Off-line analysis in the manganese catalysed epoxidation of ethylene-propylene-diene rubber (EPDM) with hydrogen peroxide

The epoxidation of ethylene-propylene-diene rubber (EPDM) with 5-ethylidene-2-norbornene (ENB) as the diene to epoxidized EPDM (eEPDM) creates additional routes to cross-linking and reactive blending, as well as increasing the polarity and thereby the adhesion to polar materials, e.g., mineral fillers such as silica. The low solubility of apolar, high molecular weight polymers in the polar solvents constrains the catalytic method for epoxidation that can be applied. Here we have applied an in situ prepared catalyst comprising a manganese(ii) salt, sodium picolinate and a ketone to the epoxidation of EPDM rubber with hydrogen peroxide (H(2)O(2)) as the oxidant in a solvent mixture, that balances the need for polymer and catalyst/oxidant miscibility and solubility. Specifically, a mixture of cyclohexane and cyclohexanone is used, where cyclohexanone functions as a co-solvent as well as the ketone reagent. Reaction progress was monitored off-line through a combination of Raman and ATR-FTIR spectroscopies, which revealed that the reaction profile and the dependence on the composition of the catalyst are similar to those observed with low molar mass alkene substrates, under similar reaction conditions. The combination of spectroscopies offers a reliable method for off-line reaction monitoring of both the extent of the conversion of unsaturation (Raman) and the extent of epoxidation (FTIR) as well as determining side reactions, such as epoxide ring opening and further, aerobic oxidation. The epoxidation of EPDM described, in contrast to currently available methods, uses a non-scarce manganese catalyst and H(2)O(2), and avoids side reactions, such as those that can occur with peracids

Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes

Correction published on October 23, 2020 https://doi.org/10.1021/acsomega.0c04910The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine (2BB) was employed to prepare copper complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)CuI]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of [(2BB)CuII(H2O)2](OTf)2 indicates that [(2BB)CuII(H2O)2]2+ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)CuII(H2O)x}2(μ-OH)2]2+ is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [(2BB)CuI]OTf/O2 or [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 with KO2 afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)CuII(OH)(H2O)]+ was detected by mass spectrometry as the final form of the complex

Genetic association study of QT interval highlights role for calcium signaling pathways in myocardial repolarization.

The QT interval, an electrocardiographic measure reflecting myocardial repolarization, is a heritable trait. QT prolongation is a risk factor for ventricular arrhythmias and sudden cardiac death (SCD) and could indicate the presence of the potentially lethal mendelian long-QT syndrome (LQTS). Using a genome-wide association and replication study in up to 100,000 individuals, we identified 35 common variant loci associated with QT interval that collectively explain ∼8-10% of QT-interval variation and highlight the importance of calcium regulation in myocardial repolarization. Rare variant analysis of 6 new QT interval-associated loci in 298 unrelated probands with LQTS identified coding variants not found in controls but of uncertain causality and therefore requiring validation. Several newly identified loci encode proteins that physically interact with other recognized repolarization proteins. Our integration of common variant association, expression and orthogonal protein-protein interaction screens provides new insights into cardiac electrophysiology and identifies new candidate genes for ventricular arrhythmias, LQTS and SCD

Reversible photochromic switching in a Ru(II) polypyridyl complex

Fully reversible photoswitching of the coordination mode of the ligand MeN4Py (1,1-di(pyridin-2-yl)-N,N'-bis(pyridin-2-yl-methyl)-ethan- 1-amine) in its ruthenium(II) complex with visible light is reported. Irradiation with visible light results in dissociation of a pyridyl moiety, which is reversed by irradiation at 355 nm

Mechanisms in manganese oxidation catalysis with 1,4,7-triazacyclononane based ligands

The first report of catalytic activity of manganese complexes based on the ligand N,N′,N″-trimethyl-1,4,7-triazacyclononane (Me3tacn) in oxidations with H2O2 was in the mid-1990s. Although initially limited by extensive disproportionation of H2O2, within a short time solvents and additives were identified that could suppress this wasteful side reaction. These initial successes spurred more general interest in these complexes and over the following quarter century efforts toward broadening substrate scope and controlling (enantio)selectivity have revealed their versatility. In parallel considerable effort has been directed to elucidate the species that can form under a wide range of conditions both for their relevance to bioinorganic chemistry (enzymes and the oxygen evolving complexes) and to identifying species formed during oxidation reactions. In this chapter, the focus is on the spectroscopy and electrochemistry of these complexes and mechanistic insight gained over the last 25 years.</p

Origins of Catalyst Inhibition in the Manganese-Catalysed Oxidation of Lignin Model Compounds with H2O2

