AROMATIC HYDROXYLATION BY FENTON REAGENTS (REACTIVE INTERMEDIATE[L(X)(+)FE(II)OOH(BH+)], NOT FREE HYDROXYL RADICAL (HO-CENTER-DOT))

Several iron complexes [Fe-II(bpy)(2)(2+), Fe-II(OPPh(3))(4)(2+), and Fe-II(PA)(2)] in combination with hydrogen peroxide (HOOH) catalytically hydroxylate aromatic substrates (ArH). The base-induced nucleophilic addition of HOOH to the electrophile iron center yields the reactive intermediate of Fenton reagents [Fe(II)L(x)(2+) + HOOH reversible arrow(B) L(x) (FeOOH)-Fe-+-O-II(BH+)(1)]. The latter includes a 'stabilized' hydroxyl radical that is able to replace an aromatic hydrogen (H) with a hydroxyl group (HO) via an initial addition reaction. With PhCH(3) and PhCH(2)CH(3) as substrates free HO . (from the radiolysis of H2O) reacts via aryl addition (97 and 85%, respectively) to give Ar-Ar as the predominant product, whereas 1 favors H-atom abstraction from the alkyl group (50 and 80%, respectively) and the only detectable products from aryl addition are the respective substituted phenols (o:p-ArOH). Other substituted benzenes (PhX) undergo addition by free HO. at the ortho and para aryl carbons (o:p ratio, 2), followed by dimerization and elimination of two H2O molecules to yield substituted biphenyls. In contrast, 1 reacts with PhX to yield substituted phenol (ArOH; o:p ratio, 0.5-1.1). With phenol (PhOH) as the substrate, reaction with 1 yields mainly catechol (o-Ar(OH)(2); o:p ratio, 20). In a solvent matrix of MeCN:H2O(3:1 mol:mol ratio) the reaction efficiencies with Fe-II(bpy)(2)(2+) and Fe-II(OPPh(3))(4)(2+) for the hydroxylation of benzene to phenol are 36 and 42%, respectively (product per (HOOH).