Green Photochemistry: The synthesis of fine chemicals with sunlight

This research examines the dye-sensitised photooxygenation procedure as a case study to demonstrate ‘green’ photochemistry. There are three main streams to the research, namely the optimisation of juglone (5-hydroxy-1,4-naphthoquinone) synthesis under green conditions, the extension of the optimised procedure to a library of 1-naphthols and the investigation of photooxygenation reactions in alternative media. The dye-sensitised photooxygenation of 1,5-dihydroxynaphthalene to form juglone ([4+2]- cycloaddition reaction) was used as a model reaction to develop an entirely ‘green’ photochemical synthesis, taking into account all steps of the process. This approach looks at the reaction conditions, follow-up procedures and recycling and recovery of solvents and catalysts. Following optimisation, use of non-concentrated solar irradiation was demonstrated in Irish sunlight conditions at Dublin City University. Use of concentrated sunlight was investigated in collaboration with the German Aerospace Centre (DLR), Cologne. In all cases, solar irradiation gave yields greater than those obtained under artificial light conditions (26-70% and 19-47%, respectively). In the second part of this work, the optimised conditions from above were applied to photooxygenation of other 1-naphthol substrates. A library of 5-amido-1-naphthols were prepared and converted to the corresponding 1,4-naphthoquinones, giving yields of 12-72% for artificial light irradiation (4 hours) and 23-90% for solar synthesis (6 hours). A final study looked at synthesis of juglone using alternative reaction media such as solventfree (polymer supported) synthesis, ionic liquids, microemulsions and supercritical fluids. Of these methods microemulsions proved most effective, with isolated yields of up to 88% (4 hours)