Visible‐Light‐Photocatalyzed Pinacol Coupling in Water

We present a green and efficient approach for synthesizing vicinal diols via a metal-free photocatalyzed pinacol coupling reaction in water or aqueous media. The mild and practical reaction conditions, including a commercially available organic photocatalyst and a nontoxic solvent, provide wide functional group tolerance while furnishing the corresponding vicinal diols in high yields. Mechanistic studies reveal the crucial role of Hantzsch ester (HEH), which undergoes single-electron oxidation and subsequently acts as a proton donor to aldehydes for proton-coupled electron transfer (PCET). This work contributes to the development of environmentally friendly methodologies for carbon−carbon bond formation, aligning with the principles of green and sustainable chemistry.