Dual Photoredox–Metal Asymmetric Catalysis: Merging Light and Chiral Metal Catalysis
Dual photoredox–metal catalysis, developed since 2014 by MacMillan, Doyle, and others, merges visible-light photoredox catalysis with transition-metal catalysis to achieve transformations inaccessible to either alone. When a chiral ligand is installed on the metal component, the dual system achieves enantioselective radical C–C and C–heteroatom coupling under mild visible-light irradiation. This is one of the fastest-growing frontiers in asymmetric synthesis.
The Dual-Catalytic Cycle
A typical cycle involves:
- Photoredox catalyst (Ir(ppy)₃, Ru(bpy)₃, or organic dye) absorbs visible light and generates a strongly oxidizing or reducing excited state.
- Single-electron transfer converts a substrate (α-amino acid, carboxylic acid, alkyl trifluoroborate) into a radical.
- Metal catalyst (Ni, Cu, or Pd) with a chiral ligand captures the radical and couples it with an aryl halide, alkene, or another radical.
- The chiral ligand controls the stereochemistry of the C–C bond-forming step.
The radical is generated under neutral, room-temperature conditions — far milder than classical radical initiation (AIBN, heat, peroxides).
Nickel–Photoredox Asymmetric Coupling
Chiral Ni-BOX and Ni-pyridoxime catalysts couple photogenerated alkyl radicals with aryl halides in an enantioconvergent fashion. This platform enables:
- Decarboxylative arylation of α-amino acids — access to non-proteinogenic amino acids.
- C(sp³)–C(sp²) coupling of alkyl trifluoroborates with aryl bromides.
- Cross-electrophile coupling of alkyl and aryl halides.
Copper–Photoredox Asymmetric Catalysis
Chiral Cu-phosphoramidite and Cu-BOX complexes catalyze enantioselective photoredox:
- Conjugate addition of photogenerated radicals to enones and enals.
- Asymmetric cyanation and azidation of benzylic radicals.
- Hydrofunctionalization of alkenes with photogenerated radical precursors.
Recommended Products
- (S)-t-Bu-BOX / (R)-t-Bu-BOX — bisoxazoline for Ni- and Cu-photoredox dual catalysis
- (S)-MonoPhos — phosphoramidite for Cu-photoredox conjugate addition
- (R)-BINOL / (S)-BINOL — backbone for phosphoramidite and chiral phosphoric acid co-catalysts
- (S)-TADDOL / (R)-TADDOL — diol for chiral Ti and Zr Lewis acid co-catalysts in photoredox systems
Why Dual Photoredox–Metal Catalysis Matters
- Mild conditions — room temperature, visible light, no strong oxidants/reductants.
- Novel disconnections — radical couplings inaccessible to two-electron catalysis.
- Step economy — direct functionalization of abundant carboxylic acids and amines.
- Green chemistry — reduced energy input, recyclable photocatalysts.
Frontier and Outlook
The field is rapidly expanding into: (1) metallaphotoredox C–H activation, (2) enantioselective C–N and C–O bond formation, (3) photoredox/polyoxometalate hybrid catalysis, and (4) continuous-flow photoredox with immobilized chiral metal catalysts. As ligand design for Ni and Cu photoredox systems matures, dual photoredox–metal asymmetric catalysis is becoming a mainstream platform for green enantioselective synthesis in both academia and process chemistry.