Copper‑Mediated Ortho ‑C─H Halogenation of Arylaldehydes Enabled by a Transient Directing Group

The transient directing group (TDG) strategy enables C─H functionalization without the need for pre‐installing and subsequently removing a directing group. In this study, we present a novel copper‐mediated, TDG‐promoted ortho‐ C─H halogenation of arylaldehydes. This strategy employs an economical copper mediated in conjunction with a β ‐amino acid derivative as the TDG, thereby achieving highly regioselective halogenation under mild conditions. The method demonstrates a wide substrate scope and exhibits a selectivity profile that is distinct from classical electrophilic aromatic substitution. Mechanistic studies have elucidated the existence of a key bromine radical intermediate and the involvement of a concerted metalation‐deprotonation (CMD) process.