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Mykhailovych Anatolii Trush - One of the best experts on this subject based on the ideXlab platform.
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role of cu zn superoxide dismutase in xenobiotic activation i chemical reactions involved in the cu zn superoxide dismutase accelerated oxidation of the benzene metabolite 1 4 hydroquinone
1996Co-Authors: Yunbo Li, Periannan Kuppusamy, Jay L Zweier, Mykhailovych Anatolii TrushAbstract:Cu/Zn-superoxide dismutase (Cu/Zn-SOD) has been shown to modulate the Autoxidation of a variety of phenoic compounds, including 1,4-hydroquinone (HQ), a benzene-derived metabolite. The acceleration of Autoxidation of HQ by Cu/Zn-SOD results in the production of 1,4-benzoquinone (BQ). It has been proposed that the chemical mechanism involved in the Cu/Zn-SOD-catalyzed Autoxidation of HQ may be occur through either its conventional activity as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. However, Cu/Zn-SOD-accelerated oxidation of HQ has not been resolved experimentally. In this study, with ESR spectroscopy we investigated further the chemical reactions involved in the SOD-accelerated oxidation of HQ. In phosphate-buffered saline (PSB), HQ underwent a slow Autoxidation to BQ, which was accelerated by Cu/Zn-SOD, Mn-SOD, or Fe-SOD with similar efficiency. In contrast, among free metals, only Cu(II) strongly mediated the oxidation of HQ to BQ. Mn(II) exhibited a slight capacity to oxidize HQ, whereas neither FE(II) nor FE(III) was capable of modulating the Autoxidation of HG. The presence of either form of SOD also dramatically enhanced the formation of semiquinone anion radicals SQ-. from HQ. The SOD-accelerated oxidation of HQ was also accompanied by the generation of H202. In PBS containing bovine serum albumin (BSA) (PBS/BSA), HQ did not undergo Autoxidation to SQ-., and as such the presence of SOD was unable to induce the formation of either SQ-. or BQ or the consumption of O2. The addition of 10 microM BQ to HQ (100 or 1000 microM) in PBS/BSA resulted in the formation of SQ-. and initiated a slow rate of oxidation of HQ to BQ. In this case, the presence of Cu/Zn-SOD strongly accelerated the oxidation of HQ to SQ-. and BQ and the utilization of O2. Furthermore, the enhancement by Cu/Zn-SOD of the generation of SQ-. or BQ from HQ in PBS/BSA was extensively inhibited under anaerobic conditions. The enhancement of SQ-. generation from HQ by all three forms of SOD does not support the possibility that Cu/Zn-SOD can oxidize SQ-. to BQ. Taken together, this study demonstrates that unlike free copper, Cu/Zn-SOD does not directly interact with HQ to cause its oxidation to BQ. Rather, the Autoxidation of HQ to SQ-. is a prerequisite for the enhancing capacity of Cu/Zn-SOD, and the dismutation of superoxide anion radicals generated from the SQ-. in the presence of O2 appears to be the underlying mechanism responsible for the enhancement by Cu/Zn-SOD of the oxidation of HQ.
