Significant effect of spin flip on the oxygen atom transfer reaction from (oxo)manganese(v) corroles to thioanisole: insights from density functional calculations

The electronic and structural features of (oxo)manganese(v) corroles and their catalyzed oxygen atom transfers to thioanisole in different spin states have been investigated by the B3LYP functional calculations. Calculations show that these corrole-based oxidants and their complexes with thioanisole generally have the singlet ground state, and their triplet forms are also accessible in consideration of the spin-orbit coupling interaction. Due to strong d-π conjugation interactions between Mn and the corrole ring arising from the π electron donation of the corrole moiety, the five-coordinated Mn approximately has the stable 18-electron configuration. The predicted free energy barriers for the singlet oxygen atom transfer reactions are generally larger than 22 kcal mol(-1), while the spin flip in reaction may remarkably increase the reactivity. In particular, the bromination on β-pyrrole carbon atoms of the meso-substituted (oxo)manganese(v) corrole strikingly enhances the spin-orbit coupling interaction and results in the dramatic increase of reactivity. The multiple spin changes are predicted to be involved in the low-energy reaction pathway. The present results show good agreement with the experimental observation and provide a detailed picture for the oxygen atom transfer reaction induced by the (oxo)manganese(v) corroles.     

Manganese(V)-oxo corroles in hydride-transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to manganese(V)-oxo corroles proceeds via proton-coupled electron transfer, followed by rapid electron transfer. The redox potentials (E(red)) of manganese(V)-oxo corroles exhibit a good correlation with their reactivity in hydride-transfer reactions.     

Hydrogen atom transfer reactions of imido manganese(V) corrole: one reaction with two mechanistic pathways

Hydrogen atom transfer (HAT) reactions of (tpfc)MnNTs have been investigated (tpfc = 5,10,15-tris(pentafluorophenyl)corrole and Ts = p-toluenesulfonate). 9,10-Dihydroanthracene and 1,4-dihydrobenzene reduce (tpfc)MnNTs via HAT with second-order rate constants 0.16 +/- 0.03 and 0.17 +/- 0.01 M(-1) s(-1), respectively, at 22 degrees C. The products are the respective arenes, TsNH(2) and (tpfc)Mn(III). Conversion of (tpfc)MnNTs to (tpfc)Mn by reaction with dihydroanthracene exhibits isosbestic behavior, and formation of 9,9',10,10'-tetrahydrobianthracene is not observed, suggesting that the intermediate anthracene radical rebounds in a second fast step without accumulation of a Mn(IV) intermediate. The imido complex (tpfc)Mn(V)NTs abstracts a hydrogen atom from phenols as well. For example, 2,6-di-tert-butyl phenol is oxidized to the corresponding phenoxyl radical with a second-order rate constant of 0.32 +/- 0.02 M(-1) s(-1) at 22 degrees C. The other products from imido manganese(V) are TsNH(2) and the trivalent manganese corrole. Unlike reaction with dihydroarenes, when phenols are used isosbestic behavior is not observed, and formation of (tpfc)Mn(IV)(NHTs) is confirmed by EPR spectroscopy. A Hammett plot for various p-substituted 2,6-di-tert-butyl phenols yields a V-shaped dependence on sigma, with electron-donating substituents exhibiting the expected negative rho while electron-withdrawing substituents fall above the linear fit (i.e., positive rho). Similarly, a bond dissociation enthalpy (BDE) correlation places electron-withdrawing substituents above the well-defined negative slope found for the electron-donating substituents. Thus two mechanisms are established for HAT reactions in this system, namely, concerted proton-electron transfer and proton-gated electron transfer in which proton transfer is followed by electron transfer.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Stabilization of high-valent metals by corroles: oxo[tris(pentafluorophenyl)corrolato]chromium(V)

The aerobic reaction of Cr(CO)6 with tris(pentafluorophenyl)corrole (H3(TpFPC)) in toluene gives the dark red oxochromium(V) compound (TpFPC)Cr(O), which has been characterized by X-ray crystallography, electrochemistry, and EPR spectroscopy. Short Cr-N (1.927-1.943 A) bonds as well as relatively large 14N and small 53Cr coupling constants suggest that sigma (N-->Cr) donation is responsible for the unusual stability of chromium(V) in this complex. The CrV/IV reduction potential (0.11 V vs Ag/AgCl) is 0.65 V below that of oxo(tetramesitylporphinato)chromium(V).     

