visible light
mediated photocyclization of 2-N-arylamino-cyclohexenones bearing an ortho-iodo
substituent. These substrates undergo cyclization exclusively on the carbon bearing the iodine
substituent, which upon loss of the iodine atom, leads to the formation of iminium ions capable
of trapping numerous nucleophiles. The reactivity displayed by these substrates is completely
orthogonal to that of their 2-aryloxy analogues, which lead to the cyclization at the ipso-H
position. Here we investigate the mechanism of this transformation with the aid of quantum
chemical calculations.</p>
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