Metal‐free Semiconductor Photocatalysis for sp² C−H Functionalization with Molecular Oxygen

Abstract

Designing metal‐free catalysts for solar energy conversion is a long‐standing challenge in semiconductor photoredox catalysis (SPC). With visible‐light‐responsive hexagonal boron carbon nitride (h‐BCN) as a non‐metal photocatalyst, this system affords C−H/N−H coupling products with broad substitution tolerance and high efficiency with molecular oxygen as the terminal oxidant. The catalyst exhibits remarkable performance for the selective C−H functionalization of electron‐rich arenes to C−N products (yields up to 95 %) and good stability (6 recycles). Both nitrogen heteroarenes and amine salts are competent coupling nucleophiles. Mechanically, the reactive oxygen species are superoxide anion radical (O2−.) and H2O2, which are proved by electron spin resonance (ESR) data, KI‐starch, and control experiments. In addition, kinetic isotope effect (KIE) experiments indicate that C−H bond cleavage is not involved in the rate limiting step. This semiconductor‐based photoredox system allows for C−H amination free of any metals, ligands, strong oxidants, and additives. It provides a complementary avenue to C−H functionalizations and enables synthetic applications efficiently in a sustainable manner.

Metal-Free Photocatalysis: Two-Dimensional Nanomaterial Connection toward Advanced Organic Synthesis

“… 55 , 56 As a result, h-BCN is an appealing option for photocatalytic applications, 57 and, apart from water splitting evolution and CO 2 reduction, 56 , 57 h-BCN has recently attracted attention as a catalyst for synthetically relevant photoredox reactions. 58 , 59 König, Wang, and co-workers studied the photochemical C–H functionalization of electron-rich arenes catalyzed by an h-BCN with notable activity, 60 whereas other photo-oxidation and photoreduction reactions were possible by simply tuning the relative content of the h-BCN precursors (typically glucose and boric acid, Figure 6 ). 61 − 63 Despite the above-mentioned encouraging case studies, organic photocatalysis by BCN is still in its infancy; therefore, there is fertile soil for future developments.…”

Photocatalytic C−H Azolation of Arenes Using Heterogeneous Carbon Nitride in Batch and Flow

“…A photoelectrocatalytic approach was developed by Hu and co‐workers using hematite as photoanode and allowed to avoid the use of homogeneous photocatalysts [13] . Recently, Wang, König and co‐workers developed a metal‐free semiconductor strategy using hexagonal boron carbon nitride as a photocatalyst [14] . Despite these seminal advances, a general protocol displaying a broad substrate scope and using an easy‐to‐recover photocatalyst remains a challenge.…”

Photocatalysis in Dual Catalysis Systems for Carbon‐Nitrogen Bond Formation

“…Very recently, an alternate approach for CÀ H amination was reported by König and Wang. [29] They introduced a semiconductor-based hexagonal boron nitride (h-BCN) as a non-metallic photocatalyst which displayed high stability up to 6 recycles. This protocol was found to be an excellent pathway for C(sp 2 )À H functionalization.…”