Intermediates in the Epoxidation of Alkenes by Cytochrome P-450 Models. 3. Mechanism of Oxygen Transfer from Substituted Oxochromium(V) Porphyrins to Olefinic substrates

Article abstract of DOI:10.1021/ja00183a030

Studies were initiated with the following chromium(IV) oxo porphyrin species: <5,10,15,20-tetraphenylporphinato>chromium(IV) oxide ((TPP)Cr(IV)(O)), <5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphinato>chromium(IV) oxide ((Me₁₂TPP)Cr(IV)(O)), <5,10,15,20-tetrakis(2,6-dichlorophenyl)porphinato>chromium(IV) oxide ((Cl₈TPP)Cr(IV)(O)), <5,10,15,20-tetrakis(2,6-diflurophenyl)porphinato>chromium(IV) oxide ((F₈TPP)Cr(IV)(O)), and <5,10,15,20-tetrakis(pentaflurophenyl)porphinato>chromium(IV) oxide ((F₂₀TPP)Cr(IV)(O)).Cyclic voltammetric (CH2Cl2, V, SCE) 1e oxidationpotentials for the couples (Porph)Cr(IV)(O)/(Porph)Cr(V)(O)(ClO4) have been determined <(TPP)Cr(IV)(O), 0.790; (Me₁₂TPP)Cr(IV)(O), 0.825; (Cl₈TPP)Cr(IV)(O), 0.895; (F₈TPP)Cr(IV)(O), 0.975; (F₂₀TPP)Cr(IV)(O), chemically irreversible>.Standard solutions of (TPP)Cr(V)(O)(ClO4), (Me₁₂TPP)Cr(V)(O)(ClO4), (Cl8TPP)Cr(V)(O)(ClO4), and (F₈TPP)Cr(V)(O)(ClO4) were obtained by controlled-potential bulk electrolysis of the corresponding chromium(IV) oxo porphyris, and these solutions were employed in kinetic and product studies of the oxidation of a number of alkenes (CH2Cl2 solvent; 30 deg C).Under the pseudo-first-order condition of >> <(Porph)Cr(V)(O)(ClO4)>, the following reactions have been show to take place: (i) (Porph)Cr(V)(O)(ClO4)+alkene --> (Porph)Cr(III)(X)+epoxide (k₁); (ii) (Porph)Cr(V)(O)(ClO4)+(Porph)Cr(III)(X) <*> (Porph)Cr(IV)(O)+(Porph)Cr(IV)(X) (k₂/k₃); and (iii) spontaneous conversion of (Porph)Cr(V)(O) to (Porph)Cr(IV)(O) possibly by oxidation of solvent or supporting electrolyte (k₄).The reversibility (k₂/k₃) of (ii) must be related to the formation of two different chromium(IV) porphyrinspecies, one of which is stabilized by oxo axial ligation ((Porph)Cr(IV)(O)) and one of which is not ((Porph)Cr(IV)(X); where X=Cl(-) or ClO4(-)).This is supported by the observation that the spectrum of (Me₁₂TPP)Cr(IV)(O) remains unchanged in CH2Cl2 solutions of norbornene.Values of the second-order rate constants (k₁) for alkene epoxidations were determined by both the slopes of plots of vs the pseudo-first-order rate constants (k_obsd) for disappearance of <(Porph)Cr(V)(O)(ClO4)> and by computer simulation of the time courses for disappearance and appearance of the species (Porph)Cr(V)(O)(ClO4), (Porph)Cr(III)(X), and (Porph)Cr(IV)(O) according to reactions (i)-(iii).The two methods provided like values of k₁.Values of k₁ (M^-1 s^-1), determined from dependence of k_obsd on , for reactions with (Me₁₂TPP)Cr(V)(O)(ClO4) are the following: 5.4x1E-2, norbornene; 8.78x1E-4, cis-cyclooctene; 6.82x1E-3, cis-stilbene; and 5.19x1E-13, cyclohexene.With (Cl₈TPP)Cr(V)(O)(ClO4) the values of k₁ (M^-1 s^-1) are 9.56x1E-1 for norbornene and 2.13x1E-2 for cis-cyclooctene.Use of computer simulation of...