Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes

Abstract

Photoexcited electron-hole pairs on a semiconductor surface can engage in redox reactions with two different substrates. Similar to conventional electrosynthesis, the primary redox intermediates afford only separate oxidized and reduced products or, more rarely, combine to one addition product. Here, we report that a stable organic semiconductor material, mesoporous graphitic carbon nitride (mpg-CN), can act as a visible-light photoredox catalyst to orchestrate oxidative and reductive interfacial electron transfers to two different substrates in a two- or three-component system for direct twofold carbon–hydrogen functionalization of arenes and heteroarenes. The mpg-CN catalyst tolerates reactive radicals and strong nucleophiles, is straightforwardly recoverable by simple centrifugation of reaction mixtures, and is reusable for at least four catalytic transformations with conserved activity.

A Retrosynthetic Approach for Photocatalysis

“…MacMillan and co‐workers reported a similar procedure using an additional iridium based photocatalyst and blue light instead of only the nickel complex . Our group was able to realize this transformation using a heterogeneous photocatalyst instead of an iridium‐based one . Johannes and co‐workers expanded MacMillan's procedure from alkylamines to mono‐arylamines .…”

“…In this case, acetone cyanohydrin could be used as a cheap and readily available source of cyanide . Furthermore, our group reported the use of mesoporous graphitic carbon nitride (mpg‐CN) photocatalyst as an operationally simple strategy for arene C–H and C‐X functionalization, including the cyanation of arenes …”

Let there be light

“…5 In a separate effort from the same laboratory, Indrajit Ghosh and König applied a photochemical method to replace otherwise inert C-H bonds in heteroarenes -aromatic compounds including a non-carbon atom -to build up chemical complexity with a minimum number of steps. 6 They used an organic semiconductor, mesoporous graphitic carbon nitride (mpg-CN), as a photocatalyst. This semiconductor material can be made in multigram scales from very cheap starting materials -for example, urea -is stable under a diverse range of reaction conditions, and the range of redox reactions it can catalyse after excitation with visible light is at least as wide as with commonly used metal complexes.…”

Photochemical Functionalization of Helicenes

“…In the past decade, visible‐light‐mediated photoredox catalysis has evolved into a valuable method for organic synthesis, and changed the way we activate chemical bonds for chemical transformations . Recently, we and others have functionalized arenes and heteroarenes by visible‐light‐mediated photoredox activation of C(sp 2 )‐halogen or C(sp 2 )‐hydrogen bonds .…”

“…[27] In the past decade, visible-light-mediated photoredox catalysis has evolved into a valuable method for organic synthesis, and changed the way we activate chemical bonds for chemical transformations. [28][29][30][31][32][33][34] Recently, we and others have functionalized arenes and heteroarenes by visible-light-mediated photoredox activation of C(sp 2 )-halogen [35][36][37][38] or C(sp 2 )-hydrogen bonds. [33,39] The former transformation, depending on the choice of the substrate and on the reaction conditions, proceed through single-electron reduction using polypyridyl transition-metal complexes, [40] simple organic dyes, [35,36,41] or using potassium tert-butoxide as a base under ultraviolet (UV, l Ex 350 nm) irradiation (S RN 1 reactions).…”