Chemoselective Radical Dehalogenation and C–C Bond Formation on Aryl Halide Substrates Using Organic Photoredox Catalysts

Abstract

Despite the number of methods available for dehalogenation and carbon-carbon bond formation using aryl halides, strategies that provide chemoselectivity for systems bearing multiple carbon-halogen bonds are still needed. Herein, we report the ability to tune the reduction potential of metal-free phenothiazine-based photoredox catalysts and demonstrate the application of these catalysts for chemoselective carbon-halogen bond activation to achieve C-C cross-coupling reactions as well as reductive dehalogenations. This procedure works both for conjugated polyhalides as well as unconjugated substrates. We further illustrate the usefulness of this protocol by intramolecular cyclization of a pyrrole substrate, an advanced building block for a family of natural products known to exhibit biological activity.

Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

“…The main challenge in this case is the competing hydrogenatom abstraction by the reactive aryl radicals either from the radical cation of the amine or from the solvent to form the reduction product, as was also observed for other photoredox catalytic C-H arylation reactions of aryl halides. [12,13,32,[36][37][38] However, when a reaction mixture containing 2-bromobenzonitrile, Aq-OH, N,N-diisopropylethylamine (DIPEA), and benzene was irradiated with blue LEDs, the desired product 3m was obtained in 14 % yield. [45] Similarly, when furan and thiophene were introduced as the radical trapping reagents, the desired products 3k and 3l were obtained in useful yields.…”

“…As the excited states of radical anions act as powerful reductants, [12,13,28,29,31] we en- [40] In the inset, the luminescence spectrum (λ Ex = 427 nm) of Aq-OH-H -is shown. For the formation of the radical anion and the semiquinone anion of Aq-OH upon photoirradiation in the presence of Et 3 N, see Figure 3. visioned that anthraquinone derivatives could be employed for the photoredox catalytic reduction of aryl halide substrates, including aryl chlorides, to obtain aryl radicals either for metalfree dehalogenation reactions [12,[32][33][34][35] or for synthetically important carbon-carbon [12,32,[36][37][38] bond-formation reactions. Such photoredox catalytic methods [12,13,[32][33][34][35][36][37][38][39] are valuable alternatives to well-established transition-metal-based activation methods.…”

Established and emerging strategies for polymer chain‐end modification

“…Building on small molecule photochemical reactions for the removal of halides at the termini of polymers (Fig. ), a reducing organic photoredox catalyst—10‐phenylphenothiazine (PTH)—has been developed for the light‐mediated removal of halogen chain‐ends from styrenic, acrylic and methacrylic polymers . This quantitative method is readily applicable to thin films, enabling the facile preparation of hierarchical patterned polymer brush films …”

A Conjugated Microporous Polymer for Palladium‐Free, Visible Light‐Promoted Photocatalytic Stille‐Type Coupling Reactions

“…Especially room temperature (RT)‐active, visible light‐generated photocatalytic alkylation2 or arylation3, 4, 5 reactions have been demonstrated as a versatile tool for C—C bond formations 6, 7. In the past, a number of visible light‐active catalytic systems have been developed.…”

“…For the aryl radical‐mediated arylation reactions reported in the literature,4, 5 the aryl radicals were obtained via dehalogenation reaction of aryl halides, which was driven by the photogenerated electron of the photocatalyst. In contrast, for the here studied photocatalytic Stille‐type C—C bond formation reaction, we envision to report the formation of the aryl radical from the aryl stannane via oxidation driven by the photogenerated hole of the photocatalyst.…”