Slow photogenerated Rh–H prefers aldehydes
Proflavine plus a Cp*Rh mediator makes dilute Rh(III)–H that reduces aldehydes while ketones wait, unlike fast formate hydride transfer.
Source
Visible light photocatalytic reduction of aldehydes by Rh(iii)-H: a detailed mechanistic study
Inspect source passages
Slow photogenerated Rh–H prefers aldehydes
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Highest benzyl alcohol yields used 10 mol% of both PF and Rh cat.
The yields of benzyl alcohol were highest in case of 10 mol% of both PF and Rh cat (, entry 5).
A flow reactor accelerates the reaction by a factor of 5 without losing selectivity.
The reaction rate could be accelerated by a factor of 5, without affecting the selectivity (, entries 1–3) using a flow reactor, which delivers the incident light more efficiently to the whole volume of the reaction mixture.
What they did
Authors optimized PF/Rh cat in DMF/water at 455 nm, screened aldehyde/ketone pairs (including a keto-aldehyde), compared formate-generated Rh–H, and used Stern–Volmer, UV-vis, headspace H2 and pump–probe spectra.
What they found
10 mol% of both catalysts in 1:1 DMF/H2O is optimal; a flow reactor speeds the reaction 5× without losing selectivity. Rh(III)–H appears at 612 nm. Φ = (0.14 ± 0.05)% at 455 nm. Triplet 3PFH+* (~2 μs) is reduced by TEOA to PF·– (~530 nm), which then reduces Rh. Aerated runs still give ~30% via photoionization.
The limits
What it doesn't show
Quantum yield is low because of fluorescence, H2 evolution and Rh(II) disproportionation; one bifunctional substrate gives a pinacol side product; imine reductions need dry DMSO/thiourea and are a separate optimisation.
Key terms
- Proflavine (PF)
- 3,6-Diaminoacridine dye; PFH+ absorbs at 443 nm and is the photocatalyst.
- Rh cat
- [Cp*Rh(bpy)Cl]n-type mediator that becomes [Cp*Rh(bpy)(H2O)]2+ then Rh(III)–H.
- Rh(III)–H
- Metal hydride that delivers H– to aldehydes faster than to ketones at low concentration.
- TEOA
- Triethanolamine sacrificial donor that reduces 3PFH+* (E0 ≈ +0.76 V vs SCE).
- Photoionization pathway
- Oxygen-tolerant route in which 1PFH+* ejects an electron that still reduces Rh cat.
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Common questions
Why aldehydes over ketones?
Slow in situ [Rh–H] lets kinetics, not thermodynamics, pick the aldehyde.
Is H2 the reductant?
No—headspace H2 is a side product; H2 atmosphere gives no alcohol.
Must the solution be degassed?
Helps (triplet path) but aerated runs still give ~30% via photoionization.
Where is water needed?
Proton source for Rh(III)–H; dry organics fail.
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