Participation of Electron Transfer Process in Rate-Limiting Step of Aromatic Hydroxylation Reactions by Compound I Models of Heme Enzymes

Hydroxylation reactions of aromatic rings are key reactions in various biological and chemical processes. In spite of their significance, no consensus mechanism has been established. Here we performed Marcus plot analysis for aromatic hydroxylation reactions with oxoiron­(IV) porphyrin π-cation radical complexes (compound I). Although many recent studies support the mechanism involving direct electrophilic attack of compound I, the slopes of the Marcus plots indicate a significant contribution of an electron transfer process in the rate-limiting step, leading us to propose a new reaction mechanism in which the electron transfer process between an aromatic compound and compound I is in equilibrium in a solvent cage and coupled with the subsequent bond formation process

Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex

The Co­(salen)­(X) complex, where salen is chiral <i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butylsalicylidene)-1,2-cyclohexanediamine and X is an external axial ligand, has been widely utilized as a versatile catalyst. The Co­(salen)­(X) complex is a stable solid that has been conventionally described as a Co^III(salen)­(X) complex. Recent theoretical calculations raised a new proposal that the Co­(salen)­(H₂O)­(SbF₆) complex contains appreciable contribution from a Co^II(salen^•+) electronic structure (Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 10557–10571), while other theoretical calculations for Co­(salen)­(Cl) indicated a triplet Co^III(salen) electronic structure (Kemper, S.; Hrobárik, P.; Kaupp, M.; Schlörer, N. E. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 4172–4173). However, there have been no experimental data to evaluate these theoretical proposals. We herein report key experimental data on the electronic structure of the Co­(salen)­(X) complex (X = CF₃SO₃^–, SbF₆^–, and <i>p</i>-MeC₆H₄SO₃^–). The X-ray crystallography shows that Co­(salen)­(OTf) has a square-planar N₂O₂ equatorial coordination sphere with OTf as an elongated external axial ligand. Magnetic susceptibility data indicate that Co­(salen)­(OTf) complexes belong to the <i>S</i> = 1 spin system. ¹H NMR measurements provide convincing evidence for the Co^II(salen^•+)­(X) character, which is estimated to be about 40% in addition to 60% Co^III(salen)­(X) character. The CH₂Cl₂ solution of Co­(salen)­(X) shows an intense near-IR absorption, which is assigned as overlapped transitions from a ligand-to-metal charge transfer in Co^III(salen)­(X) and a ligand-to-ligand charge transfer in Co^II(salen^•+)­(X). The present experimental study establishes that the electronic structure of Co­(salen)­(X) contains both Co^II(salen^•+)­(X) and Co^III(salen)­(X) character

Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex

The Co­(salen)­(X) complex, where salen is chiral <i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butylsalicylidene)-1,2-cyclohexanediamine and X is an external axial ligand, has been widely utilized as a versatile catalyst. The Co­(salen)­(X) complex is a stable solid that has been conventionally described as a Co^III(salen)­(X) complex. Recent theoretical calculations raised a new proposal that the Co­(salen)­(H₂O)­(SbF₆) complex contains appreciable contribution from a Co^II(salen^•+) electronic structure (Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 10557–10571), while other theoretical calculations for Co­(salen)­(Cl) indicated a triplet Co^III(salen) electronic structure (Kemper, S.; Hrobárik, P.; Kaupp, M.; Schlörer, N. E. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 4172–4173). However, there have been no experimental data to evaluate these theoretical proposals. We herein report key experimental data on the electronic structure of the Co­(salen)­(X) complex (X = CF₃SO₃^–, SbF₆^–, and <i>p</i>-MeC₆H₄SO₃^–). The X-ray crystallography shows that Co­(salen)­(OTf) has a square-planar N₂O₂ equatorial coordination sphere with OTf as an elongated external axial ligand. Magnetic susceptibility data indicate that Co­(salen)­(OTf) complexes belong to the <i>S</i> = 1 spin system. ¹H NMR measurements provide convincing evidence for the Co^II(salen^•+)­(X) character, which is estimated to be about 40% in addition to 60% Co^III(salen)­(X) character. The CH₂Cl₂ solution of Co­(salen)­(X) shows an intense near-IR absorption, which is assigned as overlapped transitions from a ligand-to-metal charge transfer in Co^III(salen)­(X) and a ligand-to-ligand charge transfer in Co^II(salen^•+)­(X). The present experimental study establishes that the electronic structure of Co­(salen)­(X) contains both Co^II(salen^•+)­(X) and Co^III(salen)­(X) character

The dynamics when ACh is gradually decreased according to an exponential function.

<p>In order to simulate more physiologically plausible situations, we evaluated the network dynamics when ACh is decreased according to an exponential function. The top-down Glu spike volleys are injected to the 9th and 10th units for . ACh is also injected at . It is observed that pattern 2 is successfully retrieved while ACh is effective. Therefore, our scenario would hold as well even if the injection of ACh is temporary.</p

The effect of top-down Glu spike volleys to the apical distal dendrites as well as the release of ACh in cortical layer 1.

<p>The top-down Glu spike volleys are projected only to the 9th and 10th units for . Note that both the 9th and 10th units, indicated by two arrows in each figure, are active only in pattern 2. Moreover, decreases for associated with the release of ACh. (A) In the ongoing state with , after the injection of top-down Glu spike volleys at , the network temporarily retrieves pattern 2. However, the retrieved pattern in the network soon transits to pattern 1 at around because pattern 2 is a quasi-attractor. (B) For , the trajectory also moves to pattern 2 but the staying time increases because of the effect of ACh. (C) In the network with , each quasi-attractor is stabilized to be an attractor. Therefore, once the trajectory transits to pattern 2, it does not move to other patterns.</p

Schematic diagram of cortical layers in the early visual cortex (V1/V2).

