Reductive Activation and Hydrofunctionalization of Olefins by Multiphoton Tandem Photoredox Catalysis

Abstract

The conversion of olefin feedstocks to architecturally complex alkanes represents an important strategy in the expedient generation of valuable molecules for the chemical and life sciences. Synthetic approaches are reliant on the electrophilic activation of unactivated olefins, necessitating functionalization with nucleophiles. However, the reductive functionalization of unactivated and less activated olefins with electrophiles remains an ongoing challenge in synthetic chemistry. Here, we report the nucleophilic activation of inert styrenes through a photoinduced direct single electron reduction to the corresponding nucleophilic radical anion. Central to this approach is the multiphoton tandem photoredox cycle of the iridium photocatalyst [Ir(ppy) 2 (dtbbpy)]PF 6 , which triggers in situ formation of a high-energy photoreductant that selectively reduces styrene olefinic π bonds to radical anions without stoichiometric reductants or dissolving metals. This mild strategy enables the chemoselective reduction and hydrofunctionalization of styrenes to furnish valuable alkane and tertiary alcohol derivatives. Mechanistic studies support the formation of a styrene olefinic radical anion intermediate and a Birch-type reduction involving two sequential single electron transfers. Overall, this complementary mode of olefin activation achieves the hydrofunctionalization of less activated alkenes with electrophiles, adding value to abundant olefins as valuable building blocks in modern synthetic protocols.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

“…This tandem photoredox catalysis also allowed for the development of a carbonylative amidation of aryl and alkyl halides, and of a reduction/reductive functionalization of olefins, in more recent reports from this group. 906 , 907 …”

Photocatalytic C(sp3) radical generationviaC–H, C–C, and C–X bond cleavage

“…In 2021, Polyzos published a photocatalysed styrene reductive hydrofunctionalisation with proton or ketone as electrophiles and amine as the sacricial reductant (Scheme 31B). 133 Interestingly, a multiphoton tandem photoredox cycle of the Ir-photocatalyst [Ir(ppy) 2 (dtbbpy)]PF 6 was Scheme 32 Rc generation via C-N cleavage of quaternary nitrogen salt.…”

“…In 2021, Polyzos published a photocatalysed styrene reductive hydrofunctionalisation with proton or ketone as electrophiles and amine as the sacrificial reductant (Scheme 31B). 133 Interestingly, a multiphoton tandem photoredox cycle of the Ir-photocatalyst [Ir(ppy) 2 (dtbbpy)]PF 6 was proposed. Central to this method was the photocatalytic generation of a new Ir species with a partially reduced bipyridine ligand, which could initiate another photocatalytic cycle and deliver higher-energy electrons to overcome the high reduction barrier of styrene (typically, E red 1/2 = <−2.0 V vs. SCE in MeCN).…”

Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes

“… 50 Polyzos and co-workers showed the reduction of aromatic olefins via direct photoinduced electron transfer under highly reductive conditions and light intensity. 51 …”

“…50 Polyzos and co-workers showed the reduction of aromatic olens via direct photoinduced electron transfer under highly reductive conditions and light intensity. 51 Ideally, AP systems should employ water as a source of hydrogen atoms and efficiently use the photoredox catalyst in combination with molecular catalysts to facilitate the selective reduction of alkenes. Such an approach should identify catalysts' hits to develop more complex AP schemes.…”