Selective Oxygen Transfers with Iron(III) Porphyrin Nitrite

Abstract

The reaction of octaethylporphyrin iron(III) chloride with potassium crown ether (18-crown-6) nitrite in N-methylpyrrolidone−1% acetic acid under argon generates the iron(III) nitrite salt (PFeNO₂). The latter is a unique and selective oxygen atom transfer reagent. The reaction of a broad range of substrates (S) proceeds quantitatively to yield the oxidized substrate and the iron(II) porphyrin−nitrosyl adduct: PFeNO₂ + S → PFeNO + SO. Diatomic molecules to which oxygen is directly transferred from PFeNO₂ are NO, CO, and O₂. The conversion NO to NO₂ is shown via ¹⁵NO₂^- labeling experiments to proceed exclusively by the O atom transfer process. The ozone, generated from dioxygen, was trapped with nitrite ion and the two olefins 2-methyl-2 butene and 2,3-dimethyl-2 butene. These substances are inert to PFeNO₂ under argon. However, in an oxygen-saturated reaction mixture, nitrite produced nitrate. The olefins, following reduction of the reaction mixture with Zn/HOAc, yielded 1 mol of acetone and acetaldehyde and 2 mol of acetone, respectively. Other simple O atom transfers under argon were observed with dimethyl sulfide and triphenylphosphine. The PFeNO₂ reagent shows a preference for O insertion into allylic, benzylic, and aldehydic C−H bonds. Thus, no olefin containing these moieties is epoxidized. However, styrene and cis-stilbene are converted to styrene oxide and cis-stilbene oxide, respectively. The double oxidation of allylbenzene to trans-cinnamaldehyde entails an allylic rearrangement that suggests radical character to the O insertion process. However, no kinetic evidence for this was obtained. The reaction is an overall third-order process, rate = k(PFe^III)(NO₂^-)(S). There was no correlation of observed rates with relevant C−H bond dissociation energies of substrates. The fastest reacting substrate was nitric oxide (k₂₂° = 52 M^-2 s^-1) and the slowest was toluene (k₅₀° = 6.3 × 10^-4 M^-2 s^-1). The range and selectivity of these O atom transfers sets them apart from the catalytic oxidations brought about by reactions of iron(III) porphyrins with peroxides, iodosoaryls, hypochlorite, and other oxidants. The driving force for the relatively mild oxidations with PFeNO₂ resides in the thermodynamic stability of the heme−NO adduct. Given the broad presence of nitrite in the environment and the ubiquity of porphyrins in the biosphere, the activation of nitrite by iron porphyrins has both an environmental and biochemical significance.