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Final Report Summary - VISTA-CARB (VISIBLE LIGHT MEDIATED CARBONYLATIONS)

Visible light is a clean, inexpensive, and ‘infinite’ source of energy which has attracted considerable interest from the chemistry community only recently. The Italian chemist Ciamician had already recognized a century ago that sunlight could be utilized as a promoter of organic reactions. However, one fundamental impediment that significantly limits the development of photochemical processes is the inability of most common organic molecules to absorb light in the visible range. For that purpose, many investigations have been devoted to develop photoredox catalysts that are able to absorb visible light and mediate the desired chemical transformations by means of rapid electron-transfer processes. In the last few years, the application of organometallic and organic dyes to several chemical processes has emerged as a powerful synthetic technology enabling several organic transformations that are either impossible or impractical with conventional protocols. This approach largely relies on the ability of metal complexes and organic dyes to engage in single electron transfer (SET) processes with organic substrates (mainly reactive substrates with C–I, C–Br and π-bonds) upon photo-excitation with visible light. Many of the common visible light photocatalysts are oligopyridyl complexes of ruthenium and iridium. Organic dyes such as methylene blue, rose Bengal, eosin, triphenylpyrylium salts or 9-mesityl-10-methylacridinium ion can also act as visible light photocatalysts giving rise to the population of excited states which undergo rapid SET with a multitude of organic substrates. However, there are still only very few powerful synthetic applications of metal-free photocatalysts, so that the development of new protocols with cheap organic dyes will certainly make a strong impact on the art of sustainable synthesis. In this context, the most challenging objective of this project was the implementation of two-photon excitation events into organic photoredox catalysis in order to expand the accessible reactivity window to less activated substrates. We mostly directed our attention at triplet-triplet annihilations (TTA) which requires two photoactive dye molecules but operates with less energy than two-photon absorptions. During this project phase, Dr. Perez-Ruiz has single-handedly investigated the photophysical and photochemical aspects of combinations of TTA events between two dyes with reductive activations of aryl electrophiles with simple organic dyes (diketone, oxazoline, anthracene). This framework involved detailed spectroscopic, theroretical, and synthetic studies and ultimately led to the development of a new set of photoredox-catalytic hydrodebrominations, hydrodechlorinations, arene-alkene coupling reactions, and related transformations. For the first time, TTA has been embedded into chemical reactions involving electron transfer activation of organic molecules. In extension of this work, applications of metallic photocatalysts to similar transformations were studied which resulted in two further protocols of reductive arene functionalization with platinum and ruthenium based catalysts. This new mechanistic paradigm holds great potential for challenging bond activations while retaining the benefit of mild reaction conditions by the use of lower-energy visible light.
Dr. Perez-Ruiz has also contributed to the design and application of supramolecular fibrillar gel networks for the assembly of photoactive materials by the incorporation of donor/acceptor pairs while preserving the structural integrity of the bulk material. In collaboration with Prof. Diaz Diaz at the University of Regensburg, the first intragel photoreduction of aryl halides via a TTA mechanism was discovered. The gel network provided a stable microenvironment for the challenging multi-step process under aerobic conditions, room temperature and without additional additives. Intragel photoreductions of aryl halides in air were investigated with platinum(II) octaethyl-porphyrin (PtOEP) as sensitizer and 9,10-diphenylanthracene (DPA) as emitter in supramolecular gel networks (N,N’-bis(octadecyl)-L-boc-glutamic diamide (G-1) and N,N’-((1S,2S)-cyclohexane-1,2-diyl)didodecanamide (G-2)). These results demonstrated that low weight molecular (LWM) gelators could be used as confined reaction media or micro/nanoreactors, providing the background for more demanding photophysical processes.
In a third line of activities, Dr. Perez-Ruiz investigated the photophysical properties and chemiluminescence mechanism of luminol derivatives. This collaboration with the groups of Prof. Jacobi von Wangelin (EU host at UREG) and Dr. Diego Sampedro (University of La Rioja, Spain) demonstrated an unprecedented 20-fold increase in chemiluminescence for 6,8-dialkyl luminol derivatives. This significant result could be interpreted as a “steric gearing” effect which facilitates the transition from the intermediate endoperoxide to the electronically excited phthalate species. This concept of a mostly steric modulation complements those involving strong electronic variations of the luminophore or extension of the conjugation length. The application of such rather subtle structural manipulation of photoactive molecules holds great potential for the design and amplification of various photophysical processes and eventually chemiluminescence properties. This might lead to improvements of the efficiency of current luminescence tools for the detection of small molecules or imaging.
Finally, Dr. Perez-Ruiz undertook detailed spectroscopic studies on photochemical oxygenations of hydrocarbons. Together with two graduate students, he discovered the high reactivity of cis-fused bicyclic alkenes in Schenck ene reactions. Mechanistic investigations revealed a strict conformational control of photo-oxygenation reactivity through the population of pseudo-axial C-H bond conformers. The rationalizations of this under-estimated effect led to the great extension of the scope of Schenck ene oxidations to a variety of substituted cyclohexenes which has so far been considered to be sluggish substrates.

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