Visible-light-initiated air-oxygenation of 5-hydroxymethylfurfural to maleic acid by iron(III) chloride

Developing methodologies for biomass-based 5-hydroxymethylfurfural (HMF) transformation into value-added molecules could provide potential solutions for HMF utilization in both academic and industrial communities and open up new pathways for chemicals production. Herein, we firstly describe a visible-light-induced FeCl3-catalyzed transformation of HMF into maleic acid (MA) at ambient temperature using air as oxidant. 100 % HMF conversion and 55 % yield of MA obtained using cheap and abundant iron catalyst under a mild reaction system (air, 456 nm light, ambient temperature, 5 h). Detailed mechanism research reveals that the chlorine radical, produced by ligand-to-metal charge transfer (LMCT) of FeCl3, is the key active species to initiate the HMF transformation. A new chlorine radical-involved mechanism of HMF oxidation including LMCT, hydrogen atom transfer (HAT), dehydroxymethylation, decarbonylation and β-scission processes is established through detection of crucial intermediates. This study could provide a new catalytic mode for biomass transformation using mild and efficient photocatalytic methodologies.