Stephanie Grace Elizabeth Amos
Over the past twenty years, photochemical transformations have gained in importance in organic chemistry. Indeed, the development of photocatalysts has allowed the use of visible light as an energy source for chemical transformations. More specifically, photoredox chemistry has emerged as a valuable tool for the generation of free radical intermediates, enabling the organic chemist to envisage new bond disconnections and bond formations.For the organic chemist, the triple CïºC bond, or alkyne, is a versatile functional group. Alkynes are valuable building blocks for accessing more complex scaffolds. They also have multiple applications in medicinal chemistry, materials science, and chemical biology. Hence, the development of strategies that give access to alkynes is highly relevant. Due to the innate electronics of the alkyne, its transfer was initially limited to the alkynylation of electron-poor positions (electrophiles), however the development of alkyne transfer reagents have allowed the alkynylation of electron-rich positions (nucleophiles), and, more recently, the alkynylation of radicals. Specifically, ethynylbenziodoxolones (EBXs), hypervalent iodine (III) reagents bearing an alkyne, have frequently been used for the alkynylation of radicals. In combination with photochemical conditions they have enabled the activation of multiple functional groups (for example: carboxylates, potassium alkyl trifluoroborates salts, or α-oxo C-H bonds) for the introduction of an alkyne.The first objective of this research was to enable the photochemical difunctionalisation of double bonds to allow the introduction of an alkyne and a second functional group using EBXs. Indeed, difunctionalisation strategies are highly valuable as they can allow a rapid increase in molecular complexity in a single step. We found that enamides and enol ethers could undergo an alkynylative difunctionalisation with EBXs in presence of a photocatalyst and a hypervalent iodine additive. This method gave access to 1-alkynyl-1,2-aminoalcohols and diols scaffolds, which can be selectively deprotected to deliver the free alcohol or free amine.The second objective of this thesis was to develop a deoxyalkynylation strategy to allow the conversion of alcohols into alkynes. Starting from cesium oxalates, in presence of a photocatalyst and the EBX reagent, we could access a broad scope of aliphatic arylalkynes. Interestingly, the key to success for this project was found in the choice of light source: to ensure high yields, two high intensity blue LED lamps (440 nm, ca. 80 W) were required. During our mechanistic studies we discovered that this light source could also generate an excited state EBX species. We then decided to explore this discovery: we were delighted to find that the EBX alone under irradiation could promote the photomediated alkynylation of carboxylates, trifluoroborates, enamides, imines and α-oxo C-H bonds. Although the understanding of this process is still in the preliminary stages, we believe that the discovery of the direct photoexcitation of EBXs will enable facile reaction discovery and complement photocatalysed strategies.
EPFL2022
