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10.4 Merging Photoredox with Palladium Catalysis for Oxidation

DOI: 10.1055/sos-SD-229-00180

Wang, M.; Li, P.; Wang, L.Science of Synthesis: Photocatalysis in Organic Synthesis, (20181335.

Lignocellulose exists widely in plant cell walls and lignin is the largest renewable feedstock containing an aromatic backbone; hence, the development of efficient processes for its exploitation as a renewable resource is a well-recognized objective.[‌93‌,‌94‌] Although considerable effort has been devoted to chemical depolymerization of lignin,[‌95‌‌100‌] its rigid, irregular, and highly cross-linked structure renders effective degradation a challenging goal. Recently, a novel approach toward the valorization of lignin and related systems has been developed by combining photoredox and palladium catalysis, which allows for the efficient oxidation of lignin-related systems at room temperature. Some simplified β-O-4 model systems 44 are oxidized to the corresponding ketones 45 in high yields using iridium photocatalyst 43 {[Ir{dF(CF3)ppy}2(dtbbpy)]PF6; dF(CF3)ppy = 3,5-difluoro-2-[5-(trifluoromethyl)pyridin-2-yl]phenyl; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridyl} and sodium persulfate. This method constitutes a vital step toward converting lignin into value-added, low-molecular weight aromatics (Scheme 12).[‌101‌]

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