Visible‐Light‐Driven Fluoroalkylation of Tryptophan Residues in Peptides

Trifluoromethylated and fluoroalkylated cyclic λ 3 ‐iodanes and their acyclic salts were used for visible light‐driven fluoroalkylation of tryptophan and tryptophan‐containing peptides in aqueous media. In comparison to previously reported fluoroalkylation using similar reagents and sodium ascorbate as reductant, the photochemical process did not require any additive or catalyst and was more selective for Trp versus other aromatic amino acids due to the gradual production of fluoroalkyl radicals over the whole irradiation period. However, in the presence of Cys residues, both methods were not selective and cysteine sulfhydryl groups were fluoroalkylated in side reactions. Spectroscopic and photochemical investigations as well as quantum chemical calculations provided insight into the reaction mechanism. The process was found to be photoinduced involving the formation of fluoroalkyl radical from the excited state of λ 3 ‐iodane.