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Photocatalytic Generation of CarbAnions for Organic Synthesis

Periodic Reporting for period 2 - PHAROS (Photocatalytic Generation of CarbAnions for Organic Synthesis)

Reporting period: 2019-03-01 to 2020-08-31

Visible light is a sustainable source of energy. The conversion of visible light into electrical energy is already highly developed and widely applied, but the direct use of visible light to drive reactions in chemical synthesis was only recently rediscovered as a technology. The principle of photocatalysis resembles the process of biological photosynthesis, but in a much simpler technical way and directed to organic synthesis. Over the last decade, chemical photocatalysis for organic synthesis developed rapidly and the method can now be considered as an enabling technology for the synthesis of pharmaceuticals or performance chemicals. However, the full potential of light driven synthetic chemistry has not been realized. Visible light is not a dense form of energy and its use imposes an energy limit to the method, which needs to be overcome by accumulation of the energy of several photons. In addition, most known photocatalytic reactions proceed via mechanisms involving radicals, which are chemical reactive intermediates with unpaired electrons. However, the space of chemical reaction pathways is much larger. Innovative and new techniques must be developed to make chemical photocatalysis applicable to all areas of synthetic organic chemistry allowing an even broader use of the method in academic and industrial laboratories in research, in development and in chemical production.
Clean and efficient chemical technologies are essential for chemical production and the synthesis of drugs. All future technologies should be safe, efficient and effective. Chemical photocatalysis complies with all of these prerequisites and is therefore an ideal candidate for the next generation of a more sustainable chemical synthesis. Innovative technologies are the essential core to sustain economic, environmental and societal development.
To pave the way for a broader use of chemical photocatalysis as synthesis tool and in industrial applications the limits of the method should be extended as far as possible and a solid understanding of underlying concepts must be developed, which are the goals of this project. One limit to overcome is the low energy content of visible light compared to chemical bonds that should be activated. We use the same strategy as biological photosynthesis and accumulate the energy of several photons for a single chemical step. Another limit to expand is the light-induced transfer of more than one electron leading to different reactive intermediates and new chemistry.
We develop new concepts in chemical photocatalysis and demonstrate their use in the efficient and sustainable synthesis of organic molecules including pharmaceuticals and performance chemicals. The project will provide new tools for chemical synthesis that find applications in chemical research, product development and industrial production.
We have focused our work in the first half of the project on two specific tasks: We aimed to achieve stronger reducing power by accumulating the energy of several visible light photons in new generations of photocatalysts and we established new reaction modes for the photogeneration of carbanions for synthesis. Several new photocatalysts with the potential to enable consecutive photoinduced electron transfer (conPET) or reaching strong excited state reduction potentials by other means were investigated. Particular promising proved the compound class of anthrone dyes, which after deprotonation and excitation reach extreme reduction potentials. Heterogeneous organic semiconductors had been used before in photocatalysis, but mostly in energy applications. We explore its use in organic synthesis and discovered new reaction modes. Particular the high stability and easy recycling of the material makes it the ideal photocatalyst for larger scale applications. Carbanion generation and use in synthesis was realized first from carboxylic acids and the method was then extended to the activation of C-H bonds. Optimization of the procedure allowed the efficient incorporation of carbon dioxide via anionic reaction intermediates into organic molecules.
Technological developments include improved photoreactor set ups with better temperature control and higher light intensities.
With a new class of homogeneous photocatalysts we have expanded the limits of reductive photcatalysis and can now perform transformations that were impossible before. The use of organic heterogeneous photocatalysts in synthesis demonstrate a way to realize larger scale photocatalytic applications. The redox neutral C-H carboxylation can be considered as a “dream” reaction; carbon dioxide is incorporated into organic molecules without additional reagents in the presence of a photocatalyst and visible light. The generation of carbanion intermediates for organic synthesis has been realized and demonstrated for several substrate classes.
Until the end of the project, we aim to extend the scope of substrates that can be photoconverted into carbanion intermediates. We continue our efforts to establish photocatalytic versions of organometallic reagents and extend activities towards oxidative photoprocesses. It is our aim to establish a broadly applicable energy efficient visible light driven chemistry enabling new reaction pathways and new transformations in organic chemistry.