BIOMIMETIC OXIDATION STUDIES .5. MECHANISTIC ASPECTS OF ALKANE FUNCTIONALIZATION WITH FE,FE2O, AND FE4O2 COMPLEXES IN THE PRESENCE OF HYDROGEN-PEROXIDE

The biomimetic oxidation reactions of a variety of hydrocarbons with iron complexes, Fe2O(OAc)2(bpy)2Cl2 (1), Fe4O2(OAc)7(bpy)2(ClO4) (2), Fe2O(OAc)(tmima)2(ClO4)3 (3) (tmima = tris[(1-methylimidazol-2-yl)methyl]amine), and Fe(ClO4)3.6H2O (4), using H2O2/O2 as the oxidant were studied. Functionalization of cyclohexane gave cyclohexanol (CyOH) and cyclohexanone (CyONE). Complex 4 was the most effective and selective, 15-20 mmol of CyOH, 5-8 mmol of CyONE/mmol of Fe complex, and CyOH/CyONE ratios of approximately 2; however, the pseudo-first-order rate constants for the formation of CyOH and CyONE for 1 and 2 were approximately 1.4-1.6 times greater than those for 4. Complexes 1-3 gave 2-7 and 3-8 mmol of product/mmol of Fe complex, respectively, with CyOH/CyONE ratios of 0.6-1.1. The presence of an oxidizing intermediate was suggested by iodometric titration in the functionalization of cyclohexane with complexes 1-4 and H2O2. This intermediate was isolated from the reaction mixture and identified by C-13 NMR as cyclohexyl hydroperoxide (CyOOH) as compared to an independently prepared sample. The decomposition of CyOOH by 1-4 and H2O2 gave CyOH/CyONE ratios of 0.7, 0.9, 0.7, and 2.8, respectively, in the ranges observed in the actual cyclohexane oxidation reactions. These hydrocarbon oxidation reactions were also inhibited by 2,4,6-tri-tert-butylphenol. Reactions run under a sweep of argon gave mmol of product/mmol of Fe complex 0-31% of the normal values. These results are consistent with a free-radical chain mechanism in which an initially formed cyclohexyl radical is trapped by oxygen gas to give a cyclohexyl peroxyl radical, which abstracts a hydrogen atom to give CyOOH and carry the chain. The tertiary hydrogen of adamantane was selectively abstracted with complexes 1-4 to obtain normalized C3/C2 values of 3.5, 3.3, 3.4, and 5.6, respectively. Toluene was transformed to a mixture of benzyl alcohol, benzaldehyde, and o; m; and p-cresols with benzylic/aromatic activation ratios of 3.4, 4.2, 0.9, and 20, respectively, and indicate that hydroxyl radicals (aromatic C-H functionalization) may also participate. Functionalization of methane, ethane, and propane was also observed.