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role of cu zn superoxide dismutase in xenobiotic activation i chemical reactions involved in the cu zn superoxide dismutase accelerated oxidation of the benzene metabolite 1 4 hydroquinone
1996Co-Authors: Yunbo Li, Periannan Kuppusamy, Jay L Zweier, Mykhailovych Anatolii TrushAbstract:Cu/Zn-superoxide dismutase (Cu/Zn-SOD) has been shown to modulate the Autoxidation of a variety of phenoic compounds, including 1,4-hydroquinone (HQ), a benzene-derived metabolite. The acceleration of Autoxidation of HQ by Cu/Zn-SOD results in the production of 1,4-benzoquinone (BQ). It has been proposed that the chemical mechanism involved in the Cu/Zn-SOD-catalyzed Autoxidation of HQ may be occur through either its conventional activity as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. However, Cu/Zn-SOD-accelerated oxidation of HQ has not been resolved experimentally. In this study, with ESR spectroscopy we investigated further the chemical reactions involved in the SOD-accelerated oxidation of HQ. In phosphate-buffered saline (PSB), HQ underwent a slow Autoxidation to BQ, which was accelerated by Cu/Zn-SOD, Mn-SOD, or Fe-SOD with similar efficiency. In contrast, among free metals, only Cu(II) strongly mediated the oxidation of HQ to BQ. Mn(II) exhibited a slight capacity to oxidize HQ, whereas neither FE(II) nor FE(III) was capable of modulating the Autoxidation of HG. The presence of either form of SOD also dramatically enhanced the formation of semiquinone anion radicals SQ-. from HQ. The SOD-accelerated oxidation of HQ was also accompanied by the generation of H202. In PBS containing bovine serum albumin (BSA) (PBS/BSA), HQ did not undergo Autoxidation to SQ-., and as such the presence of SOD was unable to induce the formation of either SQ-. or BQ or the consumption of O2. The addition of 10 microM BQ to HQ (100 or 1000 microM) in PBS/BSA resulted in the formation of SQ-. and initiated a slow rate of oxidation of HQ to BQ. In this case, the presence of Cu/Zn-SOD strongly accelerated the oxidation of HQ to SQ-. and BQ and the utilization of O2. Furthermore, the enhancement by Cu/Zn-SOD of the generation of SQ-. or BQ from HQ in PBS/BSA was extensively inhibited under anaerobic conditions. The enhancement of SQ-. generation from HQ by all three forms of SOD does not support the possibility that Cu/Zn-SOD can oxidize SQ-. to BQ. Taken together, this study demonstrates that unlike free copper, Cu/Zn-SOD does not directly interact with HQ to cause its oxidation to BQ. Rather, the Autoxidation of HQ to SQ-. is a prerequisite for the enhancing capacity of Cu/Zn-SOD, and the dismutation of superoxide anion radicals generated from the SQ-. in the presence of O2 appears to be the underlying mechanism responsible for the enhancement by Cu/Zn-SOD of the oxidation of HQ.
Elisabeth Bouwman - One of the best experts on this subject based on the ideXlab platform.
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the oxidative drying of alkyd paint catalysed by metal complexes
2005Co-Authors: Remy Van Gorkum, Elisabeth BouwmanAbstract:The focus of this review paper is the transition metal chemistry involved in alkyd (paint) drying. A general introduction is given on the composition of a common alkyd paint and the latest insights in the structure of the dried alkyd film are discussed. The drying of an alkyd paint is an Autoxidation process catalysed by transition metal salts. Thorough investigations have shed light on the role of the transition metals as catalysts in the drying process and a wealth of spectroscopic techniques have been used to monitor the Autoxidation process and to analyse the Autoxidation products. More recently, these techniques have been used to develop a test reaction for the rapid screening of new manganese and iron based drier catalysts. Promising new driers based on simple manganese complexes have been proposed as replacements for the environmentally unfriendly cobalt driers that are presently used in alkyd paints.
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effect of mn acac 3 and its combination with 2 2 bipyridine on the Autoxidation and oligomerisation of ethyl linoleate
2005Co-Authors: Z O Oyman, Remy Van Gorkum, Weihua Ming, R Van Der Linde, Elisabeth BouwmanAbstract:In this study we investigated the Autoxidation and oligomerisation of ethyl linoleate (EL) catalysed by [Mn(III)(acac)3] (acac=2,4-pentanedionate) and its combination with 2,2-bipyridine (bpy), in comparison with the EL catalysed by Co(II) 2-ethylhexanoate (Co-EH). EL is a model compound for the alkyd resin in alkyd paints, Co-EH is a common drying catalyst for alkyd paints, and [Mn(acac)3] and the [Mn(acac)3]/bpy combination are potential new drying catalysts. The Autoxidation of EL was studied through time-resolved Raman spectroscopy, oxygen uptake measurements, and peroxide amount determination. To follow the oligomerisation of EL in time, size exclusion chromatography was used. Head-space GC-MS measurements were performed to determine the amounts of hexanal and pentanal that were formed as volatile byproducts during the Autoxidation of EL. The Autoxidation rates of EL in the presence of Co-EH and [Mn(acac)3]/bpy were found to be similar, while the rate in the presence of [Mn(acac)3] was slower. The extent of EL oligomerisation was much higher for [Mn(acac)3] compared to the other catalysts. Different mechanisms are proposed for the mode of action for each of the catalysts: Co-EH is primarily a hydroperoxide decomposition catalyst, as is [Mn(acac)3], only less active. The [Mn(acac)3]/bpy combination probably forms the very reactive complexes [Mn(III)(acac)2(bpy)]+ and [Mn(II)(acac)2(bpy)], which are responsible for a very high Autoxidation rate, but also for significant degradation of the formed EL oligomers via β-scission reactions due to the promotion of alkoxy radical formation.
