Synthesis, characterization, and reactivity of oxoiron(IV) porphyrin π-cation radical complexes bearing cationic N-methyl-2-pyridinium group

Electric field effect of positive and negative charges of substituents on electronic structure and reactivity of oxoiron(IV) porphyrin π-cation radical complex

Electric field effect by the positive and negative changes near the active site is an important factor for controlling the reactivity of metalloenzymes. Previously, we reported that the positive charge of the N-methyl-2-pyridinium cation increases the reactivity of oxoiron(IV) porphyrin π-cation radical complex (Compound I), due to the attractive Coulomb interaction with electrons in Compound I. To further investigate the electric field effect, we study here the effect of the negative charge of the sulfonate group on the electronic structure and reactivity using Compound I of meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrin (TMPS-I). Although Compound I has been known as a very unstable complex, TMPS-I is very stable in 0.1 M acetate buffer at pH = 6. The half-life of TMPS-I is estimated to be 6.9 × 10³ s, which is the longest in Compound I previously reported. The redox potential of TMPS-I is estimated to be 0.76 V vs SCE in phosphate buffer, pH = 10. Kinetic analysis with stopped-flow technique indicates TMPS-I is less reactive than Compounds I reported previously. However, ¹H NMR and EPR spectra of TMPS-I are very close to those of Compounds I reported previously. The DFT calculations show that the orbital energy of Compound I is drastically altered by the positive and negative charges on the meso-phenyl group, suggesting the electric field effect. The difference of the reactivity of Compound I can be rationalized with the change of the orbital energy caused by the intramolecular electric field effect of the positive and negative charges.

Heme compound II models in chemoselectivity and disproportionation reactions

Heme compound II models bearing electron-deficient and -rich porphyrins, [Fe^IV(O)(TPFPP)(Cl)]⁻ (1a) and [Fe^IV(O)(TMP)(Cl)]⁻ (2a), respectively, are synthesized, spectroscopically characterized, and investigated in chemoselectivity and disproportionation reactions using cyclohexene as a mechanistic probe. Interestingly, cyclohexene oxidation by 1a occurs at the allylic C–H bonds with a high kinetic isotope effect (KIE) of 41, yielding 2-cyclohexen-1-ol product; this chemoselectivity is the same as that of nonheme iron(iv)-oxo intermediates. In contrast, as observed in heme compound I models, 2a yields cyclohexene oxide product with a KIE of 1, demonstrating a preference for C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C epoxidation. The latter result is interpreted as 2a disproportionating to form [Fe^IV(O)(TMP⁺˙)]⁺ (2b) and Fe^III(OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a, 1a does not disproportionate under the present reaction conditions. DFT calculations confirm that compound II models prefer C–H bond hydroxylation and that disproportionation of compound II models is controlled thermodynamically by the porphyrin ligands. Other aspects, such as acid and base effects on the disproportionation of compound II models, have been discussed as well.

Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.