Hydroxylation of Benzene with Hydrogen Peroxide Catalyzed by Fe(III) Complexes of β-Cyclodextrin-Protocatechuate
Hidetake SAKURABA*, Masaaki HORII and Toshiki TAKEZUTSUMI
Department of Industrial Chemistry, Kanto Gakuin University; 4834 Matsuura, Kanazawa-ku, Yokohama-shi 236-8501 Japan
β-Cyclodextrin derivatives as ligands (β-CD-6-PA and β-CD-2-PA) were prepared by the monoesterification of the hydroxy group at the C-6 or C-2 position of β-CD with 3,4-dihydroxybenzoic acid (protocatechuic acid, PAH) with KF in DMF. Direct hydroxylation of benzene with hydrogen peroxide was performed by the iron(III) coordinated the β-CD ligands (the improved Hamilton system). The β-CD-6-PA and iron(III) formed a highly active catalyst for the hydroxylation, composed of 1:2 complex (Fe:ligand)at pH of 2.5, but an inert one (1:3 complex) at pH of 6.0. The stability of the iron(III)-β-CD complex (1:3 (λmax 536 nm, ε=2030, log K´ =13.8)) was higher than that of 1:2 complex (λmax 667 nm, ε=860, log K´=8.66). At the optimum pH of 2.5, when the β-CD-6-PA-Fe(III) catalyst was used for the hydroxylation, yields of catechol, hydroquinone, and phenol were 73, 5 and 22%, respectively. On the other hand,when the β-CD-2-PA-Fe(III) catalyst was used, these yields were 10, 24 and 12%, respectively. The observed selectivity of products in the present hydroxylation is discusssed in terms of a position of PA substituent of β-CD-PA-Fe(III) catalyst.
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