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fast Autoxidation of ethyl linoleate catalyzed by mn acac 3 and bipyridine a possible drying catalyst for alkyd paints
2004Co-Authors: Remy Van Gorkum, Elisabeth Bouwman, Jan ReedijkAbstract:It has been found that [Mn(acac)3], by functioning both as a radical initiator and a hydroperoxide decomposition catalyst, has a very high activity in the Autoxidation of ethyl linoleate (EL), a model compound for the binder molecule in household alkyd paint. Adding 1 equiv of bipyridine (bpy) to a reaction mixture of EL and [Mn(acac)3] significantly enhances the Autoxidation rate. The redox properties of [Mn(acac)3] and [Mn(acac)2(bpy)] have been compared with cyclic voltammetry; the observed peak potentials show that addition of bpy facilitates reduction of Mn(III) to Mn(II), which may explain the increase in Autoxidation activity.
Yunbo Li - One of the best experts on this subject based on the ideXlab platform.
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role of cu zn superoxide dismutase in xenobiotic activation i chemical reactions involved in the cu zn superoxide dismutase accelerated oxidation of the benzene metabolite 1 4 hydroquinone
1996Co-Authors: Yunbo Li, Periannan Kuppusamy, Jay L Zweier, Mykhailovych Anatolii TrushAbstract:Cu/Zn-superoxide dismutase (Cu/Zn-SOD) has been shown to modulate the Autoxidation of a variety of phenoic compounds, including 1,4-hydroquinone (HQ), a benzene-derived metabolite. The acceleration of Autoxidation of HQ by Cu/Zn-SOD results in the production of 1,4-benzoquinone (BQ). It has been proposed that the chemical mechanism involved in the Cu/Zn-SOD-catalyzed Autoxidation of HQ may be occur through either its conventional activity as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. However, Cu/Zn-SOD-accelerated oxidation of HQ has not been resolved experimentally. In this study, with ESR spectroscopy we investigated further the chemical reactions involved in the SOD-accelerated oxidation of HQ. In phosphate-buffered saline (PSB), HQ underwent a slow Autoxidation to BQ, which was accelerated by Cu/Zn-SOD, Mn-SOD, or Fe-SOD with similar efficiency. In contrast, among free metals, only Cu(II) strongly mediated the oxidation of HQ to BQ. Mn(II) exhibited a slight capacity to oxidize HQ, whereas neither FE(II) nor FE(III) was capable of modulating the Autoxidation of HG. The presence of either form of SOD also dramatically enhanced the formation of semiquinone anion radicals SQ-. from HQ. The SOD-accelerated oxidation of HQ was also accompanied by the generation of H202. In PBS containing bovine serum albumin (BSA) (PBS/BSA), HQ did not undergo Autoxidation to SQ-., and as such the presence of SOD was unable to induce the formation of either SQ-. or BQ or the consumption of O2. The addition of 10 microM BQ to HQ (100 or 1000 microM) in PBS/BSA resulted in the formation of SQ-. and initiated a slow rate of oxidation of HQ to BQ. In this case, the presence of Cu/Zn-SOD strongly accelerated the oxidation of HQ to SQ-. and BQ and the utilization of O2. Furthermore, the enhancement by Cu/Zn-SOD of the generation of SQ-. or BQ from HQ in PBS/BSA was extensively inhibited under anaerobic conditions. The enhancement of SQ-. generation from HQ by all three forms of SOD does not support the possibility that Cu/Zn-SOD can oxidize SQ-. to BQ. Taken together, this study demonstrates that unlike free copper, Cu/Zn-SOD does not directly interact with HQ to cause its oxidation to BQ. Rather, the Autoxidation of HQ to SQ-. is a prerequisite for the enhancing capacity of Cu/Zn-SOD, and the dismutation of superoxide anion radicals generated from the SQ-. in the presence of O2 appears to be the underlying mechanism responsible for the enhancement by Cu/Zn-SOD of the oxidation of HQ.