Solvent effects on oxygen atom transfer reaction between manganese(V)-oxo corrole and alkene

Pseudo-first order reaction rate constants of 5,10,15-tris（pentafluorophenyl）corrole Mn（V）-oxo (F₁₅CMn（V）-oxo),5,15-bis（pentafluorophenyl）-10-（phenyl）corrole Mn（V）-oxo(F₁₀CMn（V）-oxo),5,15- bis（phenyl）-10-（pentafluorophenyl）corrole Mn（V）-oxo（F₅CMn（V）-oxo） and 5,10,15-tris（phenyl）corrole Mn（V）-oxo（F₀CMn（V）-oxo） with a series of alkene substrates in different solvents were determined by UV-vis spectroscopy.The results indicated that the oxygen atom transfer pathway between Mn（V）-oxo corrole and alkene is solvent-dependent.     

Ligand displacement and oxidation reactions of methyl(oxo)rhenium(V) complexes

Compounds that contain the anion [MeReO(edt)(SPh)](-) (3-) were synthesized with the countercations 2-picolinium (PicH+3-) and 2,6-lutidinium (LutH+3-), where edt is 1,2-ethanedithiolate. Both PicH+3- and MeReO(edt)(tetramethylthiourea) (4) were crystallographically characterized. The rhenium atom in each of these compounds exists in a five-coordinate distorted square pyramid. In the solid state, PicH+3- contains an anion with a short (d(SH) = 232 pm) and nearly linear hydrogen-bonded (N-H.S) interaction to the cation. Ligand substitution reactions were studied in chloroform. Displacement of PhSH by PPh(3) follows second-order kinetics, d[MeReO(edt)(PPh(3))]/dt = k[PicH+3-][PPh3], whereas with pyridines an unusual form was found, d[MeReO(edt)(Py)]/dt = k[PyH+3-][Py](2), in which the conversion of PicH+3- to PyH+3- has been incorporated. Further, added Py accelerates the formation of [MeReO(edt)(PPh3)], v = k.[PicH+3-].[PPh3].[Py]. Compound 4, on the other hand, reacts with both PPh(3) and pyridines, L, at a rate given by d[MeReO(edt)(L)]/dt = k.[4].[L]. When PicH+3- reacts with pyridine N-oxides, a three-stage reaction was observed, consistent with ligand replacement of SPh(-) by PyO, N-O bond cleavage of the PyO assisted by another PyO, and eventual decomposition of MeRe(O)(edt)(OPy) to MeReO(3). Each of first two steps showed a large substituent effect; Hammett analysis gave rho(1) = -5.3 and rho(2) = -4.3.     

Hydrogen peroxide oxidation of mustard-model sulfides catalyzed by iron and manganese tetraarylporphyrines. Oxygen transfer to sulfides versus H(2)O(2) dismutation and catalyst breakdown.

Fe(III)- and Mn(III)-meso-tetraarylporphyrin catalysis of H(2)O(2) oxidation of dibenzyl and phenyl-2-chloroethyl sulfides, 1, is investigated in ethanol with the aim of designing catalytic systems for mustard decontamination. The sulfide conversion, the sulfoxide and sulfone yields, the oxygen transfer from H(2)O(2) to the sulfide, and the catalyst stability depend markedly on the metal, on the substituents of its ligand, and on the presence or the absence of a cocatalyst, imidazole or ammonium acetate. With Fe, sulfones, the only oxidation products, are readily obtained whatever the ligand (TPP, F(20)TPP, or TDCPP) and the cocatalyst; the oxygen transfer is fairly good, up to 95% when the catalyst concentration is small ([1]/[Cat] = 420); the catalyst breakdown is insignificant only in the absence of any cocatalyst. With Mn, the sulfide conversion is achieved completely when the ligand is TDCPP or TSO(3)PP, but not F(20)TPP or TPP; a mixture of sulfoxide, 2, and sulfone, 3, is always obtained with [2]/[3] = 3.5-0.85 depending on the ligand and the cocatalyst (electron withdrawing substituents favor 3 and NH(4)OAc, 2). The catalyst stability is very good, but the oxygen transfer is poor whatever the ligand and the cocatalyst. These results are discussed in terms of a scheme in which sulfide oxygenation, H(2)O(2) dismutation, and oxidative ligand breaking compete. It is shown that the efficiency of the oxygen transfer is related not only to the rate constant of the dismutation route but also to the concentration of the active metal-oxo intermediate, most likely a perferryl or permanganyl species, i.e., to the rate of its formation.     

