Formal base-free homolytic aromatic substitutions via photoredox catalysis

Abstract

We developed a simple and convenient method to assemble biaryls exploiting a photoredox catalyst and visible light.

Dual gold and photoredox catalysed C–H activation of arenes for aryl–aryl cross couplings

“… 14 , 15 To the best of our knowledge, however, aryl–aryl couplings via C(sp 2 )-H activation using dual gold and photoredox catalysis has yet to be achieved, although it was recently attempted by Maestri and Malacria. 16 Under their conditions, they instead discovered that the coupling between unactivated arenes and diazonium salts could occur under photocatalysis-only conditions (no gold) through mechanistically distinct formal homolytic aromatic substitutions, which does not involve C–H activation. However, poor regioselectivities (mixtures of ortho , meta and para coupling) were observed and 40 equivalents of arene were generally required for this radical reaction.…”

“…However, poor regioselectivities (mixtures of ortho , meta and para coupling) were observed and 40 equivalents of arene were generally required for this radical reaction. 16 Therefore, aryl–aryl couplings via C(sp 2 )-H activation involving dual gold and photoredox catalysis is clearly desirable, as it will not only prove for the first time that catalytic oxidants can be utilised in the general field of gold-catalysed C–H activations, but it should also significantly improve the regioselectivities and arene equivalents in the aryl–aryl couplings, compared to the mechanistically distinct photocatalysis-only reaction.…”

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

“…Similarly, the tribromomethyl radical was generated from carbon tetrabromide using Ru(II)-photoredox catalysis, and undergoes substitution then methanolysis in the presence of base to give the 2-substituted methylester of indoles (Scheme 17). Visible light Ru(II)-photoredox catalysis has been used to mediate the intermolecular coupling of aryldiazonium salts with arenes, 60 and N-heteroarenes, 61,62 including caffeine to give 8-aryl substituted derivatives 7 in moderate to good yields (Scheme 18). 62 The substitution of the nucleophilic aryl radical is facilitated by activating the N-heteroarene with formic acid in order to quaternize the basic N-atom of caffeine.…”

Merging Transition-Metal Catalysis with Photoredox Catalysis: An Environmentally Friendly Strategy for C–H Functionalization

“…Very recently, Lee's group 49 reported an arylation of aryl C-H bonds with diazonium salts via gold/photoredox dual catalysis (Scheme 39). Compared with previous reports on the use of a photocatalyst alone, 50 this gold/photoredox dual catalytic system significantly improved the regioselectivity with respect to the arene substrates. In these reactions, Au(I)/Au(III) catalytic pathways were successfully achieved under mild conditions.…”