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role of cu zn superoxide dismutase in xenobiotic activation i chemical reactions involved in the cu zn superoxide dismutase accelerated oxidation of the benzene metabolite 1 4 hydroquinone
1996Co-Authors: Yunbo Li, Periannan Kuppusamy, Jay L Zweier, Mykhailovych Anatolii TrushAbstract:Cu/Zn-superoxide dismutase (Cu/Zn-SOD) has been shown to modulate the Autoxidation of a variety of phenoic compounds, including 1,4-hydroquinone (HQ), a benzene-derived metabolite. The acceleration of Autoxidation of HQ by Cu/Zn-SOD results in the production of 1,4-benzoquinone (BQ). It has been proposed that the chemical mechanism involved in the Cu/Zn-SOD-catalyzed Autoxidation of HQ may be occur through either its conventional activity as a superoxide:superoxide oxidoreductase or as a semiquinone:superoxide oxidoreductase. However, Cu/Zn-SOD-accelerated oxidation of HQ has not been resolved experimentally. In this study, with ESR spectroscopy we investigated further the chemical reactions involved in the SOD-accelerated oxidation of HQ. In phosphate-buffered saline (PSB), HQ underwent a slow Autoxidation to BQ, which was accelerated by Cu/Zn-SOD, Mn-SOD, or Fe-SOD with similar efficiency. In contrast, among free metals, only Cu(II) strongly mediated the oxidation of HQ to BQ. Mn(II) exhibited a slight capacity to oxidize HQ, whereas neither FE(II) nor FE(III) was capable of modulating the Autoxidation of HG. The presence of either form of SOD also dramatically enhanced the formation of semiquinone anion radicals SQ-. from HQ. The SOD-accelerated oxidation of HQ was also accompanied by the generation of H202. In PBS containing bovine serum albumin (BSA) (PBS/BSA), HQ did not undergo Autoxidation to SQ-., and as such the presence of SOD was unable to induce the formation of either SQ-. or BQ or the consumption of O2. The addition of 10 microM BQ to HQ (100 or 1000 microM) in PBS/BSA resulted in the formation of SQ-. and initiated a slow rate of oxidation of HQ to BQ. In this case, the presence of Cu/Zn-SOD strongly accelerated the oxidation of HQ to SQ-. and BQ and the utilization of O2. Furthermore, the enhancement by Cu/Zn-SOD of the generation of SQ-. or BQ from HQ in PBS/BSA was extensively inhibited under anaerobic conditions. The enhancement of SQ-. generation from HQ by all three forms of SOD does not support the possibility that Cu/Zn-SOD can oxidize SQ-. to BQ. Taken together, this study demonstrates that unlike free copper, Cu/Zn-SOD does not directly interact with HQ to cause its oxidation to BQ. Rather, the Autoxidation of HQ to SQ-. is a prerequisite for the enhancing capacity of Cu/Zn-SOD, and the dismutation of superoxide anion radicals generated from the SQ-. in the presence of O2 appears to be the underlying mechanism responsible for the enhancement by Cu/Zn-SOD of the oxidation of HQ.
Remy Van Gorkum - One of the best experts on this subject based on the ideXlab platform.
