Issue 3, 2015

Visible light photocatalytic reduction of aldehydes by Rh(iii)–H: a detailed mechanistic study


The chemoselective photoreduction of aldehydes in the presence of ketones was achieved using triethanolamine (TEOA) as sacrificial electron donor, proflavine (PF) as photocatalyst and [Cp*Rh(III)(bpy)Cl]Cl (Rhcat) as mediator. The reducing agent, which reacts with the carbonyl group was found to be [Cp*Rh(III)(bpy)H]Cl (Rh(III)–H). Contrary to formate-based reduction, its slow photochemical in situ generation enables to kinetically distinguish aldehydes from ketones. The inherent reactivity difference of the carbonyl compounds is transferred by the method into synthetically useful reaction selectivities. The substrate scope is broad with excellent yields. A detailed study of the reaction mechanism reveals that the photoreduction of the PF triplet and the subsequent reduction of the Rhcat leading to Rh(III)–H represents the major reaction pathway, which is highly oxygen sensitive. The oxidative quenching of the PF singlet state by Rhcat is a competing mechanism, which prevails in non-degassed systems.

Graphical abstract: Visible light photocatalytic reduction of aldehydes by Rh(iii)–H: a detailed mechanistic study

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Article information

Article type
Edge Article
30 Nov 2014
06 Jan 2015
First published
06 Jan 2015
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2015,6, 2027-2034

Author version available

Visible light photocatalytic reduction of aldehydes by Rh(III)–H: a detailed mechanistic study

T. Ghosh, T. Slanina and B. König, Chem. Sci., 2015, 6, 2027 DOI: 10.1039/C4SC03709J

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