<p>The bottom-up spike signals via layer 4 from the thalamus core circuit and the top-down spike signals onto layer 1 (and layer 6) interact in the perception of external sensory stimuli in the early cortex (V1/V2). Moreover, acetylcholine (ACh) is transiently released from the nucleus basalis of Meynert to all the layers associated with top-down attention. In layers 2/3, pyramidal neurons (PYRs) that project their apical distal dendrites to layer 1, interneurons (INs), and fast spiking neurons exist. Moreover, it is also known that ACh to layer 1 depolarizes calretinin positive () INs in layer 1 through nicotinic receptors. However, we do not consider the latter effect in our model for simplicity.</p

Preparation, Characterization and Reactivity of a Bis-hypochlorite Adduct of a Chiral Manganese(IV) Salen Complex

A bis-hypochlorite adduct of a manganese­(IV) salen complex having a chiral (<i>R</i>,<i>R</i>)-cyclohexane-1,2-diamine linkage (<b>2-</b>^<b>t</b><b>Bu</b>) is successfully prepared and characterized by various spectroscopic methods. The reactions of <b>2-</b>^<b>t</b><b>Bu</b> with various organic substrates show that <b>2-</b>^<b>t</b><b>Bu</b> is capable of sulfoxidation, epoxidation, chlorination, and hydrogen abstraction reactions. However, the enantioselectivity of the epoxidation reactions by <b>2-</b>^<b>t</b><b>Bu</b> is much lower than that reported for the catalytic reactions by Jacobsen’s catalyst. The low enantioselectivity is consistent with a planar conformation of the salen ligand, which is suggested by circular dichroism spectroscopy. This study suggests that <b>2-</b>^<b>t</b><b>Bu</b> is not a reactive intermediate of Jacobsen’s enantioselective epoxidation catalysis

Di-μ-oxo Dimetal Core of Mn^IV and Ti^IV as a Linker Between Two Chiral Salen Complexes Leading to the Stereoselective Formation of Different <i>M</i>- and <i>P</i>‑Helical Structures

Because of restricted rotational freedom along the metal–metal axis, a di-μ-oxo dimetal core could be an excellent building block to create dinuclear compounds with well-defined stereochemistry, but their stereoselective synthesis remains a challenge. We herein report the formation of di-μ-oxo dimanganese­(IV) complexes with tetradentate salen ligands bearing different degrees of steric bulk, in order to study stereochemical aspects of the dimerization reaction that potentially generates multiple stereoisomers. X-ray crystallography shows that the di-μ-oxo dimanganese­(IV) complex with salen, where salen is (<i>R</i>,<i>R</i>)-<i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butylsalicylidene)-1,2-cyclohexanediamine, adopts a unique structure in which two salen complexes are arranged in an <i>M</i>-helical fashion. According to the solution study using ¹H, ²H NMR, and circular dichroism spectroscopies, the dimerization reaction is highly diastereoselective in the presence of the <i>tert</i>-butyl group at the 3/3′ position as a determinant steric factor. In contrast, the di-μ-oxo dititanium­(IV) complex with the same salen ligand was previously reported to afford an opposite <i>P</i>-helical dimer. The present DFT study clarifies that a less-covalent Ti–O bonding causes a distortion of the di-μ-oxo dititanium­(IV) core structure, generating a completely different framework for interligand interaction. The present study provides a solid basis to understand the stereochemistry for the formation of the di-μ-oxo dimetal core

Schematic diagram of our model.

<p>(A) As an elemental model of a small network in layers 2/3 in the early visual cortex, we construct a unit model that is composed of PYRs and INs, each of which is modeled as a phase neuron connected to all the other neurons globally. We set and or we take the limit in the analysis of the Fokker-Planck equations (see the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053854#s4" target="_blank">Methods</a> section). (B) The multiple units in cortical layers 2/3. ACh decreases inhibitions to PYRs through the presynaptic, muscarinic disinhibitions, and it stabilizes the quasi-attractors. The top-downs Glu spike volleys to the apical distal dendrites of PYRs contribute to the selection of attractors. (C) Connections between two units. Only the connections from the left unit to the right one are shown for simplicity.</p

Formation of Iron(III) <i>meso</i>-Chloro-isoporphyrin as a Reactive Chlorinating Agent from Oxoiron(IV) Porphyrin π-Cation Radical

Iron­(III) isoporphyrin, a tautomer of porphyrin with a saturated <i>meso</i> carbon, is one of the isoelectronic forms of oxoiron­(IV) porphyrin π-cation radical, which is known as an important reactive intermediate of various heme enzymes. The isoporphyrin has been believed to be incapable of catalyzing oxygenation and oxidation reactions. Here, we report that an oxoiron­(IV) porphyrin π-cation radical can be converted to iron­(III) <i>meso</i>-chloro-isoporphyrin in the presence of trifluoroacetic acid and chloride ion. More importantly, this study shows the first evidence that iron­(III) <i>meso</i>-chloro-isoporphyrin is an excellent reactive agent for chlorinating aromatic compounds and olefins. The results of this study suggest that the mechanism involves electrophilic chlorination of substrate with iron­(III) <i>meso</i>-chloro-isoporphyrin