Acylperoxo Complex Research Articles

Structure and reactivity of a seven-coordinate ruthenium acylperoxo complex.

The use of metal-acylperoxo complexes as oxidants has been little explored. Herein we report the synthesis and characterization of the first seven-coordinate Ru-acylperoxo complex, [RuIV(bdpm)(pic)2(mCPBA)]+ (H2bdpm = [2,2'-bipyridine]-6,6'-diylbis(diphenylmethanol); pic = 4-picoline; HmCPBA = m-chloroperbenzoic acid). This complex is a highly reactive oxidant for C-H bond activation and O-atom transfer reactions.

A mononuclear nonheme manganese(III)‐acylperoxo complex: Synthesis, characterization, and reactivity studies

AbstractA mononuclear nonheme manganese(III)‐acylperoxo complex bearing a simple picolylamine‐based linear ligand was synthesized and characterized by various spectroscopic methods along with theoretical calculations. The reactivity of the manganese(III)‐acylperoxo complex was investigated in the oxygen atom transfer, CH and OH bond activation reactions. The oxidations of organic substrates followed first‐order kinetics with respect to substrates and featured nonradical‐type, suggesting that the manganese(III)‐acylperoxo complex was the reactive oxidant. The CH bond activation of hydrocarbon and the OH bond activation of alcohol proceeded via a rate‐limiting H‐atom abstraction reaction in a concerted manner.

Mononuclear Nonheme High-Spin Iron(III)-Acylperoxo Complexes in Olefin Epoxidation and Alkane Hydroxylation Reactions.

Mononuclear nonheme high-spin iron(III)-acylperoxo complexes bearing an N-methylated cyclam ligand were synthesized, spectroscopically characterized, and investigated in olefin epoxidation and alkane hydroxylation reactions. In the epoxidation of olefins, epoxides were yielded as the major products with high stereo-, chemo-, and enantioselectivities; cis- and trans-stilbenes were oxidized to cis- and trans-stilbene oxides, respectively. In the epoxidation of cyclohexene, cyclohexene oxide was formed as the major product with a kinetic isotope effect (KIE) value of 1.0, indicating that nonheme iron(III)-acylperoxo complexes prefer C═C epoxidation to allylic C-H bond activation. Olefin epoxidation by chiral iron(III)-acylperoxo complexes afforded epoxides with high enantioselectivity, suggesting that iron(III)-acylperoxo species, not high-valent iron-oxo species, are the epoxidizing agent. In alkane hydroxylation reactions, iron(III)-acylperoxo complexes hydroxylated C-H bonds as strong as those in cyclohexane at -40 °C, wherein (a) alcohols were yielded as the major products with high regio- and stereoselectivities, (b) activation of C-H bonds by the iron(III)-acylperoxo species was the rate-determining step with a large KIE value and good correlation between reaction rates and bond dissociation energies of alkanes, and (c) the oxygen atom in the alcohol product was from the iron(III)-acylperoxo species, not from molecular oxygen. In isotopically labeled water (H2(18)O) experiments, incorporation of (18)O from H2(18)O into oxygenated products was not observed in the epoxidation and hydroxylation reactions. On the basis of mechanistic studies, we conclude that mononuclear nonheme high-spin iron(III)-acylperoxo complexes are strong oxidants capable of oxygenating hydrocarbons prior to their conversion into iron-oxo species via O-O bond cleavage.

Theoretical Study on Sulfoxidation Reactivity by a Nickel Acylperoxo Complex

Theoretical Study on Sulfoxidation Reactivity by a Nickel Acylperoxo Complex

Structural Characterization and Oxidation Reactivity of a Nickel(II) Acylperoxo Complex

The nickel(II)-acylperoxo complex [Ni(Tp(CF3Me))(κ(2)-mCPBA)] (1(CF3Me)) [Tp(CF3Me) = hydrotris(3-trifluoromethyl-5-methylpyrazolyl)borate, mCPBA = m-chloroperbenzoate] was isolated and fully characterized. The electrophilic oxygenation ability of 1(CF3Me) toward sulfides and olefins was confirmed. The Michaelis-Menten-type behavior of thioanisole oxygenation indicates the existence of a pre-equilibrium of substrate association in the reaction. In addition, 1(CF3Me) retains H-atom abstraction ability for hydrocarbons with activated methylene C-H bonds (e.g., fluorene). The oxidations of styrenes and these readily oxidizable hydrocarbons follow second-order kinetics, first-order each with respect to 1(CF3Me) and substrate. The lack of clear acceleration in the decay of 1(CF3Me) in the presence of substrates with high C-H bond dissociation energies (e.g., cyclohexane) suggests that another reaction pathway contributes through the O-O-cleaved intermediate.

