Ghosh T, Slanina T, König B
Institute of Organic Chemistry , University of Regensburg , D-93040 Regensburg , Germany . Email:
Department of Chemistry , Faculty of Science , Masaryk University , Kamenice 5 , 62500 Brno , Czech Republic.
Chem Sci. 2015 Mar 1;6(3):2027-2034. doi: 10.1039/c4sc03709j. Epub 2015 Jan 6.
The chemoselective photoreduction of aldehydes in the presence of ketones was achieved using triethanolamine (TEOA) as sacrificial electron donor, proflavine (PF) as photocatalyst and [CpRh(iii)(bpy)Cl]Cl (Rh) as mediator. The reducing agent, which reacts with the carbonyl group was found to be [CpRh(iii)(bpy)H]Cl (Rh(iii)-H). Contrary to formate-based reduction, its slow photochemical 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 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 is a competing mechanism, which prevails in non-degassed systems.
在酮存在的情况下，使用三乙醇胺（TEOA）作为牺牲电子供体、硫酸原黄素（PF）作为光催化剂以及[CpRh(iii)(bpy)Cl]Cl（Rh）作为介质，实现了醛的化学选择性光还原。发现与羰基反应的还原剂是[CpRh(iii)(bpy)H]Cl（Rh(iii)-H）。与基于甲酸盐的还原不同，其缓慢的光化学生成使得能够从动力学上区分醛和酮。羰基化合物固有的反应性差异通过该方法转化为具有合成用途的反应选择性。底物范围广泛，产率优异。对反应机理的详细研究表明，PF三线态的光还原以及随后Rh还原生成Rh(iii)-H是主要反应途径，该途径对氧气高度敏感。Rh对PF单重态的氧化猝灭是一种竞争机制，在未除气的体系中占主导。