Electron transfer reaction of oxo(salen)chromium(V) ion with anilines

The kinetics of oxidation of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with lambda(max) at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a rho value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product analysis studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calculated values are compliant with the measured values.     

Direct kinetic studies of atom transfer and electron transfer reactions of hydroperoxo and high-valent oxo complexes of chromium

A macrocyclic hydroperoxo chromium complex reacts with triphenyl phosphine in an acid-catalyzed oxygen-atom transfer reaction, k = 850 M-2 s-1. In a competing process, the hydroperoxo complex is converted to a dioxo chromium(V) species, which reacts with PPh3 by electron transfer, k = 4.4 x 105 M-1 s-1.     

Hydrogen atom abstraction by a high-valent manganese(V)-oxo corrolazine

High-valent metal-oxo complexes are postulated as key intermediates for a wide range of enzymatic and synthetic processes. To gain an understanding of these processes, the reactivity of an isolated, well-characterized Mn(V)-oxo complex, (TBP8Cz)MnVO (1), (TBP8Cz = octakis(para-tert-butylphenyl)corrolazinato(3-)) has been examined. This complex has been shown to oxidize a series of substituted phenols (4-X-2,6-t-Bu2C6H2OH, X = C(CH3)3 (3), H, Me, OMe, CN), resulting in the production of phenoxyl radicals and the MnIII complex [(TBP8Cz)MnIII] (2). Kinetic studies have led to the determination of second-order rate constants for the phenol substrates, which give a Hammett correlation ((log k'x/k'H) vs sigmap+) with rho = -1.26. A plot of log k versus BDE(O-H) also reveals a linear correlation. These data, combined with a KIE of 5.9 for 3-OD, provide strong evidence for a concerted hydrogen-atom-abstraction mechanism. Substrates with C-H bonds (1,4-cyclohexadiene and 9,10-dihydroanthracene) are also oxidized via H-atom abstraction by 1, although at a much slower rate. Given the stability of 1, and in particular its low redox potential, (-0.05 V vs SCE), the observed H atom abstraction ability is surprising. These findings support a hypothesis regarding how certain heme enzymes can perform difficult H-atom abstractions while avoiding the generation of high-valent metal-oxo intermediates with oxidation potentials that would lead to the destruction of the surrounding protein environment.     

New oxorhenium(V) compound for catalyzed oxygen atom transfer from picoline N-oxide to triarylphosphines

The synthesis and characterization of a new oxorhenium(V) compound is reported; it is [MeReO(edt)(bpym)], 8, where edt = 1,2-ethanedithiolate and bpym = 2,2'-bipyrimidine. Compound 8 was characterized by NMR spectroscopy and single-crystal X-ray analysis. It exists as a six-coordinate Re(V) compound comparable to the previously known [MeReO(edt)(bpy)] and [MeReO(mtp)(bpy)]. Compound 8 catalyzes the oxygen-atom-transfer reaction PicO + PZ3 --> Pic + Z3PO, whereas the other two do not. The kinetics of this reaction with catalyst 8 follows the rate law -d[PicO]/dt = k[8][PicO]/(1 + c[PZ3]). With different phosphines, the rate law has the same k value, 4.17 L mol(-1) s(-1), but different c values. For tritolylphosphine, c = 67.5 L mol(-1) in benzene at 25 degrees C. A mechanism has been proposed to account for these findings. The data establish that an open coordination site on rhenium is necessary for oxygen-atom-transfer reactions.     

