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Eric J Thomas - One of the best experts on this subject based on the ideXlab platform.
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concerning the 1 5 stereocontrol in tin iv chloride promoted reactions of 4 and 5 alkoxyalk 2 enylstannanes trapping intermediate allyltin trichlorides using phenyllithium
Organic and Biomolecular Chemistry, 2012Co-Authors: Lindsay A Hobson, Eric J ThomasAbstract:Transmetallation of the 5-benzyloxy-4-methylpent-2-en-1-yl(tributyl)- and -(triphenyl)stannanes 1 and 8 using tin(IV) chloride generates an allyltin trichloride that reacts with aldehydes to give (Z)-1,5-anti-6-benzyloxy-5-methylhex-3-en-1-ols 2. The allyltin trichloride believed to be the key intermediate in these reactions has been trapped by phenyllithium to give anti-5-benzyloxy-4-methylpent-1-en-3-yl(triphenyl)stannane 9. Transmetallation of this anti-5-benzyloxy-4-methylpent-1-en-3-yl(triphenyl)stannane 9 generated an allyltin trichloride that reacted with aldehydes to give the (Z)-1,5-syn-6-benzyloxy-5-methylhex-3-en-1-ols 23 and was trapped by phenyllithium to give syn-5-benzyloxy-4-methylpent-1-en-3-yl(triphenyl)stannane 24. Similar stereoselectivity was observed for tin(IV) chloride promoted reactions of this syn-5-benzyloxy-4-methylpent-1-en-3-yl(triphenyl)stannane 24 with aldehydes and with phenyllithium. The allyltin trichlorides generated by transmetallation of 4-hydroxy- and 4-benzyloxy-pent-2-enyl(triphenyl)stannanes 34 and 35 were similarly trapped by phenyllithium to give 4-hydroxy- and 4-benzyloxy-pent-1-en-3-ylstannanes 36 and 37 whose configurations were established by correlation with known compounds. This work confirmed the configurations of the intermediate allyltin trichlorides involved in tin(IV) chloride promoted reactions of 4- and 5-alkoxypent-2-enylstannanes with aldehydes and showed that the high levels of remote stereocontrol were due mainly to kinetically controlled transmetallation. A fuller mechanistic scheme is proposed for the reactions in the 5-benzyloxy-4-methylpent-2-enylstannane series together with relevant 119Sn NMR data.
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1 5 asymmetric induction in reactions between aldehydes and 4s 5 tert butyldimethylsilyloxy 4 hydroxypent 2 enyl tributyl stannane promoted by tin iv chloride
Journal of The Chemical Society-perkin Transactions 1, 1995Co-Authors: Robert J Maguire, Eric J ThomasAbstract:[(4S)-5-(tert-Butyldimethylsilyloxy)-4-hydroxypent-2-enyl](tributyl)stannane 18 has been prepared from di-O-isopropylidene-D-mannitol 8. Oxidative cleavage of the mannitol derivative followed by condensation with triethyl phosphonoacetate and reduction gave the alcohol 10 which was converted into the xanthate 11. Deprotection gave the dihydroxy xanthate 12 which was protected as its bis-tert-butyldimethylsilyl ether 13. This rearranged on heating in toluene to give the dithiocarbonate 15 which reacted with tributyltin hydride under free radical conditions to give the [(4S)-4,5-bis(tert-butyldimethylsilyloxy)pent-2-enyl](tributyl)stannane 16 as an approximately 9:1 mixture of E- and Z-isomers. Deprotection and selective protection of the primary hydroxy group provided the [(4S)-5-(tert-butyldimethylsilyloxy)-4-hydroxypent-2-enyl]stannane 18. As a shorter route, the primary hydroxy group of the dihydroxy xanthate 12 was protected as its tert-butyldimethylsilyl ether 14 which underwent clean rearrangement into the dithiocarbonate 19 when heated in toluene. Reaction with tributyltin hydride under free radical conditions then gave the (5-tert-butyldimethylsilyloxy-4-hydroxypent-2-enyl)stannane 18. Treatment of this 4,5-disubstituted pentenylstannane with tin(IV) chloride generated an allyltin trichloride which reacted with aldehydes with excellent 1,5-asymmetric induction to give 1,5-syn-products, e.g. 20, 29–31 and 41. The stereoselectivity of these reactions would appear to be controlled by the 4-hydroxy substituent rather than by the 5-tert-butyldimethylsilyl group. Aspects of the chemistry of these products, in particular their conversion into 2,6-disubstituted 5,6-dihydro-2H-pyrans, was investigated.
