Continuous flow photochemical C-H functionalization of 2-oxindole for C-3 selective fluorination.

A simple, efficient, metal-free protocol for the selective mono-fluorination of 2-oxindoles has been developed using Selectfluor in a continuous-flow setup under black light irradiation conditions. This transformation provides direct access to a broad range of fluorinated 2-oxindole derivatives in quantitative yields. The products are isolated in high purity through a straightforward workup, eliminating the need for tedious column chromatography. Gram-scale synthesis of 3-fluorooxindole was achieved with 95% isolated yield, demonstrating the scalability of the method. The fluorinated oxindoles were further employed in diverse transformations to highlight the method's broad applicability and synthetic utility. Experimental evidence supports a plausible radical mechanism for this fluorination reaction.