U.S. Food and Drug Administration-Certified Food Dyes as Organocatalysts in the Visible Light-Promoted Chlorination of Aromatics and Heteroaromatics

Abstract

Seven FDA-certified food dyes have been investigated as organocatalysts. As a result, Fast Green FCF and Brilliant Blue FCF have been discovered as catalysts for the chlorination of a wide range of arenes and heteroarenes in moderate to excellent yields and high regioselectivity. Mechanistic investigations of the separate systems indicate that different modes of activation are in operation, with Fast Green FCF being a light-promoted photoredox catalyst that is facilitating a one-electron oxidation of N-chlorosuccinimide (NCS) and Brilliant Blue FCF serving as a chlorine-transfer catalyst in its sulfonphthalein form with 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) as stoichiometric chlorine source. Dearomatization of naphthol and indole substrates was observed in some examples using the Brilliant Blue/DCDMH system.

Photocatalytic Late-Stage C–H Functionalization

“…An alternative means of activation for N -haloamide compounds (e.g., NBS, NCS), termed photoredox electrophilic amplification, was pioneered by König and co-workers . Lamar and co-workers developed, in a series of reports, aerobic bromination and chlorination protocols that proved suitable toward the late-stage C­(sp 2 )–H halogenation of complex molecules (Scheme A). − As shown in the mechanistic outline, N -halogenated species 69.3 can undergo single-electron oxidation to generate haloiminium radical-cation 69.4 , which is well-poised for electrophilic aromatic substitution with 69.1 to deliver the final product 69.2 . Single-electron reduction of the N -centered radical 69.5 affords anion 69.6 and restores the organonophotoredox catalyst system.…”

Visible-Light-Induced Catalytic Selective Halogenation with Photocatalyst

“…In 2020, Lamar and co-workers reported FDA-certified food dye mediated-chlorination of aromatic and heteroaromatic substrates [ 55 ]. The chlorination of aromatic compound was achieved via reaction with N -chlorosaccharin (NCS) as a chloride source in the presence of Fast Green FCF as photoredox catalyst under irradiation of white LED in acetonitrile ( Scheme 53 ).…”

Visible light-enabled regioselective chlorination of coumarins using CuCl2via LMCT excitation

“…For example, Lama's group developed an organic dye activation of NXS or trichloroisocyanuric acid (TCCA) to chlorinate (hetero)arenes with electron-withdrawing substituents. 10…”