Catalysis of oxo transfer to prochiral sulfides by oxovanadium(v) compounds that model the active center of haloperoxidases

The catalytic properties of a new class of chiral vanadium compounds--[(S,S,S)-VO(OMe)L1] (5), [(S,S)-VO(OMe)L2] (6), [(S,S)-VO(OMe)L3] (7), and [(R,R,R)-VO(OMe)L4] (8), as well as the system VO(OiPr)(3)/(R,R,R)-H(2)L4 [H(2)L1=(S,S)-bis(2-hydroxypropyl)-(S)-1-phenylethylamine, 1; H(2)L2=(S,S)-bis(2-hydroxypropyl)benzylamine, 2; H(2)L3=(S,S)-bis(2-hydroxypropyl)isopropylamine), 3; (H(2)L4)=(R,R)-bis(2-phenylethanol)-(R)-1-phenylethylamine, 4]--in the asymmetric oxidation of prochiral sulfides by organic hydroperoxides have been investigated. Particular attention has been paid to the factors that guide the discrimination between the two prochiral faces of the sulfides (methyl p-tolyl sulfide and benzyl phenyl sulfide), to steric implications stemming from the oxidant (cumyl hydroperoxide and tert-butyl hydroperoxide), and to the specific complex used. As an example, (S)-methyl p-tolyl sulfoxide was obtained in a 31 % enantiomeric excess by use of cumyl hydroperoxide as oxidant and complex 5 as the catalyst, after 150 min at 0 degrees C and with 100 % conversion of the sulfide. The crystal and molecular structures of 5 and 6 reveal the close relationship between these complexes and the active center of vanadate-dependent haloperoxidases: the vanadium is in a slightly distorted trigonal-bipyramidal environment with the nitrogen and the methoxy group in the axial positions, and the oxo and alkoxide functions of L2 and L3 are the plane. The presence and equilibrium situation of isomers of the catalysts in solution has been investigated by (51)V EXSY and variable-temperature multinuclear NMR spectroscopy. An intermediately formed peroxo (ROO(-)) vanadium complex was detected by (51)V NMR spectroscopy.     

Electron transfer reactions relevant to atom transfer radical polymerization

The continuous development of more active and stable catalysts in atom transfer radical polymerization (ATRP) has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. Special attention is provided in this short review to such processes, including disproportionation, most pronounced in Cu-mediated ATRP, the reduction of radicals to carbanions or oxidation to carbocations, and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.     

Oxygen atom transfer from nitrite mediated by Fe(III) porphyrins in aqueous solution

Aqueous solutions of the iron(III) porphyrin complex FeIII(TPPS) (1, TPPS = tetra(4-sulfonatophenyl)-porphyrinato) and nitrite ion react with various substrates S to generate the ferrous nitrosyl complex FeII(TPPS)(NO) (2) plus oxidized substrate. When S is a water-soluble sulfonated phosphine, the product is the resulting monoxide. When air is introduced to the product solutions, 2 is rapidly reoxidized to 1; however, even in the absence of air, there is a slow regeneration of the ferric species with concomitant production of nitrous oxide. Thus, in an anaerobic aqueous environment, FeIII(TPPS) catalyzes oxygen atom transfer from nitrite ion to substrates with the eventual formation of N2O.     

Pyridylazole chelation of oxorhenium(V) and imidorhenium(V). Rates and trends of oxygen atom transfer from Re(V)O to tertiary phosphines

