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.