Strongly reducing Ir photoredox on aryl chlorides
Ir(ppy)2(NacNac) photocatalysts with BIH hydrodehalogenate unactivated aryl bromides, chlorides, fluorides, and alkyl bromides, including with green light.
Source
Photoredox catalysis on unactivated substrates with strongly reducing iridium photosensitizers
What they did
They used two β-diketiminate iridium complexes (Ir1, Ir2) plus 1,3-dimethyl-2-phenylbenzimidazoline (BIH) under blue or green LEDs. Scope included hydrodebromination, hydrodechlorination, alkyl bromides, radical cyclization, and ether C–O cleavage. Quantum yields and Stern–Volmer quenching rates were measured.
What they found
Excited-state potentials are −2.6 V (Ir1) and −2.4 V (Ir2) vs Fc+/Fc, ~300–500 mV more reducing than fac-Ir(ppy)3. With 1 mol% Ir and BIH, aryl bromides work in 24 h; aryl chlorides give quantitative yields with Ir1/blue light. Quantum yields are 4.7% (S2) and 1.4% (S7). Light/dark cycling shows catalysis needs light.
The limits
What it doesn't show
Quantum yields <100% do not fully exclude short radical chains. Many reactions were set up in a glovebox; ambient stability of stock solutions is limited. Isolated-yield tables for every substrate are not all in the main text.
Key terms
- Photoredox catalysis
- A photosensitizer absorbs visible light and does single-electron transfer to start an organic reaction.
- β-Diketiminate (NacNac)
- Anionic N,N-chelate that raises the Ir HOMO, making the excited complex a stronger reductant.
- BIH
- 1,3-Dimethyl-2-phenylbenzimidazoline, a sacrificial electron and hydrogen-atom donor (Eox = −0.07 V vs Fc).
- Hydrodehalogenation
- Replacement of C–X (Br, Cl, F) with C–H after reductive cleavage to a radical.
- Oxidative quenching
- Excited Ir* donates an electron to the substrate (or mediator), becoming Ir(IV) before BIH regenerates Ir(III).
Flashcards
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The excited-state reduction potential of Ir1 versus Fc+/Fc is
Common questions
Why are these Ir complexes stronger photoreductants?
NacNac destabilizes the HOMO so E(IrIV/*IrIII) reaches −2.6 / −2.4 V vs Fc, ~0.3–0.5 V beyond fac-Ir(ppy)3.
What sacrificial reagent replaced TMEDA?
BIH, allowing 1 mol% catalyst and 24 h instead of 2.5 mol% / 48 h.
Can they reduce aryl chlorides with visible light?
Yes—four substrates, quantitative with Ir1 and blue LEDs; Ir2/green is slightly lower.
Is a long radical chain operating?
Φ = 4.7% and 1.4% and dark periods stop conversion, so chains are not dominant.
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