Metal [Fe(II), Cu(I), Co(II), Mn(III)] hydroperoxide-induced activation of dioxygen (center dot O-2 center dot) for the ketonization of hydrocarbons: oxygenated Fenton Chemistry

Recent reports have established that Fenton chemistry involves the formation of hydroperoxide (ROOH) adducts (1) of reduced transition metals [iron(II), copper(I), and cobalt(II)] via nucleophilic addition {e.g. (FeLx)-L-II + ROOH reversible arrow(B) [(Lx-FeOOR)-O-II, BH+](1)} (B=py or H2O). These reactive intermediates (1) react with (a) excess catalyst ((FeLx)-L-II) to form (LxFeOH)-O-III(R); (b) excess ROOH to form O-2, H2O and ROH; (c) excess hydrocarbon (RH) to form ROH (Fenton chemistry); and (d) ambient dioxygen (O-2) to form adducts of 1 {e.g, 1(O-2)=[(Lx-FeIII)(O-2)OOR(BH+)] (5)}. (Note: All bracketed species are speculative) but consistent with the product profiles.) The latter (5) reacts selectivity with (a) methylenic carbon centers of hydrocarbons (-CH2-) to form ketones [e.g. c-C6H12 -->(5) c-C6H10(O)] (oxygenated Fenton chemistry) and (b) arylolefins via dioxygenation [e.g. cis-PhCH=CHPh -->(5) 2PhCH(O)]. The species 5 formed by t-BuOOH and O-2 with Fe-II(PA)(2) (in 2:1 py/HOAc) (PAH = picolinic acid; 2-carboxyl pyridine), Fe-II(bpy)(2)(2+) (MeCN), Fe-II(OPPh3)(4)(2+) (MeCN), Cu-I(bpy)(2)(+) (4:1 MeCN/py) and Co-II(bpy)(2)(3+) (4:1 MeCN/py) ketonize the methylenic centers of hydrocarbons (e.g. c-C6H12, PhCH2CH3, c-C6H10). With the allylic centers of cyclohexene (c-C6H10) the various species 5 exhibit catalytic turnover of O-2 [e.g. 10 mM Fe-II(bpy)(2)(2+)/20 mM t-BuOOH yields 86 mM c-C6H8(O) (4.3 O-2 turnovers per t-BuOOH)]. For analogous combinations with other complexes the turnover numbers are: Fe-II(OPPh3)(4)(2+), 3.0; Fe-II(PA)(2), 3.0; Cu-I(bpy)(2)(+), 1.3: Co-II(bpy)(2)(2+), 0.7; Mn-II(salen)(OAc), 8.0; Mn-III(bpy)(2)(OAC)(3), 7.0. With PhCH2CH3 as the substrate the O-2-turnover numbers are: Fe-II(PA)(2), 3.2; Fe-II(bpy)(2)(+), 1.8; Fe-II(OPPh3)(4)(2+), 1.9; Cu-I(bpy)(2)(+), 1.4: Co-II(bpy)(2)(2+), 0.9; Mn-III(salen)(OAC) (salen-Schiff base from two salicyladehydes and ethylenediamine), 2.0; Mn-III(bpy)(2)(OAc)(3), 1.6. The several species ! react as hydroxylases (including aromatic substrates) and dehydrogenases, and species 5 react as ketonization agents and dioxygenases. With excess HOOH its MLx-induced disproportionation to O-2 and H2O results in production of species 5 without external sources of O-2. (C) 1997 Elsevier Science S.A.