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High‐valent metal‐oxo species play critical roles in enzymatic catalysis yet their properties are still poorly understood. In this work we report a combined experimental and computational study into biomimetic iron(IV)‐oxo and iron(III)‐oxo complexes with tight second‐coordination sphere environments that restrict substrate access. The work shows that the second‐coordination sphere slows the hydrogen atom abstraction step from toluene dramatically and the kinetics is zeroth order in substrate. However, the iron(II)‐hydroxo that is formed has a low reduction potential and hence cannot do OH rebound favorably. The tolyl radical in solution then reacts further with alternative reaction partners. By contrast, the iron(IV)‐oxo species reacts predominantly through OH rebound to form alcohol products. Our studies show that the oxidation state of the metal influences reactivities and selectivities with substrate dramatically and that enzymes will likely need an iron(IV) center to catalyze C−H hydroxylation reactions. [ABSTRACT FROM AUTHOR]

Aryloxyalkanoate dioxygenases are unique herbicide biodegrading nonheme iron enzymes found in plants and hence, from environmental and agricultural point of view they are important and valuable. However, they often are substrate specific and little is known on the details of the mechanism and the substrate scope. To this end, we created enzyme models and calculate the mechanism for 2,4‐dichlorophenoxyacetic acid biodegradation and 2‐methyl substituted analogues by density functional theory. The work shows that the substrate binding is tight and positions the aliphatic group close to the metal center to enable a chemoselective reaction mechanism to form the C2‐hydroxy products, whereas the aromatic hydroxylation barriers are well higher in energy. Subsequently, we investigated the metabolism of R‐ and S‐methyl substituted inhibitors and show that these do not react as efficiently as 2,4‐dichlorophenoxyacetic acid substrate due to stereochemical clashes in the active site and particularly for the R‐isomer give high rebound barriers. [ABSTRACT FROM AUTHOR]

Hydroxylation of benzene by molecular oxygen (O2) occurs efficiently with 10-methyl-9,10-dihydroacridine (AcrH2) as an NADH analogue in the presence of a catalytic amount of Fe(ClO4)3 or Fe(ClO4)2 with excess trifluoroacetic acid in a solvent mixture of benzene and acetonitrile (1:1 v/v) to produce phenol, 10-methylacridinium ion and hydrogen peroxide (H2O2) at 298 K. The catalytic oxidation of benzene by O2 with AcrH2 in the presence of a catalytic amount of Fe(ClO4)3 is started by the formation of H2O2 from AcrH2, O2, and H+. Hydroperoxyl radical (HO2.) is produced from H2O2 with the redox pair of Fe3+/Fe2+ by a Fenton type reaction. The rate-determining step in the initiation is the proton-coupled electron transfer from Fe2+ to H2O2 to produce HO. and H2O. HO. abstracts hydrogen rapidly from H2O2 to produce HO2. and H2O. The Fe3+ produced was reduced back to Fe2+ by H2O2. HO2. reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH2 to give the corresponding hydroperoxide, accompanied by generation of acridinyl radical (AcrH.) to constitute the radical chain reaction. Hydroperoxyl radical (HO2.), which was detected by using the spin trap method with EPR analysis, acts as a chain carrier for the two radical chain pathways: one is the benzene hydroxylation with O2 and the second is oxidation of an NADH analogue with O2 to produce H2O2. [ABSTRACT FROM AUTHOR]

Hydroxylation of mesitylene by a nonheme manganese(IV)-oxo complex, [(N4Py)MnIV(O)]2+ ( 1), proceeds via one-step hydrogen-atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic acid-bound manganese(IV)-oxo complex, [(N4Py)MnIV(O)]2+-(HOTf)2 ( 2), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly, when the reaction temperature is lowered to less than 263 K in the reaction of 2, however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around −0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT. [ABSTRACT FROM AUTHOR]

The formation of a bis(μ-oxido)dicopper complex with the ligand 2-(diethylaminoethyl)-6-phenylpyridine (PPN) and its subsequent hydroxylation of the pendant phenyl group (studied earlier by Holland et al., Angew. Chem. Int. Ed.­ 1999, 38, 1139-1142) has been reinvestigated to gain a better understanding of such systems in view of the development of new synthetic applications. To this end, we prepared a simple copper imine complex system that also affords selective o-hydroxylation of aromatic aldehydes by using dioxygen as the oxidant: Applying the ligand N′-benzylidene- N, N-diethylethylenediamine (BDED), salicylaldehyde was prepared in good yields and we show that this reaction also occurs through an intermediate bis-μ-oxido copper complex. The underlying reaction mechanism for the PPN-supported complex was studied at the BLYP-D/TZVP level of density functional theory and the results for representative stationary points along reaction paths of the BDED-supported complex reveal a closely related mechanistic scenario. The results demonstrate a new facile synthetic way to introduce OH groups into aromatic aldehydes. [ABSTRACT FROM AUTHOR]

A highly regioselective ortho-benzoxylation of N-alkyl benzamides with aromatic acids in the presence of [{RuCl2( p-cymene)}2], AgSbF6, and (NH4)2S2O8 in 1,2-dichloroethane at 100 °C for 24 h affording ortho-benzoxylated N-alkyl benzamides by CH bond activation is described. Further, Ru-catalyzed alkenylation is done at the ortho CH bond of benzoxylated N-alkyl benzamides with alkenes in water solvent. Subsequently, the benzoxyl moiety of N-alkyl benzamides was converted into a hydroxyl group in the presence of base or acid. A possible reaction mechanism was proposed to account for the present coupling reaction. [ABSTRACT FROM AUTHOR]

