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. 2020 Mar 24;5(13):7693-7704.
doi: 10.1021/acsomega.0c00631. eCollection 2020 Apr 7.

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

David A Rogers  1 , Megan D Hopkins  1 , Nitya Rajagopal  1 , Dhruv Varshney  1 , Haley A Howard  1 , Gabriel LeBlanc  1 , Angus A Lamar  1
Affiliations

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

David A Rogers et al. ACS Omega. .

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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Organocatalytic Methods for Arene Chlorination Using N-Chloro Reagents (NCS Is Shown As a Representative Reagent)
Figure 1
Figure 1
FDA-approved food dyes.
Figure 2
Figure 2
Substrate scope of Fast Green FCF-catalyzed arene and heteroarene chlorination.
Scheme 2
Scheme 2. Plausible Mechanism for Fast Green/NCS Chlorination
Figure 3
Figure 3
Substrate scope of DCDMH/Brilliant Blue FCF chlorination of arenes and heteroarenes.
Scheme 3
Scheme 3. Dearomatization of Arenes and Heteroarenes Using the DCDMH/Brilliant Blue FCF Chlorination System
Figure 4
Figure 4
UV–Vis absorbance of Brilliant Blue FCF (shown in blue), Brilliant Blue stirred with 1 equiv of DCDMH for 24 h (shown in red), and DCDMH (shown in green).
Figure 5
Figure 5
Monitoring of the chlorination of 2-methylnaphthalene by DCDMH with pre-formed (18 h) Brilliant Blue/DCDMH mixture (red curve) versus Brilliant Blue/DCDMH performed under standard conditions (no pre-mixing; blue curve).
Scheme 4
Scheme 4. Plausible Mechanism of Brilliant Blue/DCDMH Chlorination of Arenes
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