C-H Bond Activation Relay (CHAR) of Proline Ester Derivatives Promoted by In Situ Triarylamine Radical Cation: Selective Synthesis of 4-Bromopyrrole Derivatives.

Using the in situ generated triarylamine radical cation as an initiator, the sp C-H bond of proline esters was smoothly oxidized and brominated through C-H activation relay (CHAR), giving a series of 4-bromopyrroles in good yields with high regioselectivity. The mechanistic study revealed that the oxidation of the active C-H bond initiated the followed 1,5-HAT and bromination, which provides a new method to realize the functionalization of the remote C-H bond.