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1 5 asymmetric induction in reactions between aldehydes and 4s 5 tert butyldimethylsilyloxy 4 hydroxypent 2 enyl tributyl stannane promoted by tin iv chloride
Journal of The Chemical Society-perkin Transactions 1, 1995Co-Authors: Robert J Maguire, Eric J ThomasAbstract:[(4S)-5-(tert-Butyldimethylsilyloxy)-4-hydroxypent-2-enyl](tributyl)stannane 18 has been prepared from di-O-isopropylidene-D-mannitol 8. Oxidative cleavage of the mannitol derivative followed by condensation with triethyl phosphonoacetate and reduction gave the alcohol 10 which was converted into the xanthate 11. Deprotection gave the dihydroxy xanthate 12 which was protected as its bis-tert-butyldimethylsilyl ether 13. This rearranged on heating in toluene to give the dithiocarbonate 15 which reacted with tributyltin hydride under free radical conditions to give the [(4S)-4,5-bis(tert-butyldimethylsilyloxy)pent-2-enyl](tributyl)stannane 16 as an approximately 9:1 mixture of E- and Z-isomers. Deprotection and selective protection of the primary hydroxy group provided the [(4S)-5-(tert-butyldimethylsilyloxy)-4-hydroxypent-2-enyl]stannane 18. As a shorter route, the primary hydroxy group of the dihydroxy xanthate 12 was protected as its tert-butyldimethylsilyl ether 14 which underwent clean rearrangement into the dithiocarbonate 19 when heated in toluene. Reaction with tributyltin hydride under free radical conditions then gave the (5-tert-butyldimethylsilyloxy-4-hydroxypent-2-enyl)stannane 18. Treatment of this 4,5-disubstituted pentenylstannane with tin(IV) chloride generated an allyltin trichloride which reacted with aldehydes with excellent 1,5-asymmetric induction to give 1,5-syn-products, e.g. 20, 29–31 and 41. The stereoselectivity of these reactions would appear to be controlled by the 4-hydroxy substituent rather than by the 5-tert-butyldimethylsilyl group. Aspects of the chemistry of these products, in particular their conversion into 2,6-disubstituted 5,6-dihydro-2H-pyrans, was investigated.
Fuhui Wang - One of the best experts on this subject based on the ideXlab platform.
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in situ study of the formation process of Stannate conversion coatings on az91d magnesium alloy using electrochemical noise
Corrosion Science, 2010Co-Authors: Fuhui Wang, Tao Zhang, Yawei Shao, Guozhe MengAbstract:The formation process of Stannate conversion coating (CC) on AZ91D alloy was in-situ investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R(n)) and spectral noise resistance (R(sn)), had been employed to analyze the EN data. It was revealed that there exist two distinguishing stages of Stannate CC formation process on AZ91D alloy, including an incubation stage companying with the nucleation and nuclei dissolution process, a periodical growth stage involving hemispherical particles growth and coating dissolution process. Furthermore, the results demonstrated that EN was a powerful tool to investigate rapid electrochemical process, such as CC formation process. (C) 2009 Elsevier Ltd. All rights reserved.
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corrosion of az91d magnesium alloy with a chemical conversion coating and electroless nickel layer
Corrosion Science, 2004Co-Authors: Ying Li, Fuhui WangAbstract:A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline Stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the Stannate treated AZ91D alloy showed the presence of MgSnO3 (.) H2O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution. (C) 2003 Elsevier Ltd. All rights reserved.
