3,4,5‐Trifluorophenyldiazonium–A Unique Radical Source for the Visible Light Induced, Catalyst‐Free Arylation of Tyrosine Residues in Peptides

The direct radical arylation of tyrosine residues in peptides has been achieved under highly attractive conditions, requiring only the peptide substrate, a 3,4,5-trifluorophenyldiazonium salt, and irradiation with visible light. Being well-balanced in its reactivity to form strong charge-transfer complexes with the phenolic side chain, which are required for initiation and aryl radical formation, the trifluoro substitution still shows sufficient stability toward nucleophilic aromatic substitution in aqueous solution. Besides allowing selective arylation, the introduction of the 3,4,5-trifluorophenyl unit enables further functionalization as well as potential applications in NMR and MRT studies.