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 [(CH 3 ) 4 N] 4 PMo 11 VO 40 /ascorbic acid/TEMPO/O 2 catalytic system was carefully investigated. UV–vis, ESR and 1 H 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 [(CH 3 ) 4 N] 4 PMo 11 VO 40 /ascorbic acid/TEMPO/O 2 catalytic system with sufficient ascorbic acid after 400   min.