Photochemical Nickel-Catalyzed C–H Arylation: Synthetic Scope and Mechanistic Investigations

Abstract

An iridium photocatalyst and visible light facilitate a room temperature, nickel-catalyzed coupling of (hetero)aryl bromides with activated α-heterosubstituted or benzylic C(sp3)–H bonds. Mechanistic investigations on this unprecedented transformation have uncovered the possibility of an unexpected mechanism hypothesized to involve a Ni–Br homolysis event from an excited-state nickel complex. The resultant bromine radical is thought to abstract weak C(sp3)–H bonds to generate reactive alkyl radicals that can be engaged in Ni-catalyzed arylation. Evidence suggests that the iridium photocatalyst facilitates nickel excitation and bromine radical generation via triplet–triplet energy transfer.

Photosensitized, energy transfer-mediated organometallic catalysis through electronically excited nickel(II)

“…1). In this context, it is important to note that Molander and Doyle and co-workers have described the light-mediated arylation of α-oxy C–H bonds, wherein dual-catalysis photosensitization or direct transition metal photoexcitation were proposed, respectively, as the operative catalytic mechanism (7, 24). …”

Mild, Redox‐Neutral Formylation of Aryl Chlorides through the Photocatalytic Generation of Chlorine Radicals

“…In contrast to systems reported by MacMillan [11] and Molander [12] , which primarily employ aryl bromides and iodides, our method utilizes aryl chlorides by design. Here we demonstrate that this manifold can be leveraged to enable redox-neutral formylation by selective 2-arylation of the inexpensive and abundant solvent 1,3-dioxolane with aryl chlorides followed by a mild acidic workup (Figure 1B).…”

Alkyl‐C‐C‐Bindungsbildung durch Nickel/Photoredox‐Kreuzkupplung

“…[59,60] Der experimentelle Nachweis deutet an, dass die Kupplungen über stabile und isolierbare Ni II -Arylhalogenid-Komplexe der oxidativen Addition (I)v erlaufen (Abbildung 4A). [59,60] Der experimentelle Nachweis deutet an, dass die Kupplungen über stabile und isolierbare Ni II -Arylhalogenid-Komplexe der oxidativen Addition (I)v erlaufen (Abbildung 4A).…”