Isolation of a MnIV acylperoxo complex and its monooxidation ability

The first example of monooxygenation by a high-valent Mn(IV) complex with a peroxide is described. A key Mn(IV) acylperoxo intermediate, which uses m-chloroperoxybenzoic acid as the oxygen donor, is directly observed by electro-spray ionization mass spectrometry and resonance Raman spectroscopy.

The nature of active species in catalytic systems based on non-heme iron complexes, hydrogen peroxide, and acetic acid for selective olefin epoxidation

The catalytic systems [(BPMEN)FeII(CH3CN)2](ClO4)2/H2O2/CH3OOH and [(TPA)FeII(CH3CN)2](ClO4)2/H2O2/CH3OOH, where BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane and TPA = tris(2-pyridylmethyl)amine, provide selective olefin epoxidation. Proton NMR studies showed that the mononuclear iron(IV) oxo complexes [(L)FeIV=O]2+, with L = BPMEN or TPA, are present in the cited catalytic systems. These intermediates are the decomposition products of the acylperoxo complexes [(L)FeIII-O3CCH3]2+. Such a complex was observed by the 2H NMR technique at low temperatures. The [(L)FeIV=O]2+ and [(L)FeV=O]3+ oxo complexes are possible active species in the studied catalytic systems.

Reaction of Ferric Cytochrome P450cam with Peracids: KINETIC CHARACTERIZATION OF INTERMEDIATES ON THE REACTION PATHWAY

Reactions of substrate-free ferric cytochrome P450cam with peracids to generate Fe=O intermediates have previously been investigated with contradictory results. Using stopped-flow spectrophotometry, the reaction with m-chloroperoxybenzoic acid demonstrated an Fe(IV)=O + porphyrin pi-cation radical (Cpd I) (Egawa, T., Shimada, H., and Ishimura, Y. (1994) Biochem. Biophys. Res. Commun. 201, 1464-1469). By contrast, with peracetic acid, Fe(IV)=O plus a tyrosyl radical were observed by freeze-quench Mossbauer and EPR spectroscopy (Schunemann, V., Jung, C., Trautwein, A. X., Mandon, D., and Weiss, R. (2000) FEBS Lett. 479, 149-154). Our detailed kinetic studies have resolved these contradictory results. At pH >7, a significant fraction of Cpd I is formed transiently, whereas at low pH only a species with a Soret band at 406 nm, presumably Fe(IV)=O + tyrosyl radical, is observed. Evidence for formation of an acylperoxo complex en route to Cpd I was obtained. Because of rapid heme destruction, steps subsequent to formation of the highly oxidized forms could not be fully characterized. Heme destruction was avoided by including peroxidase substrates (e.g. guaiacol), which were oxidized to characteristic peroxidase products as the Fe(III)-P450 was regenerated. Addition of ascorbate to either of the high valent species also reforms the Fe(III) state with only a small loss of heme absorbance. These results indicate that typical peroxidase chemistry occurs with P450cam and offer an explanation for the contrasting results reported earlier. The delineation of improved conditions (pH, temperature, choice of peracid) for generating highly oxidized species with P450cam should be valuable for their further characterization.

Cooperative Pull and Push Effects on the O−O Bond Cleavage in Acylperoxo Complexes of [(Salen)MnIIIL]: Ensuring Formation of Manganese(V) Oxo Species

The acidity (pull) and the axial ligand (push) effects on the O-O bond cleavage in the [(Salen)Mn(III)(RCO(3))L] acylperoxo complexes, with model L = none, NH(3), and HCO(2)(-) (1), have been studied with B3LYP density functional calculations. The acidic conditions have been mimicked by explicit protonation of 1 to afford a variety of [(Salen)Mn(III)(RCO(3)H)L] (2) and [(SalenH)Mn(III)(RCO(3))L] (3) complexes in ground quintet states. The protonation assists the O-O bond heterolysis, thus primarily forming highly reactive Mn(V)(O) species, and consequently suppresses formation of the less reactive Mn(IV)(O) species through homolytic channel described earlier in 1 [Khavrutskii, I. V.; Rahim, R. R.; Musaev, D. G.; Morokuma, K. J. Phys. Chem. B 2004, 108, 3845-3854]. In addition to the qualitative change of the O-O bond cleavage mode, the protonation affects the rate of the O-O bond cleavage. Therefore, varying the acidity of the reaction media helps control the O-O bond cleavage mode and rate.

