Photocatalytic Phosphine‐mediated Hydroxymethylation of Alkenes With Formaldehyde Hydrate

The mild and convenient generation of unprotected hydroxymethyl radical from simple C1 feedstocks remains a fundamental challenge. Here we report a visible-light-induced, phosphine-mediated hydroxymethylation of electron-deficient alkenes using aqueous formaldehyde as the hydroxymethyl radical precursor. The reaction is enabled by single-electron oxidation of phosphine to phosphine radical cation (PRC), which engages formaldehyde hydrate to release hydroxymethyl radical via β-scission. Subsequent regioselective radical addition to C─C double bonds, single-electron reduction, and protonation deliver hydroxymethylated products under mild conditions. The method accommodates electronically diverse cinnamates, heteroaryl alkenes, trisubstituted alkenes, and complex substrates, demonstrating broad functional-group tolerance and potential application for late-stage functionalization. This work provides a practical entry to radical hydroxymethylation from commercially available aqueous formaldehyde and establishes phosphine-mediated deoxygenation as a good platform for C1 radical generation.