Reductive Radical Conjugate Addition of Alkyl Electrophiles Catalyzed by a Cobalt/Iridium Photoredox System

Abstract

Alkyl and aryl halides have been studied extensively as radical precursors; however, mild and less toxic conditions for the activation of alkyl bromides toward alkyl radicals are still desirable. Reported here is a reductive radical conjugate addition that allows for the formation of alkyl radicals via activation of alkyl bromides through cobalt/iridium catalysis. The developed conditions are emphasized in the broad substrate scope presented, including benzylic halides and halides containing free alcohols, silanes, and chlorides.

Scalable Total Synthesis of (−)-Triptonide: Serendipitous Discovery of a Visible-Light-Promoted Olefin Coupling Initiated by Metal-Catalyzed Hydrogen Atom Transfer (MHAT)

“…The past two decades have witnessed major advances in the cobalt­(II) porphyrin-catalyzed cyclopropanation and C–H bond functionalization, which represent valuable additions to the toolbox for organic synthesis . The recent combinations of visible light photocatalysis and Co catalysis have also opened various new potential pathways to facilitate Co-catalyzed transformations, including MHAT-initiated processes. , In addition to the thermodynamic considerations, the match of kinetics of elementary steps in the catalytic cycles is crucial for the dual catalysis system . Even if serendipity plays a role in our work, the visible-light-promoted olefin coupling initiated by MHAT echoes recent resurgence to leverage the photoexcitable cobalt complex to weaken the Co–C bond, ,, which would inspire further interrogation and application of photoactive organometallic species …”

Dual Photoredox and Cobalt Catalysis Enabled Transformations

“…In this context, conjugate addition of alkyl radicals to electron deficient alkenes via dual photoredox-cobalt catalyzed reaction was reported by Escobar and Johannes. [47] In this protocol, an initial oxidation of organic reductant triethylamine by excited photocatalysts [Ir(III)]* gives Ir(II), which undergoes subsequent single electron transfer (SET) with Co(II) salt to generate reduced cobalt(I), which then generate alkyl radical and subsequently deliver the final conjugate addition product. Alkyl chlorides are less reactive as a radical precursor in comparison to bromide and iodide.…”

Bromo Radical‐Mediated Photoredox Aldehyde Decarbonylation towards Transition‐Metal‐Free Hydroalkylation of Acrylamides at Room Temperature

“…For instance, Balsells group [7] disclosed a silyl‐mediated photoredox‐catalyzed Giese addition of non‐activated alkyl bromides to α , β ‐unsaturated amides and esters. Likewise, Johannes [8] and Wang [9] independently realized visible‐light‐initiated hydroalkylation of unactivated alkyl iodides to electron‐deficient olefins. Besides, a wide range of visible‐light mediated hydroalkylation of alkenes by using light alkanes as alkyl sources was developed in recent years (Figure 1b) [10] .…”

“…For instance, Balsells group [7] disclosed a silyl-mediated photoredox-catalyzed Giese addition of non-activated alkyl bromides to α,βunsaturated amides and esters. Likewise, Johannes [8] Figure 1. The strategies of hydroalkylation of alkenes.…”