The past, present, and future of the Yang reaction

Visible-Light-Activated Asymmetric β-C-H Functionalization of Acceptor-Substituted Ketones with 1,2-Dicarbonyl Compounds

We report a visible-light-activated asymmetric β-C(sp3)-H functionalization of 2-acyl imidazoles and 2-acylpyridines with 1,2-dicarbonyl compounds (typically α-ketoesters) catalyzed by a tailored stereogenic-at-rhodium Lewis acid catalyst. The C-C bond formation products are obtained in high yields (up to 99%) and with excellent stereoselectivities (up to >20:1 dr and up to >99% ee). Experimental and computational studies support a mechanism in which a photoactivated Rh-enolate transfers a single electron to the 1,2-dicarbonyl compound followed by proton transfer and a subsequent stereocontrolled radical-radical recombination.

Photoredox ketone catalysis for the direct C-H imidation and acyloxylation of arenes

The photoexcited aryl ketone-catalyzed C-H imidation of arenes and heteroarenes is reported. Using 3,6-dimethoxy-9H-thioxanthen-9-one as a catalyst in combination with a bench-stable imidating reagent, C-N bond formation proceeds with high efficiency and a broad substrate scope. A key part of this method is that the thioxanthone catalyst acts as an excited-state reductant, thus establishing an oxidative quenching cycle for radical aromatic substitution. The synthetic potential of this photoexcited ketone catalysis is further demonstrated by application to the direct C-H acyloxylation of arenes.

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development

Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted. The pursuit of electrophilic coupling partners beyond (hetero)aryl bromides has also led to the incorporation of several new classes of C(sp²)-hybridized substrates into light-mediated cross-coupling. Advances to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals that were previously inaccessible from trifluoroborates and silicates are also emphasized. Additionally, several organic photocatalysts have been investigated as replacements for their expensive iridium- and ruthenium-based counterparts. Lastly, the net C-H functionalization of the radical partner in an effort to improve atom economy is presented. An underlying theme in all of these studies is the value of generating radicals in a catalytic manner, rather than stoichiometrically.