NATURE OF THE REACTIVE INTERMEDIATES FROM THE IRON-INDUCED ACTIVATION OF HYDROGEN-PEROXIDE - AGENTS FOR THE KETONIZATION OF METHYLENIC CARBONS, THE MONOOXYGENATION OF HYDROCARBONS, AND THE DIOXYGENATION OF ARYLOLEFINS

The reaction efficiencies and product selectivities for a group of ML/HOOH/substrate/solvent systems have been evaluated. The catalysts include FeII(PA)2, FeII(DPA)22-, FeIIICl3, [FeII(OPPh3)4](ClO4)2, [FeII(bpy)2](ClO4)2, [FeII-(MeCN)4](ClO4)2, [FeII(O2bpy)2](ClO4)2, and [CoII(bpy)2](ClO4)2, the solvents include (py)2HOAc, (MeCN)4py, and MeCN, and the substrates include c-C6H12, PhCH3, PhCH2CH3, c-C6H10, and c-PhCH = CHPh. When the HOOH/ML ratio is at least 10, all of the complexes catalytically ketonize methylenic carbons and dioxygenate arylolefins. The most effective catalyst systems are FeII(PA)2/py2(HOAc) (e.g., with c-C6H12 the process is 70% efficient and 95% selective), FeII(OPPh3)42+/MeCN, and CoII(bpy)22+/(MeCN)4py, which activate nucleophilic HOOH via their electrophilicity. The reactive intermediates appear to be [(PA)2FeIV(OH)(OOH), 3], [(Ph3PO)42+FeIV(OH)(OOH), 3'], and [(bpy)22+CoIV(OH)(OOH), 3"'], which are representative of all of the catalysts. These same intermediates also dioxygenate arylolefins. A precursor intermediate [a one-to-one ML/HOOH adduct; e.g., [(PA)2-FeIIOOH + pyH+, 1]] monooxygenates (oxidizes) hydrocarbons to alcohols and Rpy and reacts with excess HOOH to form species 3. Although the latter is the favored path for the FeII(PA)2/(py)2HOAc and CoII(bpy)22+/(MeCN)4py systems, the monooxygenation path is dominant for the FeIIICl3/MeCN and FeII(OPPh3)42+/MeCN systems. In the presence of olefins, species 1 undergoes dehydration to (PA)2FeIV = O, 2, which transforms them to epoxides. Similar intermediates and reaction paths are observed when t-BuOOH is used in place of HOOH. With t-BuOOH the reactivity of species 1 with substrate (RH) becomes dominant to give Rpy or ROH, and species 3 reacts with methylenic carbons to give mixtures of ROOBu-t and ketone. Evaluation of the kinetic isotope effect with cyclohexane and toluene for the full range of catalysts and solvents gives k(H)/k(D) values of 2.4 to > 10 for the ketonization of c-C6H12, 2.7-4.7 for PhCH3, 1.4-2.9 for the hydroxylation process, and 1.7 for the Rpy production process [FeII(PA)2/(py)2HOAc/c-C6H12].