The effect of TEMPO in the hydroxylation of benzene to phenol on the [(CH3)4N]4PMo11VO40/ascorbic acid/TEMPO/O2 catalytic system: Formation of ascorbic acid radicals through hydrogen exchange of ascorbic acid and TEMPO
Hydroxylation of benzene to phenol in the [(CH3)4N]4PMo11VO40/ascorbic acid/TEMPO/O2 catalytic system was carefully investigated. UV–vis, ESR and 1H NMR studies showed ascorbic acid radicals were formed through the hydrogen exchange of ascorbic acid with TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl radicals) during the reaction. In acetonitrile, the ascorbic acid would be partly oxidized to 2-hydroxy-2-buten-4-olide (α-tetronic acid, isotetronic acid, compound 1) without TEMPO. The interaction of TEMPO and ascorbic acid restrained the oxidative dissociation of ascorbic acid and promoted the rate of the hydroxylation. Aqueous acetic acid solvent can also restrain the oxidative dissociation of ascorbic acid. In aqueous acetic acid, the yield of phenol could reach 18.9% in the [(CH3)4N]4PMo11VO40/ascorbic acid/TEMPO/O2 catalytic system with sufficient ascorbic acid after 400 min.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (117 K)Download as PowerPoint slideHighlights► Oxidative dissociation of ascorbic acid decreased its efficiency for the reaction. ► The interaction between TEMPO and ascorbic acid increase the hydroxylation rate. ► Oxidative dissociation of the ascorbic acid did not happen in aqueous acetic acid. ► 18.9% yield of phenol was achieved with TEMPO in aqueous acetic acid.
Journal: Applied Catalysis A: General - Volumes 415–416, 16 February 2012, Pages 22–28