The concerned azoles are 2-(2-pyridyl)benzoxazole (pbo) and 2-(2-pyridyl)benzthiazole (pbt). These react with ReOCl(3)(PPh(3))(2) in benzene, affording Re(V)OCl(3)(pbo) and Re(V)OCl(3)(pbt), which undergo facile oxygen atom transfer to PPh(2)R (R = Ph, Me) in dichloromethane solution, furnishing Re(III)(OPPh(2)R)Cl(3)(pbo) and Re(III)(OPPh(2)R)Cl(3)(pbt). The oxo species react with aniline in toluene solution, yielding the imido complexes Re(V)(NPh)Cl(3)(pbo) and Re(V)(NPh)Cl(3)(pbt). The X-ray structures of pbt, ReOCl(3)(pbt), Re(OPPh(3))Cl(3)(pbt), and Re(NPh)Cl(3)(pbo) are reported. The lattice of pbt consists of stacked dimers. In all the complexes the azole ligand is N,N-chelated and the ReCl(3) moiety is meridionally disposed. In ReOCl(3)(pbt) the metal-oxo bond length is 1.607(9) A. The second-order rates and the associated activation parameters of the oxygen atom transfer reactions of the Re(V)O chelates with PPh(2)R are reported. The large and negative entropy of activation (approximately -24 eu) is consistent with an associative pathway involving nucleophilic phosphine attack. The rate increases with phosphine basicity (PPh(2)Me > PPh(3)) and azole heteroatom electronegativity (O(pbo) > S(pbt)). Logarithmic rate constants for ReOCl(3)(pbo), ReOCl(3)(pbt), and ReOCl(3)(pal) are found to correlate linearly with Re(VI)O/Re(V)O reduction potentials (pal is pyridine-2-(N-p-tolyl)aldimine). The relatively low rate constant of ReOCl(3)(pbt) compared to that of ReOCl(3)(pal) is consistent with the observed shortness of the metal-oxo bond in the former. Crystal data are as follows: (pbt) empirical formula C(12)H(8)N(2)S, crystal system orthorhombic, space group Pca2(1), a = 13.762(9) A, b = 12.952(8) A, c = 11.077(4) A, V = 1974(2) A(3), Z = 8; (ReOCl(3)(pbt)) empirical formula C(12)H(8)Cl(3)N(2)OSRe, crystal system monoclinic, space group P2(1)/c, a = 11.174(7) A, b = 16.403(10) A, c = 7.751(2) A, beta = 99.35(4) degrees, V = 1401.8(13) A(3), Z = 4; (Re(NPh)Cl(3)(pbo)) empirical formula C(18)H(13)Cl(3)N(3)ORe, crystal system monoclinic, space group P2(1)/c, a = 9.566(6) A, b = 16.082(8) A, c = 11.841(5) A, beta = 94.03(4) degrees, V = 1817(2) A(3), Z = 4.     

Oxygen transfer reactions. 4. Reaction of high valent oxoruthenium compounds with sulfides

The oxidation of methoxy substituted benzyl phenyl sulfides can be used to distinguish between oxidants that react by single electron transfer (followed by oxygen rebound) and those which react by direct oxygen atom transfer in a two-electron process. Transfer of a single electron results in the formation of an intermediate radical cation, which can undergo C-S bond cleavage and deprotonation reactions leading to the formation of methoxy substituted benzyl derivatives, methoxy substituted benzaldehydes, and diphenyl disulfide. The oxidation of 4-methoxybenzyl phenyl sulfide and 3,4,5-trimethoxybenzyl phenyl sulfide by oxidants known to participate in single electron transfers (Ce(4+), Mn(3+), and Cr(6+)) results in the formation of the corresponding benzaldehydes, benzyl alcohols, benzyl acetates, and benzyl nitrates in variable yields. However, the only products obtained from the oxidation of the same compounds with RuO(4), RuO(4-), and RuO(4)(2-) are sulfoxides and sulfones. Therefore, it is concluded that the oxidation of sulfides by oxoruthenium compounds likely proceeds by a concerted mechanism.     

Electronic and steric effects on the oxygenation of organic sulfides and sulfoxides with oxo(salen)chromium(V) complexes

The kinetics of oxygenation of several para-substituted phenyl methyl sulfides and sulfoxides with a series of 5-substituted and sterically hindered oxo(salen)chromium(V) complexes have been studied by a spectrophotometric technique. Though the reaction of sulfides follows simple second-order kinetics, sulfoxides bind strongly with the metal center of the oxidant and the oxygen atom is transferred from the oxidant-sulfoxide adduct to the substrate. The reduction potentials, E(red), of eight Cr(V) complexes correlate well with the Hammett sigma constants, and the reactivity of the metal complexes is in accordance with the E(red) values. The metal complexes carrying bulky tert-butyl groups entail steric effects. Organic sulfides follow a simple electrophilic oxidation mechanism, and the nonligated sulfoxides undergo electrophilic oxidation to sulfones using the oxidant-sulfoxide adduct as the oxidant. Sulfoxides catalyze the Cr(V)-salen complexes' oxygenation of organic sulfides, and the catalytic activity of sulfoxides is comparable to pyridine N-oxide and triphenylphosphine oxide. The rate constants obtained for the oxidation of sulfides and sulfoxides clearly indicate the operation of a pronounced electronic and steric effect in the oxygenation reaction with oxo(salen)chromium(V) complexes.     

Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate)

Mechanistic studies indicate that the oxomolybdenum(vi) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine-N-oxides in a reaction where the oxygen is delivered to molybdenum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2-benzoquinone.