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the oxidative drying of alkyd paint catalysed by metal complexes
2005Co-Authors: Remy Van Gorkum, Elisabeth BouwmanAbstract:The focus of this review paper is the transition metal chemistry involved in alkyd (paint) drying. A general introduction is given on the composition of a common alkyd paint and the latest insights in the structure of the dried alkyd film are discussed. The drying of an alkyd paint is an Autoxidation process catalysed by transition metal salts. Thorough investigations have shed light on the role of the transition metals as catalysts in the drying process and a wealth of spectroscopic techniques have been used to monitor the Autoxidation process and to analyse the Autoxidation products. More recently, these techniques have been used to develop a test reaction for the rapid screening of new manganese and iron based drier catalysts. Promising new driers based on simple manganese complexes have been proposed as replacements for the environmentally unfriendly cobalt driers that are presently used in alkyd paints.
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effect of mn acac 3 and its combination with 2 2 bipyridine on the Autoxidation and oligomerisation of ethyl linoleate
2005Co-Authors: Z O Oyman, Remy Van Gorkum, Weihua Ming, R Van Der Linde, Elisabeth BouwmanAbstract:In this study we investigated the Autoxidation and oligomerisation of ethyl linoleate (EL) catalysed by [Mn(III)(acac)3] (acac=2,4-pentanedionate) and its combination with 2,2-bipyridine (bpy), in comparison with the EL catalysed by Co(II) 2-ethylhexanoate (Co-EH). EL is a model compound for the alkyd resin in alkyd paints, Co-EH is a common drying catalyst for alkyd paints, and [Mn(acac)3] and the [Mn(acac)3]/bpy combination are potential new drying catalysts. The Autoxidation of EL was studied through time-resolved Raman spectroscopy, oxygen uptake measurements, and peroxide amount determination. To follow the oligomerisation of EL in time, size exclusion chromatography was used. Head-space GC-MS measurements were performed to determine the amounts of hexanal and pentanal that were formed as volatile byproducts during the Autoxidation of EL. The Autoxidation rates of EL in the presence of Co-EH and [Mn(acac)3]/bpy were found to be similar, while the rate in the presence of [Mn(acac)3] was slower. The extent of EL oligomerisation was much higher for [Mn(acac)3] compared to the other catalysts. Different mechanisms are proposed for the mode of action for each of the catalysts: Co-EH is primarily a hydroperoxide decomposition catalyst, as is [Mn(acac)3], only less active. The [Mn(acac)3]/bpy combination probably forms the very reactive complexes [Mn(III)(acac)2(bpy)]+ and [Mn(II)(acac)2(bpy)], which are responsible for a very high Autoxidation rate, but also for significant degradation of the formed EL oligomers via β-scission reactions due to the promotion of alkoxy radical formation.
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fast Autoxidation of ethyl linoleate catalyzed by mn acac 3 and bipyridine a possible drying catalyst for alkyd paints
2004Co-Authors: Remy Van Gorkum, Elisabeth Bouwman, Jan ReedijkAbstract:It has been found that [Mn(acac)3], by functioning both as a radical initiator and a hydroperoxide decomposition catalyst, has a very high activity in the Autoxidation of ethyl linoleate (EL), a model compound for the binder molecule in household alkyd paint. Adding 1 equiv of bipyridine (bpy) to a reaction mixture of EL and [Mn(acac)3] significantly enhances the Autoxidation rate. The redox properties of [Mn(acac)3] and [Mn(acac)2(bpy)] have been compared with cyclic voltammetry; the observed peak potentials show that addition of bpy facilitates reduction of Mn(III) to Mn(II), which may explain the increase in Autoxidation activity.
Isabel W C E Arends - One of the best experts on this subject based on the ideXlab platform.
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catalytic oxidations mediated by metal ions and nitroxyl radicals
2006Co-Authors: Roger A Sheldon, Isabel W C E ArendsAbstract:Abstract The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed, from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as TEMPO, and reactive diacylnitroxyls, derived from N -hydroxy imides, such as N -hydroxyphthalimide (NHPI). NHPI catalyzes a wide variety of free radical Autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal cocatalysts improved conversions and selectivities are obtained in the Autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. In combination with transition metal cocatalysts, notably cobalt, NHPI and related compounds, such as N -hydroxysaccharin (NHS), afford effective catalytic systems for the Autoxidation of hydrocarbons, e.g. toluenes to carboxylic acids and cycloalkanes to the corresponding ketones. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g. alcohols, with single oxygen donors such as hypochlorite via the intermediate formation of the corresponding oxoammonium cation. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols via metal-centred dehydrogenation. The role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu). In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme.
