Visible Light Mediated External Oxidant Free Selective C5 Bromination of 8‐Aminoquinoline Amides under Ambient Conditions

Abstract

Selective functionalization of quinolines is the hot topic of organic synthesis due to its wide application in natural products, medicine, etc. In this research, C5 bromination of 8-aminoquinoline amides have been achieved using the strategy of photoredox catalysis under mild conditions. Easily available 10-phenylphenothiazine is employed as the organophotoredox catalyst and CBr 4 is used as the bromine source. No external oxidant or transition metal catalyst is needed in this transformation. Both the alkyl and aryl substituted carboxamides are compatible with this method.

Activation of C–Br Bond of CBr4 and CBrCl3 Using 9-Mesityl-10-methylacridinium Perchlorate Photocatalyst

“…In both cases, triethylamine was used as the electron transfer agent. To the best of our knowledge, scanty literature is available on the homolytic bond dissociation of the aliphatic C–Br bond using visible light. − The C–Br bond dissociation energy of CBr 4 is 235 kJ mol –1 (Figure a).…”

Visible light-mediated halogenation of organic compounds

“…Carbon tetrabromide could be also exploited for the bromination of 8-aminoquinolines. 120 The more sustainable bromine source HBr, is usually employed for bromination under oxidative conditions, which can be performed with a photoredox catalyst. Fukuzumi reported an acridinium dye-catalysed bromination of electronrich arenes 77 employing HBr and oxygen-saturated MeCN (Scheme 51).…”

Organophotoredox‐Catalyzed Late‐Stage Functionalization of Heterocycles

“…In this regard, Zhang, Lei and co‐workers reported a organophotoredox‐catalyzed C‐5 selective bromination of 8‐aminoquinoline amides 88 under metal‐free redox neutral conditions using CBr 4 as the bromine source (Scheme 25). [57] Importantly, the amount of water remained critical to harness the desired high reactivity. The authors have proposed that the excited photocatalyst PTH* ( PC‐7 ) undergoes oxidative quenching by CBr 4 to ultimately lead to the generation of ⋅CBr 3 radical, which immediately abstracts H‐atom from 88 to form the radical intermediate A .…”

“…In this regard, Zhang, Lei and co-workers reported a organophotoredox-catalyzed C-5 selective bromination of 8aminoquinoline amides 88 under metal-free redox neutral conditions using CBr 4 as the bromine source (Scheme 25). [57] Scheme 22. Rose Bengal-catalyzed alkoxylation of imidazopyridines.…”