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Akira Oku - One of the best experts on this subject based on the ideXlab platform.
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stereochemistry in carbenoid formation by Bromine lithium and Bromine zinc exchange reactions of gem dibromo compounds
Tetrahedron, 1994Co-Authors: Toshiro Harada, Takeshi Katsuhira, Kazuhiro Hattori, Akira OkuAbstract:Abstract Stereochemistry in generation of lithium and zincate carbenoids by Bromine/metal exchange reactions of gem -dibromo compounds with BuLi and lithium triorganozincates, respectively, has been investigated. Both lithium and zincate carbenoids derived from 1,1-dibromoalkenes are demonstrated to be configurationally stable at low temperatures whereas, in the presence of the unreacted starting dibromoalkenes, the lithium carbenoids, but not the zincate carbenoids, undergo facile isomerization at the carbenoid carbons. Zincate carbenoids derived from 1,1-dibromocyclopropanes undergo isomerization slowly at the carbenoid carbons in the presence of the starting dibromocyclopropanes. The isomerization reactions of the lithium and zincate carbenoids are proved to proceed through a mechanism involving a Bromine/metal exchange between the carbenoids and the starting dirbromo compounds. Highly stereoselective formation of the lithium carbenoids is observed in the reaction of 1,1-dibromoalkenes under the thermodynamic conditions. Under kinetically controlled conditions, both the Bromine/lithium and Bromine/zinc exchange reactions take place preferentially at the sterically more hindered Bromine Atom of the starting gem -dibromo compounds. The observation is rationalized by an exchange mechanism involving a linear transition state of either an ate complex formation or an S N 2 reaction at the more hindered Bromine Atom where strain relief due to the elongation of the carbon-Bromine bond is expected.
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stereochemistry in carbenoid formation by Bromine lithium and Bromine zinc exchange reactions of 1 1 dibromoalkenes higher reactivity of the sterically more hindered Bromine Atom
Journal of Organic Chemistry, 1992Co-Authors: Toshiro Harada, Takeshi Katsuhira, Akira OkuAbstract:Both lithium and zincate carbenoids (R 1 -(R 2 )C=C(Br)M; M=Li and Zn(Bu) 2 Li) generated by the halogen/metal exchange reaction of 1,1-dibromoalkene 1 with BuLi and (Bu) 3 ZnLi, respectively, are configurationally stable at low temperatures, but in the presence of excess 1, the lithium carbenoids undergo facile isomerization at the carbenoid carbons. Under kinetically controlled conditions, both the Br/Li and Br/Zn exchange reactions take place preferentially at the sterically more hindered Bromine Atom of 1
Paul B Shepson - One of the best experts on this subject based on the ideXlab platform.
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direct detection of atmospheric Atomic Bromine leading to mercury and ozone depletion
Proceedings of the National Academy of Sciences of the United States of America, 2019Co-Authors: Siyuan Wang, Paul B Shepson, Stephen M Mcnamara, Christopher W Moore, Daniel Obrist, Alexandra Steffen, Ralf M Staebler, Angela R W Raso, Kerri A PrattAbstract:Bromine Atoms play a central role in atmospheric reactive halogen chemistry, depleting ozone and elemental mercury, thereby enhancing deposition of toxic mercury, particularly in the Arctic near-surface troposphere. However, direct Bromine Atom measurements have been missing to date, due to the lack of analytical capability with sufficient sensitivity for ambient measurements. Here we present direct atmospheric Bromine Atom measurements, conducted in the springtime Arctic. Measured Bromine Atom levels reached 14 parts per trillion (ppt, pmol mol−1; 4.2 × 108 Atoms per cm−3) and were up to 3–10 times higher than estimates using previous indirect measurements not considering the critical role of molecular Bromine. Observed ozone and elemental mercury depletion rates are quantitatively explained by the measured Bromine Atoms, providing field validation of highly uncertain mercury chemistry. Following complete ozone depletion, elevated Bromine concentrations are sustained by photochemical snowpack emissions of molecular Bromine and nitrogen oxides, resulting in continued atmospheric mercury depletion. This study provides a breakthrough in quantitatively constraining Bromine chemistry in the polar atmosphere, where this chemistry connects the rapidly changing surface to pollutant fate.
