"On-water" photosensitization enables redox neutral acylation and alkylation of quinones

Water-oil interfaces exhibit a high cohesive energy density and form a supramolecular hydrogen-bonding network that supports organic reactions in various ways. Here, we introduce the redox-neutral photo-Friedel-Crafts acylation and alkylation of quinones at the aqueous-organic interface. Spectroscopic evidence and computational studies indicate extensive hydrogen bonding at the water surface, enabling the stabilization of the quinone's photo-excited state and thus reducing its excited state energy, while increasing the excited state energy of the photosensitizer Eosin Y. This combined spectroscopic behavior allows photosensitization of quinone under visible light at the oil-water interface and facilitates the desired transformation. Mechanistic studies reveal that C‒C bond formation occurs via hydrogen atom transfer (HAT) from the aldehyde or alkyl reactant to the quinone, following an overall redox-neutral route, with concurrent radical recombination, efficiently producing 2-functionalized quinols. The versatility of the method is demonstrated with aromatic and aliphatic aldehydes, including natural and synthetic drug molecules, as well as ethers, thioethers, alkanes, silanes, and amines, which act as acylating or alkylating agents. The reactions have also been scaled up, and the acylated quinol products have been further functionalized to showcase their synthetic potential.