PQ and water oxy-trifluoromethylate enynes
Sunlight-excited phenanthrenequinone turns Langlois’ CF3SO2Na into CF3· and uses water as the oxygen atom to build CF3 benzofurans, benzothiophenes and indoles.
Source
Visible-light-induced oxidant and metal-free dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes with water
Study at a glance
- Design
- Animal / in-vitro — Phenanthrenequinone photoredox cascade on 1,6-enynes with 18O-water, EPR, and DFT
- N
- Substrate-scope benzofurans, benzothiophenes, and indoles rather than a single N
- Population
- Phenolic 1,6-enynes with Langlois’ CF3SO2Na and water
- Outcome
- Visible-light, metal-free cascade trifluoromethylates and oxidizes 1,6-enynes, with water supplying the ketone oxygen
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Inspect source passages
PQ and water oxy-trifluoromethylate enynes
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Electron-diverse phenolic enynes give trifluoromethylated benzofurans in 57–75% yields.
Substrates 1b–1j , with electron donating as well as electron withdrawing substituents on the ethynyl ring, showed compatibility with the optimized reaction conditions and afforded respective trifluoromethylated benzofurans 2b–2j in 57–75% yields ().
The photocatalytic quantum yield is φ = 27.
The determined QY is φ = 27 (for details, see the ESI page S17) which suggests that 27 equivalents of product 2a are formed for every photon absorbed by the photocatalyst PQ .
What they did
Authors screened diketones/oxidants on phenolic 1,6-enynes, then mapped O-, S- and N-linked substrates, 18O-water labelling, EPR spin trapping, CV of PQ, and B3LYP hydration barriers.
What they found
PQ in CH3CN/water under sunlight (or 23 W CFL) gives benzofurans 57–75%, benzothiophenes 73–36%, N-Ts indoles 67–56%. 18O lands in the ketone (m/z 307.0821). H2 (δ 4.57 ppm) and H–D from D2O evolve. EPR sextet of tBu(CF3)NO·; φ = 27 implies a chain. DFT hydration barrier +16.47 kcal mol–1.
The limits
What it doesn't show
Alkyl-substituted enynes give low yields; Boc-indoles can drop to 10%; H2 is inferred from NMR not quantified gas chromatography; chain carrier HSO2· is proposed, not isolated.
Key terms
- Langlois’ reagent
- CF3SO2Na, an inexpensive CF3-radical source after one-electron oxidation.
- 9,10-Phenanthrenequinone (PQ)
- Visible-absorbing diketone photoredox catalyst that accepts two electrons.
- 1,6-Enyne
- o-Alkynyl aryl vinyl ether/thioether/enamine that cyclizes to a benzofuran-type core.
- Oxy-trifluoromethylation
- Adding CF3 and an oxygen atom across the vinyl ether without cleaving C–heteroatom.
- Quantum yield φ = 27
- 27 product molecules per absorbed photon, evidence of radical-chain propagation.
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Oxygen atom in the C3-aryloyl ketone comes from:
Common questions
Where does the ketone oxygen come from?
Water: H2^18O labels the product (M+H 307.0821).
Why avoid O2 and peroxides?
They cleave the vinyl C–O bond to phenols.
What stops the reaction?
TEMPO traps CF3; no product without light or with scavenger.
Is PQ a one-photon catalyst only?
φ = 27 means a chain, likely HSO2· reoxidizing Langlois’ salt.
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