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Bromine Atom production and chain propagation during springtime arctic ozone depletion events in barrow alaska
Atmospheric Chemistry and Physics, 2016Co-Authors: Paul B Shepson, C R Thompson, J Liao, Greg L Huey, C A Cantrell, F Flocke, John J OrlandoAbstract:Abstract. Ozone depletion events (ODEs) in the Arctic are primarily controlled by a Bromine radical-catalyzed destruction mechanism that depends on the efficient production and recycling of Br Atoms. Numerous laboratory and modeling studies have suggested the importance of heterogeneous recycling of Br through HOBr reaction with bromide on saline surfaces. On the other hand, the gas-phase regeneration of Bromine Atoms through BrO–BrO radical reactions has been assumed to be an efficient, if not dominant, pathway for Br reformation and thus ozone destruction. Indeed, it has been estimated that the rate of ozone depletion is approximately equal to twice the rate of the BrO self-reaction. Here, we use a zero-dimensional, photochemical model, largely constrained to observations of stable atmospheric species from the 2009 Ocean–Atmosphere–Sea Ice–Snowpack (OASIS) campaign in Barrow, Alaska, to investigate gas-phase Bromine radical propagation and recycling mechanisms of Bromine Atoms for a 7-day period during late March. This work is a continuation of that presented in Thompson et al. (2015) and utilizes the same model construct. Here, we use the gas-phase radical chain length as a metric for objectively quantifying the efficiency of gas-phase recycling of Bromine Atoms. The gas-phase Bromine chain length is determined to be quite small, at
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chlorine and Bromine Atom ratios in the springtime arctic troposphere as determined from measurements of halogenated volatile organic compounds
Journal of Geophysical Research, 2006Co-Authors: Adam D Keil, Paul B ShepsonAbstract:Received 24 January 2006; revised 19 April 2006; accepted 16 May 2006; published 7 September 2006. [1] The concentrations of a suite of halogenated volatile organic compounds (HVOCs) were measured near Barrow, Alaska, from January to April 2005. The HVOCs are produced from the reaction of Bromine and chlorine Atoms with ethene and propene. During periods of decreasing ozone concentration, increases in the HVOC concentrations allowed for the calculation of the ratio of Bromine to chlorine radical concentrations, based on available kinetic data. We use these concentration data to interrogate the chemistry that results in tropospheric ozone depletion in the Arctic, the possible sources of ozone depleting halogen molecules, and the spatial scale in which ozone depletion occurs. We report calculated halogen Atom concentration ratios ([Br]/[Cl]) during partial ozone depletion events. The concentration ratio was observed to range from 80 ± 30 to 990 ± 300 when ozone concentrations were above 15 ppb. These data make it clear that chlorine and Bromine Atom chemistry is active in the Arctic troposphere beginning at twilight, even absent large-scale ozone depletion, and that the sources of the chlorine Atoms are poorly understood.
Bernard Pirotte - One of the best experts on this subject based on the ideXlab platform.
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synthesis and pharmacological activity of n 2 2 dimethyl 3 4 dihydro 2h 1 benzopyran 4 yl 4h 1 2 4 benzothiadiazine 3 carboxamides 1 1 dioxides on rat uterus rat aorta and rat pancreatic β cells
European Journal of Medicinal Chemistry, 2012Co-Authors: S Khelili, Nadjib Kihal, Mohamed Yekhlef, Pascal De Tullio, Philippe Lebrun, Bernard PirotteAbstract:N-(2,2-Dimethyl-3,4-dihydro-2H-1-benzopyran-4-yl)-4H-1,2,4-benzothiadiazine-3-carboxamides 1,1-dioxides were prepared and evaluated on rat uterus, rat aortic rings and rat pancreatic β-cells. Pharmacological studies conducted on rat uterus indicated that several of these original hybrid compounds displayed a strong myorelaxant activity. The most active compounds hold a Bromine Atom at the 6-position of the dihydrobenzopyran ring. Moreover, the compounds failed to display a marked inhibitory effect on insulin secretion and vascular myogenic activity. These features suggest that the 6-bromo compounds could be relatively selective towards the uterine smooth muscle.