In Nature, the family of copper monooxygenases comprised of peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM) is known to perform dioxygen-dependent hydroxylation of aliphatic CH bonds by using two uncoupled metal sites. In spite of many investigations, including biochemical, chemical, and computational, details of the CH bond oxygenation mechanism remain elusive. Herein we report an investigation of the mechanism of hydroxylation by PHM by using hybrid quantum/classical potentials (i.e., QM/MM). Although previous investigations using hybrid QM/MM techniques were restricted to geometry optimizations, we have carried out ab initio molecular dynamics simulations in order to include the intrinsic flexibility of the active sites in the modeling protocol. The major finding of this study is an extremely fast rebound step after the initial hydrogen-abstraction step promoted by the cupric-superoxide adduct. The hydrogen-abstraction/rebound sequence leads to the formation of an alkyl hydroperoxide intermediate. Long-range electron transfer from the remote copper site subsequently triggers its reduction to the hydroxylated substrate. We finally show two reactivity consequences inherent in the new mechanistic proposal, the investigation of which would provide a means to check its validity by experimental means. [ABSTRACT FROM AUTHOR]

A parting of the waves: A prototype microflow system for the continuous recycling of homogeneous catalysts through liquid/liquid phase separation was developed and its effectiveness demonstrated in a challenging palladium‐catalyzed hydroxylation reaction. The unmodified catalyst is recycled under standard reaction conditions by using a combination of chemical and engineering methods. [ABSTRACT FROM AUTHOR]

The mechanism of aromatic hydroxylation of aniline and phenol derivatives in a H2O2-driven microperoxidase-8(MP8)-catalyzed reaction was investigated. It was shown that the reaction was not inhibited by the addition of scavengers of superoxide anion or hydroxyl radicals, which demonstrates that the reaction mechanism differs from that of the aromatic hydroxylation catalyzed by a horseradish peroxidase/ dihydroxyfumarate system. Additional experiments with 18O-labelled H218O2 demonstrated that the oxygen incorporated into aniline to give 4-aminophenol originates from H2O2. Furthermore, it was found that the addition of ascorbic acid efficiently blocks all peroxidase-type reactions that can be catalyzed by the MP8/H2O2 system, but does not inhibit the aromatic hydroxylation of aniline and phenol derivatives Together, these observations exclude reaction mechanisms for the aromatic hydroxylation that proceed through peroxidase-type mechanisms in which the oxygen incorporated into the substrate originates from O2 or H2O. The mechanism instead seems to proceed by an initial attack of the high-valent iron-oxo intermediate of MP8 on the &pie;-electrons of the aromatic ring of the substrate leading to product formation by a cytochrome-P-450-type of σ-O-addition or oxygen-rebound mechanism This implies that MP8, which has a histidyl and not a cysteinate fifth axial ligand, is able to react by a cytochrome-P-450-1ike oxygen-transfer reaction mechanism. [ABSTRACT FROM AUTHOR]

The side chain of Tyr222 in p-hydroxybenzoate hydroxylase interacts with the carboxy moiety of the substrate. Studies on the Tyr222→Phe mutant, [F222]p-hydroxybenzoate hydroxylase, have shown that disruption of this interaction hampers the hydroxylation of 4-hydroxybenzoate. Tyr222 is possibly involved in flavin motion, which may facilitate the exchange of substrate and product during catalysis. To elucidate the function of Tyr222 in more detail, in the present study the substrate and product during catalysis. To elucidate the function of Tyr222 in more detail, in the present study the substrate and effector specificity of the Tyr222→Ala mutant, [A222]p-hydroxybenzoate hydroxylase, was investigated. Replacement of Tyr222 by Ala impairs the binding of the physiological substrate 4-hydroxybenzoate and the substrate analog 4-aminobenzoate. With these compounds, hydroxylase tightly interacts with 2,4-dihydroxybenzoate and 2-hydroxy-4-aminobenzoate. Crystallographic data [Schreuder, H.A., Mattevi, A., Oblomova, g., Kalk, K. H., Hol, W.G.J., van der Bolt, F.J.T. & van Berkel, W.J.H. (1994) Biochemsitry 33, 10161-10170] suggest that this is due to motion of the flavin ring out of the active site, allowing hydrogen-bond interaction between the 2-hydroxy group of the substrate analogs and N3 of the flaving. [A222]p-Hydroxybenzoate hydroxylase produces about 0.6 mol 2,3,4-trihydroxybenzoate from 2,4-dihydroxybenzoate/mol NADPH oxidized. This indicates that reduction of the Tyr222→Ala mutant shifts the equilibrium of flavin conformers towards the productive 'in' position. [A222]p-Hydroxybenzoate hydroxylase converts 2-fluoro-4-hydroxybenzoate to 2-fluor-3,4-dihydroxybenzoate. The regioselectivity of hydroxylation suggests that [A222]p-hydroxybenzoate hydroxylase binds the fluorinated substrate in the same orientation as wild-type. Spectral studies suggest that wild-type and [A222]p-hydroxybenzoate hydroxylase bind 2-fluoro-4-hydroxybenzoate in the phenolate form with the flavin ring preferring the 'out' conformation. Despite activation of the fluorinated substrate and in contrast to the wild-type enzyme. [A222]p-hydroxybenzoate hydroxylase largely produces hydrogen peroxide. The effector specificity of p-hydroxybenzoate hydroxylase is not changed by the Tyr222→Ala replacement. This supports the idea that the effector specificity is mainly dictated by the protein-substrate interactions at the re-side of the flavin ring. [ABSTRACT FROM AUTHOR]