Visible light photocatalysis: mechanisms and reaction development

This thesis describes the development and mechanistic investigations into novel photochemical transformations mediated by visible light. 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. 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. 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.