C R Thompson - One of the best experts on this subject based on the ideXlab platform.
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Bromine Atom production and chain propagation during springtime arctic ozone depletion events in barrow alaska
Atmospheric Chemistry and Physics, 2016Co-Authors: Paul B Shepson, C R Thompson, J Liao, Greg L Huey, C A Cantrell, F Flocke, John J OrlandoAbstract:Abstract. Ozone depletion events (ODEs) in the Arctic are primarily controlled by a Bromine radical-catalyzed destruction mechanism that depends on the efficient production and recycling of Br Atoms. Numerous laboratory and modeling studies have suggested the importance of heterogeneous recycling of Br through HOBr reaction with bromide on saline surfaces. On the other hand, the gas-phase regeneration of Bromine Atoms through BrO–BrO radical reactions has been assumed to be an efficient, if not dominant, pathway for Br reformation and thus ozone destruction. Indeed, it has been estimated that the rate of ozone depletion is approximately equal to twice the rate of the BrO self-reaction. Here, we use a zero-dimensional, photochemical model, largely constrained to observations of stable atmospheric species from the 2009 Ocean–Atmosphere–Sea Ice–Snowpack (OASIS) campaign in Barrow, Alaska, to investigate gas-phase Bromine radical propagation and recycling mechanisms of Bromine Atoms for a 7-day period during late March. This work is a continuation of that presented in Thompson et al. (2015) and utilizes the same model construct. Here, we use the gas-phase radical chain length as a metric for objectively quantifying the efficiency of gas-phase recycling of Bromine Atoms. The gas-phase Bromine chain length is determined to be quite small, at
Toshiro Harada - One of the best experts on this subject based on the ideXlab platform.
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stereochemistry in carbenoid formation by Bromine lithium and Bromine zinc exchange reactions of gem dibromo compounds
Tetrahedron, 1994Co-Authors: Toshiro Harada, Takeshi Katsuhira, Kazuhiro Hattori, Akira OkuAbstract:Abstract Stereochemistry in generation of lithium and zincate carbenoids by Bromine/metal exchange reactions of gem -dibromo compounds with BuLi and lithium triorganozincates, respectively, has been investigated. Both lithium and zincate carbenoids derived from 1,1-dibromoalkenes are demonstrated to be configurationally stable at low temperatures whereas, in the presence of the unreacted starting dibromoalkenes, the lithium carbenoids, but not the zincate carbenoids, undergo facile isomerization at the carbenoid carbons. Zincate carbenoids derived from 1,1-dibromocyclopropanes undergo isomerization slowly at the carbenoid carbons in the presence of the starting dibromocyclopropanes. The isomerization reactions of the lithium and zincate carbenoids are proved to proceed through a mechanism involving a Bromine/metal exchange between the carbenoids and the starting dirbromo compounds. Highly stereoselective formation of the lithium carbenoids is observed in the reaction of 1,1-dibromoalkenes under the thermodynamic conditions. Under kinetically controlled conditions, both the Bromine/lithium and Bromine/zinc exchange reactions take place preferentially at the sterically more hindered Bromine Atom of the starting gem -dibromo compounds. The observation is rationalized by an exchange mechanism involving a linear transition state of either an ate complex formation or an S N 2 reaction at the more hindered Bromine Atom where strain relief due to the elongation of the carbon-Bromine bond is expected.
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stereochemistry in carbenoid formation by Bromine lithium and Bromine zinc exchange reactions of 1 1 dibromoalkenes higher reactivity of the sterically more hindered Bromine Atom
Journal of Organic Chemistry, 1992Co-Authors: Toshiro Harada, Takeshi Katsuhira, Akira OkuAbstract:Both lithium and zincate carbenoids (R 1 -(R 2 )C=C(Br)M; M=Li and Zn(Bu) 2 Li) generated by the halogen/metal exchange reaction of 1,1-dibromoalkene 1 with BuLi and (Bu) 3 ZnLi, respectively, are configurationally stable at low temperatures, but in the presence of excess 1, the lithium carbenoids undergo facile isomerization at the carbenoid carbons. Under kinetically controlled conditions, both the Br/Li and Br/Zn exchange reactions take place preferentially at the sterically more hindered Bromine Atom of 1
