Harnessing Visible Light for the Development of Novel Synthetic Strategies

Identifier:  TDX:3376

Handle: https://hdl.handle.net/20.500.11797/TDX3376

Authors:  Mastandrea, Marco Michele

Abstract:
The main goal of this thesis is the development of new synthetic strategies harnessing visible light irradiation. Cooperative organo- and metal photoredox catalysis, as well as anion-pi complexes photochemistry, will be the main concepts exploited to explore new reaction pathways. After a general introduction, three research projects are collected in this thesis. The first research project, included in Chapter II, shows the development of a methodology for the asymmetric cross-dehydrogenative coupling of aldehydes and xanthenes. The key feature of this approach stands in the two-step process employed to oxidize xanthenes to their corresponding carbocations, subsequently trapped by an in situ formed enamine. The mildness and selectivity enabled by photoredox catalysis allows to reach high levels of stereocontrol, good yields and wide functional group tolerance. The second project, shown in Chapter III, illustrates a novel photoredox strategy for the synthesis of a wide range of allylic amines and ethers from carboxylic acids and alkynes. This approach relies on the activation of terminal alkynes through the photoexcitation of transiently formed copper acetylide intermediates. This process takes place through cooperative copper and organic photoredox catalysis and can be carried out in stereodivergent manner. The developed methodology has been applied to the stereoselective coupling of primary, secondary and tertiary alkyl radicals with (hetero)aromatic terminal alkynes. Finally, anion–pi interactions have been identified as the enabling step in the light-promoted amidation of aromatic systems. The available evidence indicates that an anion pi-complex between carbonate and an electron-poor aryloxy amide is elicited, facilitating the absorption of visible light. Upon irradiation, a spontaneous intracomplex electron transfer takes place, leading ultimately to the generation of amidyl radicals. These radicals have been efficiently trapped by electron-rich (hetero)arenes or in an intramolecular fashion, affording the corresponding amidated products.