In order to directly functionalize C−H bonds of complex molecules and, in particular, to control the regioselectivity of the reaction, a wide range of directing groups has been used. However, these directing groups need to be installed and removed for further applications, which may limit the use of C−H activation in synthesis. Concerning aldehydes and ketones, a transient directing group strategy has recently emerged to overcome this drawback. The addition of an additive, in general an amine, allowed the in situ formation of the real directing group to achieve C−H activation. This review presents the latest developments in the field over the period 2020–2023.
Herein, a regioselective bromination or chlorination reaction of 1‐naphthaldehydes is described. Without additive, the palladium‐catalyzed C−H halogenation showed a C8‐regioselectivity, whereas the formation of an aromatic imine intermediate allowed a switch to a C2‐reactivity. Mechanistic studies and DFT calculations were performed to explain the regioselectivity and the synthesized halogenated products were used as key building blocks to access polycyclic natural product skeletons.
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