Bromide-Promoted Visible-Light-Induced Reductive Minisci Reaction with Aldehydes

Abstract

Radical addition is a robust tool for bond formation. While ketyl radical reactivity of aldehydes by photoredox has been wellestablished, herein, we have now revealed a pathway for umpolung addition of aldehydes with or without external reductant. Hence, the reductive alkylations and challenging benzylations of nitrogen heteroarenes (i.e., Minisci reactions) are enabled by the bromidepromoted visible light-mediated photocatalysis. The present protocol offers a mild, viable method for late-stage alkylations and transitionmetal-free benzylations of biologically active nitrogen-heteroarene molecules. Mechanistic studies are indicative of a bromide-initiated acyl radical mechanism in the absence of external reductant.

Luminescent tungsten(vi) complexes as photocatalysts for light-driven C–C and C–B bond formation reactions

“…S14 † ). As the triplet excited state of W1a should be capable of oxidizing Br – ( E ( W1a* / W1a – ) = +1.1 V vs. SCE; E (Br˙/Br – ) = +0.8 V vs. SCE 42 ), W1a* may have been reduced to W1a˙ – in the reaction, though the subsequent reactions leading to product formation may be complicated and may require further study. The emissive excited state of W1a is ineffectively quenched by ethyl 4-chlorobenzoate with a rate constant of 2.3 × 10 5 M –1 s –1 only, which is consistent with the fact that the triplet excited state of W1a is thermodynamically not capable of reducing aryl chlorides.…”

Minisci C−H Alkylation of Heteroarenes Enabled by Dual Photoredox/Bromide Catalysis in Micellar Solutions**

“…However, catalytic species of a much higher reducing power can be generated via the reductive quenching of the catalyst followed by the subsequent excitation with a second visible‐light‐photon [30, 32–34] . Although the typically used reductive quenchers include tertiary amines, Hantzsch esters, alcohols, ascorbate anions etc., [35] it has recently been shown that the efficient quenching of excited Ir‐complexes can also be achieved using simple halide anions, leading to Ir II ‐species and halide radicals [36–40] . We decided to test, if Br − anions, which are released upon the single‐electron‐reduction and fragmentation of alkyl bromides, can be recycled and used as mediators in the Minisci reaction—quench the excited Ir III ‐photocatalyst and thus promote the generation of alkyl radicals.…”

Minisci C-H Alkylation of Heteroarenes Enabled by Dual Photoredox/bromide Catalysis in Micellar Solutions

“…30,[32][33][34] Although the typically used reductive quenchers include tertiary amines, Hantzsch esters, alcohols, ascorbate anions etc., 35 it has recently been shown that the efficient quenching of excited Ir-complexes can also be achieved using simple halide anions, leading to Ir(II)-species and halide radicals. [36][37][38][39][40] We decided to test, if Branions, which are released upon the single-electron-reduction and fragmentation of alkyl bromides, can be recycled and used as mediators in the Minisci reaction -quench the excited Ir(III)-photocatalyst and thus promote the generation of alkyl radicals.…”