Visible‐Light‐Induced Photocatalytic Reductive Transformations of Organohalides

Abstract

A photo opportunity: A visible‐light‐excited iridium catalyst delivers electrons from an amine to an organohalide. The electron transfer then induces reductive scission of the carbon–halogen bond, generating the corresponding alkyl, alkenyl, and aryl radical that can undergo cyclization and hydrodehalogenation reactions.

Visible‐Light Photocatalysis as an Enabling Tool for the Functionalization of Unactivated C(sp3)‐Substrates

“…[51] Mechanistically, the excited-state fac-Ir(ppy) 3 photocatalyst (29) was believed to in- With use of another Ir photocatalyst -Ir(ppy) 2 (dtbbpy)PF 6 (32) -and DIPEA (20) as sacrificial electron donor, a set of alkyl iodides could undergo the same reductive transformation with an emphasis on the intramolecular cyclization/hydrogen abstraction cascade process (Scheme 16). [52] Through a combination of Garegg-Samuelsson iodination in batch followed by visible-light-mediated reductive deiodination in flow, aliphatic alcohols 33 could be deoxygenated effectively in a useful telescoped reaction (Scheme 17). [53] Importantly, performing the reductive deiodination in flow shortened the reaction time in comparison with batch conditions, and full conversion of the alcohol was only achieved in flow.…”

“…[54] Notably, the yields were in the same range as for the alkyl iodides, further highlighting the utility of photocatalysis for reductive dehalogenations. [51,52,54] Scheme 18. Ir-catalyzed reductive dehalogenation of alkyl bromides.…”

Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

“…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.…”

The chemistry of amine radical cations produced by visible light photoredox catalysis

“…The second mode involves hydrogen atom abstraction from 2 to produce iminium ion 4 , when a good hydrogen atom acceptor is present in the reaction. The use of amine radical cation 2 as the source of a hydrogen radical has been applied to a number of visible light-mediated reductions such as reductive dehalogenation [47–51], reductive radical cyclization [52–54], reduction of activated ketones [49], and reduction of aromatic azides [55]. The third mode involves deprotonation of amine radical cation 2 to form α-amino radical 3 , which is converted to iminium ion 4 by another one-electron oxidation.…”