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: a theoretical study.

The mechanism and origin of asymmetric induction in the Mn(III)(salen)-catalyzed epoxidation by peracetic acid have been elucidated by the density functional [Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (B3LYP)] method in two different regimes: with and without an axial ligand. The acylperoxo complexes of Mn(II,III,IV) in cisON cisNO and trans geometrical configurations cannot compete with the catalyst-free Prilezhaev epoxidation. Instead, oxo species perform epoxidation following the O-O bond cleavage in the acylperoxo complexes. The epoxidation may proceed in a concerted and/or radical-mediated stepwise manner. The actual mechanism of the epoxidation depends on the electronic and oxidation state of the oxo species and the nature of the axial ligand. The olefin can approach the reactive MnO fragment of both cis and trans-l-isomers of the plain oxo species along multiple distinct directions: native approaches. The native approaches are used to rationalize the inversion of the absolute configuration of the product epoxide due to the axial ligand.

Axial Ligand and Solvent Effects on the O−O Bond Activation in Acylperoxo Complexes of [(Salen)MnIIIL]: MnIV versus MnV Oxo Species

The role of the axial ligand L (“push” effect) in the O−O bond cleavage in Mn−Salen based trans-acylperoxo complexes, [(Salen)MnIII(RCO3)L] (2) and [(Salen)MnIV(RCO3)L]+ (3), was investigated by the density functional (B3LYP) method for L = NH3 and HCO2-. It was shown that coordination of an axial ligand L to the trans isomer of 2, 2-trans, substantially reduces the triplet−quintet energy gap from +12.4 to +1.8 and −0.3 kcal/mol, and consequently decreases the overall barrier for the O−O bond cleavage producing 2-trans-L-oxo species from 19.5 to 10.2 and 10.9 kcal/mol for L = none, HCO2-, and NH3, respectively. Because the O−O bond cleavage in 2-trans-L becomes slightly more favorable than that in the isomeric 2-cis acylperoxo complexes producing 2-cis-MnV-oxo species [Khavrutskii, I. V.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 2003, 125, 13879−13889], the axial ligand may be considered as a switch between cis- and trans-oxo species formation. The product 2-trans-L-oxo unlike 2-cis-MnV-oxo species is chameleon in nature and can be viewed either as MnV−oxo species associated with RCO2- (heterolytic cleavage) or as MnIV−oxo species associated with RCO2 radical (homolytic cleavage). The mode of dissociation of this species into corresponding heterolytic or homolytic fragments is controlled by the nature of the axial ligand and the solvent employed.

Insights into the structure and reactivity of acylperoxo complexes in the Kochi-Jacobsen-Katsuki catalytic system. A density functional study.

Structural properties of the acylperoxo complexes [(Salen)Mn(III)RCO(3)] (2) and [(Salen)Mn(IV)RCO(3)] (3), the critical intermediates in the Kochi-Jacobsen-Katsuki reaction utilizing organic peracids or O(2)/aldehydes as oxygen source, have been studied with the density functional theory. Four distinct isomers, cis(O,N), cis(N,O), cis(N,N), and trans, of these complexes have been located. The isomer 2-cis(O,N) in its quintet ground state, and nearly degenerate isomers 3-cis(O,N) and 3-cis(N,O) in their quartet ground states are found to be the lowest in energy among the other isomers. The O-O bond cleavage in the cis(O,N), cis(N,O), and trans isomers of 2 and 3 has been elucidated. In complex 3, the O-O bond is inert. On the contrary, in complex 2, the O-O bond cleaves via two distinct pathways. The first pathway occurs exclusively on the quintet potential energy surface (PES) and corresponds to heterolytic O-O bond scission coupled with insertion of an oxygen atom into an Mn-N(Salen) bond to form 2-N-oxo species; this pathway has the lowest barrier of 14.9 kcal/mol and is 15.6 kcal/mol exothermic. The second pathway is tentatively a spin crossover pathway. In particular, for 2-cis(O,N) and 2-cis(N,O) the second pathway proceeds through a crucial minimum on the seam of crossing (MSX) between the quintet and triplet PESs followed by heterolytic O-O cleavage on the triplet PES, and produces unusual triplet 2-cis(O,N)- and 2-cis(N,O)-oxo ([(Salen)Mn(V)(O)RCO(2)]) species; this pathway requires 12.8 kcal/mol and is 1.4 kcal/mol endothermic. In contrast, for the 2-trans isomer, spin crossing is less crucial and the O-O cleavage proceeds homolytically to generate 2-trans-oxo [(Salen)Mn(IV)(O)] species with RCO(2) radical; this pathway, however, cannot compete with that in 2-cis because it needs 21.9 kcal/mol for activation and is 15.3 kcal/mol endothermic. In summary, the O-O cleavage occurs predominantly in the 2-cis complexes, and may proceed either through pure high spin or spin crossover heterolytic pathway to produce 2-cis-oxo and 2-N-oxo species.

