The Use of a Diagonal Photoflow System Combined with a High‐Power UV LED in High‐Speed C─H Bromination Reactions

Abstract A high‐speed photoflow benzylic C─H bromination using N ‐bromosuccinimide (NBS) was achieved by combining a diagonal photoflow reactor (LX‐1, MiChS) with a high‐power UV‐LED (365 nm) system. The LX‐1 reactor contains sloped parallel channels that allows thin‐film fluids to be effectively exposed to intensive light. Compared to other flow processes that have been reported so far for benzylic C─H bromination, this new photoflow setup enables more efficient performance as demonstrated by the significantly shortened residence time (6 to 13 s) in the system. The C‐H bromination of various alkyl arenes proceeded efficiently to give the corresponding C‐H brominated products in good yields. The synthetic utility of this protocol was demonstrated in the successful gram‐scale synthesis of the antiplatelet agent, ticlopidine, by the photoflow benzylic C─H bromination of ortho‐chlorotoluene and the subsequent nucleophilic substitution of a bromide ion by 4,5,6,7‐tetrahydrothieno[3,2‐c]pyridine.