Photochemistry
Slow photogenerated Rh–H prefers aldehydes
Open access · cc by · source: Europe PMC
Proflavine plus a Cp*Rh mediator makes dilute Rh(III)–H that reduces aldehydes while ketones wait, unlike fast formate hydride transfer.
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Slow photogenerated Rh–H prefers aldehydes
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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.
Key findings
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.
Methodology
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.
Limitations
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.
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