Role of Mn IV species in Mn(salen) catalyzed enantioselective aerobic epoxidations of alkenes: an EPR study

Using EPR spectroscopy, high valence Mn(salen) complexes were monitored in Mn III(salen)/isobutyraldehyde/O 2 and Mn III(salen)/pivalaldehyde/O 2 catalytic systems in the absence and in the presence of N-methylimidazole ( N-Me-Imd), where Mn III(salen) ( 1) is ( S, S)-(+)- N, N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) chloride. Interaction of 1 with aliphatic aldehydes in the presence of O 2 was found to give a mixture of oxo manganese(IV) species L 1(salen)Mn IV=O (2) with various axial ligands L 1 (L 1=Cl −, RCOO −). They display EPR signals having close g values of ca. 5.2 with resolved hyperfine structures from one manganese ( A=75 G, 55 Mn , I=5/2). In the presence of N-Me-Imd, new manganese(IV) complexes were detected. The first one was shown to be an antiferromagnetically coupled mixed-valence [(salen)Mn IV(μ 2-O) 2Mn III(salen)] − dimer having 16-line EPR signal at g=2. The second one is probably (salen)Mn IV(OOCOR) N-Me-Imd acylperoxo complex ( 2) ( g=4.2 (R=CH(CH 3) 2), or g=4.7 (R=C(CH 3) 3), unresolved manganese hyperfine structure). Possible roles of each complex in the enantioselective alkene epoxidations are discussed.

1H NMR and EPR spectroscopic monitoring of the reactive intermediates of (Salen)Mn III catalyzed olefin epoxidation

Using 1H NMR and EPR spectroscopy, manganese species formed in the catalytic systems 1+iodosobenzene (PhIO) and 1+meta-chloroperoxybenzoic acid ( m-CPBA), where 1 is ( R, R)-(−)- N, N′-bis(3,5-di- tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) chloride ([(Salen)Mn III]) were studied. Three types of manganese complexes were characterized in the catalytic system 1+PhIO ( 4– 6). Complex 4 is very unstable and reacts with styrene at −20°C to afford styrene oxide. It exhibits three signals of tBu groups at 1.68, 1.64 and 1.42 ppm. This pattern closely resembles that for a model complex [(Salen)Mn V≡N]. Based on these data, 4 was identified as d 2 low-spin oxomanganese(V) complex [(Salen)Mn V=O] +. Complexes 5 and 6 are relatively stable at −20°C and poorly reactive towards styrene at this temperature. They display 1H NMR spectra characteristic for antiferromagnetically coupled μ-oxo-dinuclear Mn IV species and are identified as dinuclear complexes [(Salen)LMn IV- O-Mn IV(Salen)L′] with L, L′=Cl − and PhIO. It was found by EPR that the acylperoxo complex (Salen)Mn III(OOCOAr) ( 7) was formed at the first stage of the interaction of 1 with m-CPBA in CH 2Cl 2. Complex 7 is unstable and converts into manganese(IV) oxo complex [(Salen)Mn IV(O)] ( 8). The evaluated first order rate constant of this conversion is 0.25±0.08 min −1 at −70°C. Complex 7 reacts with styrene with the rate constant 1.1±0.4 M −1 min −1 at −70°C to give epoxide and restore 1. Complex 8 is inert towards styrene at low temperature. The effect of donor ligand N-methylmorpholine- N-oxide (NMO) on the epoxidation of styrene by the system 1+ m-CPBA was studied. Addition of NMO (2–5 equiv.) to the solution of 1 in CH 2Cl 2 before interaction with m-CPBA was found to dramatically increase the rate of undesirable transformation of the reactive acylperoxo complex 7-NMO into relatively inert oxo complex- 8-NMO. However, in the presence of styrene, such undesirable conversion is entirely suppressed by very rapid reaction of 8-NMO with styrene to afford styrene oxide and restore 1-NMO.

ChemInform Abstract: Dioxygen‐Copper Reactivity: X‐Ray Structure and Characterization of an (Acylperoxo)dicopper Complex.

AbstractA stable and crystalline analogue of a hydroperoxo complex, obtained by protonation of the peroxo complex (IV), is represented by the acylperoxo complex (III), which is best prepared as shown from (I).