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

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Authors

GHOSH Tamal SLANINA Tomáš KÖNIG Burkhard

Year of publication 2015
Type Article in Periodical
Magazine / Source CHEMICAL SCIENCE
MU Faculty or unit

Faculty of Science

Citation
Web http://pubs.rsc.org/en/Content/ArticleLanding/2015/SC/C4SC03709J#!divAbstract
Doi http://dx.doi.org/10.1039/c4sc03709j
Field Organic chemistry
Keywords SODIUM-BOROHYDRIDE; CHEMOSELECTIVE REDUCTION; BIOORGANOMETALLIC CHEMISTRY; REGIOSELECTIVE REDUCTION; 1-4-NADH DERIVATIVES; HYDROGEN EVOLUTION; NADH REGENERATION; ACIDIC MEDIA; KETONES; PROFLAVIN
Description 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 (Rh-cat) 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 Rh-cat leading to Rh(III)-H represents the major reaction pathway, which is highly oxygen sensitive. The oxidative quenching of the PF singlet state by Rh-cat is a competing mechanism, which prevails in non-degassed systems.
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