A novel, environment-friendly method to prepare pyranones from furfural alcohols via photocatalytic O2 oxidation in an aqueous phase

• Published in: Green chemistry : an international journal and green chemistry resource : GC 2023-01, Vol.25 (1), p.196-210
• Main Authors: Zhou, Bei, Yun-Feng, Tao, Yu-Juan, He, Lan-Xiang, Liu, Zu-Hui, Chang, Xiang-Hong, Li, Lin, Tong, Guan-Ben Du
• Format: Article
• Language: English
• Subjects: Alcohol; Alcohols; Atmosphere; Furfural; Green chemistry; Intermediates; NMR; Nuclear magnetic resonance; Oxidation; Oxygen; Photocatalysis; Reaction kinetics; Reagents; Ring opening; Room temperature; Single electrons; Singlet oxygen
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/d2gc03344e

This study developed a novel photocatalytic method to synthesize pyranones from furfural alcohols. By using 0.2 mol% equivalent of tris-Ir(ppy)3 as a photocatalyst under visible light and O2 as the reaction atmosphere, furfural alcohols were rapidly oxidized and hydrolyzed in the aqueous reaction solution to produce diol intermediates, which further underwent hydrolysis, furan ring-opening and rearrangement to form the final product pyranones. The entire reaction was carried out at room temperature with a yield as high as 86% and a conversion efficiency of 90%. This method is general and suitable for furfural alcohols with different substituted groups on the 5- and α-OH sites. We used in situ IR and real-time NMR to examine the reaction kinetics, GC-MS and isotope-labeling experiments to track the source of atom transfer in the reaction, and EPR to identify the singlet oxygen. The results showed that the reaction followed a first-order rate equation, and O2 oxidation was the rate-determining step. The transferred H atom came from the H2O solvent, the O atom from the O2 atmosphere, and the singlet oxygen from the O2 photocatalysis. The UV-vis and cyclic voltammetry results indicated that this photocatalysis reaction involved a single electron transformation process. We also used DFT calculation to simulate the reaction routes and showed that the tris-Ir(ppy)3 photocatalysis O2 oxidation in the aqueous phase was the critical step of endoperoxide oxidation, which differs from oxygen onium ion oxidation in the classic Achmatowicz rearrangement reaction. Our method can carry out a 10.0 gram-scale reaction with a 78% yield. It avoids stoichiometric consumption of toxic or corrosive oxidative reagents in the Achmatowicz rearrangement reaction. It may provide a novel, efficient, and “green” alternative to the Achmatowicz rearrangement reaction.