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aerobic oxidation of cycloalkanes alcohols and ethylbenzene catalyzed by the novel carbon radical chain promoter nhs n hydroxysaccharin
2004Co-Authors: Xavier Baucherel, Luca Gonsalvi, Isabel W C E Arends, Simon Ellwood, Roger A SheldonAbstract:Replacement of Ishii's N-hydroxyphthalimide (NHPI) with the novel carbon radical chain promoter N-hydroxysaccharin (NHS) affords, in combination with metal salts, notably Co, or other additives, selective catalytic Autoxidation of hydrocarbons, alcohols and alkylbenzenes under mild conditions (25–100 °C, O2 1 atm). The effects of solvent, temperature and the nature of the additives were investigated to give an optimised oxidation protocol for the various systems. The NHS/Co combination was more reactive than NHPI/Co in the Autoxidation of cycloalkanes. In contrast, the opposite order of reactivity was observed in the Autoxidation of ethylbenzene and alcohols. It is suggested, on the basis of bond dissociation energy (BDE) considerations, that this is a result of a change in the rate-limiting step with the more reactive ethylbenzene and alcohol substrates. In the Autoxidation of the model cycloalkane, cyclododecane, the best results (90% selectivity to a 4 : 1 mixture of alcohol and ketone at 24% conversion) were obtained with NHS/Co(acac)3 in PhCF3 at 80 °C. Competition experiments revealed that, in contrast to what is commonly believed, formation of the dicarboxylic acid by ring opening is not a result of further oxidation of the ketone product. It is suggested that ring opened products are a result of s-scission of the cycloalkoxy radical formed via (metal-catalysed) decomposition of the hydroperoxide. This is suppressed in the presence of NHS (or NHPI) which efficiently scavenge the alkoxy radicals.
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organocatalytic oxidations mediated by nitroxyl radicals
2004Co-Authors: Roger A Sheldon, Isabel W C E ArendsAbstract:The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as the archetypal TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). The different types of reactivity observed are rationalized by considering the bond dissociation energies (BDEs) of the respective N-hydroxy precursors, substrates and reaction intermediates. Reactive diacylnitroxyl radicals are generated in situ from the corresponding N-hydroxy compound. The protagonist, NHPI, catalyzes a wide variety of free radical Autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal co-catalysts improved conversions and selectivities are obtained in the Autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. For example, cyclohexylbenzene afforded the 1-hydroperoxide in 97.6% selectivity at 32% conversion when the Autoxidation was performed in the presence of 0.5 mol % NHPI, and the product hydroperoxide as initiator, at 100 °C. This forms the basis for a potential coproduct-free route from benzene to phenol. In combination with transition metal co-catalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin NHS, afford effective catalytic systems for the effective Autoxidation of hydrocarbons, e.g., toluenes to carboxylic acids, under mild conditions. In the case of the less reactive cycloalkanes, NHS proved to be a more active catalyst than NHPI which is attributed to the higher reactivity of the intermediate nitroxyl radical, resulting from the replacement of a carbonyl group in NHPI by the more strongly electron-attracting sulfonyl group. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g., alcohols, with single oxygen donors such as hypochlorite and organic peracids. The reactions involve the intermediate formation of the corresponding oxoammonium cation as the active oxidant. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols. These reactions involve metal-centered dehydrogenations and the role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu) and oxidation of the alcohol (Cu) via hydrogen abstraction or one-electron oxidation processes. Detailed mechanistic investigations, including kinetic isotope effects, revealed that oxoammonium cations are not involved as intermediates in these reactions. In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme. Similarly, N-hydroxy compounds such as NHPI also act as mediators in laccase-catalyzed oxidations of alcohols. These reactions are assumed to involve one electron oxidation of the N-hydroxy compound, leading to the formation of a proton and the nitroxyl radical, which abstracts a hydrogen atom from the substrate. However, neither of these laccase-based systems has, as yet, attained the activity and scope of the TEMPO/hypochlorite system.