The upgrading of complex bio-renewable feedstock, such as lignocellulose, through depolymerisation benefits from the selective reactions at key functional groups. Applying homogeneous catalysts developed for selective organic oxidative transformations to complex feedstock such as lignin is challenged by the presence of interfering components. The selection of appropriate model compounds is essential in applying new catalytic systems and identifying such interferences. Here, it was shown by using as an example the oxidation of a model substrate containing a beta-O-4 linkage with H2O2 and an in situ-prepared manganese-based catalyst, capable of efficient oxidation of benzylic alcohols, that interference from compounds liberated during the reaction can prevent its application to lignocellulose depolymerisation

Reversible Deactivation of Manganese Catalysts in Alkene Oxidation and H2O2 Disproportionation

Mononuclear MnII oxidation catalysts with aminopyridine-based ligands achieve high turnover-number (TON) enantioselective epoxidation of alkenes with H2O2. Structure reactivity relations indicate a dependence of enantioselectivity and maximum TON on the electronic effect of peripheral ligand substituents. Competing H2O2 disproportionation is reduced by carrying out reactions at low temperatures and with slow addition of H2O2, which improve TONs for alkene oxidation but mask the effect of substituents on turnover frequency (TOF). Here, in situ Raman spectroscopy provides the high time resolution needed to establish that the minimum TOFs are greater than 10 s-1 in the epoxidation of alkenes with the complexes [Mn(OTf)2(RPDP)] [where R = H ( HPDP-Mn ) and R = OMe ( MeOPDP-Mn ) and RPDP = N,N′-bis(2″-(4″-R-pyridylmethyl)-2,2′-bipyrrolidine)]. Simultaneous headspace monitoring by Raman spectroscopy reveals that H2O2 disproportionation proceeds concomitant with oxidation of the substrate and that the ratio of reactivity toward substrate oxidation and H2O2 disproportionation is ligand-dependent. Notably, the rates of substrate oxidation and H2O2 disproportionation both decrease over time under continuous addition of H2O2 due to progressive catalyst deactivation, which indicates that the same catalyst is responsible for both reactions. Electrochemistry, UV/vis absorption, and resonance Raman spectroscopy and spectroelectrochemistry establish that the MnII complexes undergo an increase in oxidation state within seconds of addition of H2O2 to form a dynamic mixture of MnIII and MnIV species, with the composition depending on temperature and the presence of alkene. However, it is the formation of these complexes (resting states), rather than ligand degradation, that is responsible for catalyst deactivation, especially at low temperatures, and hence, the intrinsic reactivity of the catalyst is greater than observed TOFs. These data show that interpretation of effects of ligand substituents on reaction efficiency (and conversion) with respect to the oxidant and maximum TONs needs to consider reversible deactivation of the catalyst and especially the relative importance of various reaction pathways

Mechanisms in manganese oxidation catalysis with 1,4,7-triazacyclononane based ligands

The first report of catalytic activity of manganese complexes based on the ligand N,N′,N″-trimethyl-1,4,7-triazacyclononane (Me3tacn) in oxidations with H2O2 was in the mid-1990s. Although initially limited by extensive disproportionation of H2O2, within a short time solvents and additives were identified that could suppress this wasteful side reaction. These initial successes spurred more general interest in these complexes and over the following quarter century efforts toward broadening substrate scope and controlling (enantio)selectivity have revealed their versatility. In parallel considerable effort has been directed to elucidate the species that can form under a wide range of conditions both for their relevance to bioinorganic chemistry (enzymes and the oxygen evolving complexes) and to identifying species formed during oxidation reactions. In this chapter, the focus is on the spectroscopy and electrochemistry of these complexes and mechanistic insight gained over the last 25 years

Mechanism of Alkene, Alkane, and Alcohol Oxidation with H2O2 by an in Situ Prepared Mn-II/Pyridine-2-carboxylic Acid Catalyst

The oxidation of alkenes, alkanes, and alcohols with H2O2 is catalyzed efficiently using an in situ prepared catalyst comprised of a MnII salt and pyridine-2-carboxylic acid (PCA) together with a ketone in a wide range of solvents. The mechanism by which these reactions proceed is elucidated, with a particular focus on the role played by each reaction component: i.e., ketone, PCA, MnII salt, solvent, etc. It is shown that the equilibrium between the ketone cocatalysts, in particular butanedione, and H2O2 is central to the catalytic activity observed and that a gem-hydroxyl-hydroperoxy species is responsible for generating the active form of the manganese catalyst. Furthermore, the oxidation of the ketone to a carboxylic acid is shown to antecede the onset of substrate conversion. Indeed, addition of acetic acid either prior to or after addition of H2O2 eliminates a lag period observed at low catalyst loading. Carboxylic acids are shown to affect both the activity of the catalyst and the formation of the gem-hydroxyl-hydroperoxy species. The molecular nature of the catalyst itself is explored through the effect of variation of MnII and PCA concentration, with the data indicating that a MnII:PCA ratio of 1:2 is necessary for activity. A remarkable feature of the catalytic system is that the apparent order in substrate is 0, indicating that the formation of highly reactive manganese species is rate limiting