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corrosion of az91d magnesium alloy with a chemical conversion coating and electroless nickel layer
Corrosion Science, 2004Co-Authors: Ying Li, Fuhui WangAbstract:A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline Stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the Stannate treated AZ91D alloy showed the presence of MgSnO3 (.) H2O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution. (C) 2003 Elsevier Ltd. All rights reserved.
Hidetaka Konno - One of the best experts on this subject based on the ideXlab platform.
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Effects of pH and temperature on the deposition properties of Stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy
Electrochimica Acta, 2008Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:Abstract The effects of pH and temperature of a Stannate bath on the quality of Stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the Stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.
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effect of surface pretreatment by acid pickling on the density of Stannate conversion coatings formed on az91 d magnesium alloy
Surface & Coatings Technology, 2007Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:Abstract Hydrofluoric and hydrochloric acid solutions and a mixture of them were tested as pickling solutions for AZ91 D Mg alloy before application of Stannate coatings. Optical microscopy and energy dispersive X-ray spectroscopy (EDX) of the alloy surface after the pickling process showed that the Mg-rich α phase dissolved preferentially rather than the Al-rich β phase in hydrochloric acid solution. On the other hand, in hydrofluoric acid solution, Mg dissolved in a form of pitting corrosion. Pickling pretreatment with a mixture of these acids at an optimal concentration and an optimal pickling time resulted in relatively uniform dissolution of the alloy surface. The potentiodynamic polarization technique was used to investigate the anodic behavior of the uncoated and coated magnesium alloy in borate buffer solution. The morphology of the coatings was observed using a scanning electron microscope (SEM) before and after corrosion tests. The experimental results showed that coating film density and corrosion resistance of Stannate-coated samples prepared with pickling pretreatment were improved compared with those of the coated sample without pickling pretreatment.
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improvement in Stannate chemical conversion coatings on az91 d magnesium alloy using the potentiostatic technique
Electrochimica Acta, 2007Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:The deposition of Stannate chemical conversion coatings on AZ91 D magnesium alloy by using the potentiostatic technique was investigated to improve uniformity and corrosion protection performance of the coating film. It was found that anodic polarization during the Stannate conversion process accelerated dissolution of magnesium ions and promoted deposition of the coating film. Optimal polarization potential was explored as a function of corrosion protection performance evaluated by potentiodynamic polarization curves in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the Stannate-coated samples confirmed significant improvement in uniformity and corrosion resistance of coatings deposited by the potentiostatic technique compared to those of coatings deposited by the simple immersion method.
Marei Hacke - One of the best experts on this subject based on the ideXlab platform.
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Production and validation of model iron-tannate dyed textiles for use as historic textile substitutes in stabilisation treatment studies
Chemistry Central journal, 2012Co-Authors: Helen F. Wilson, Chris Carr, Marei HackeAbstract:For millennia, iron-tannate dyes have been used to colour ceremonial and domestic objects shades of black, grey, or brown. Surviving iron-tannate dyed objects are part of our cultural heritage but their existence is threatened by the dye itself which can accelerate oxidation and acid hydrolysis of the substrate. This causes many iron-tannate dyed textiles to discolour and decrease in tensile strength and flexibility at a faster rate than equivalent undyed textiles. The current lack of suitable stabilisation treatments means that many historic iron-tannate dyed objects are rapidly crumbling to dust with the knowledge and value they hold being lost forever. This paper describes the production, characterisation, and validation of model iron-tannate dyed textiles as substitutes for historic iron-tannate dyed textiles in the development of stabilisation treatments. Spectrophotometry, surface pH, tensile testing, SEM-EDX, and XRF have been used to characterise the model textiles. On application to textiles, the model dyes imparted mid to dark blue-grey colouration, an immediate tensile strength loss of the textiles and an increase in surface acidity. The dyes introduced significant quantities of iron into the textiles which was distributed in the exterior and interior of the cotton, abaca, and silk fibres but only in the exterior of the wool fibres. As seen with historic iron-tannate dyed objects, the dyed cotton, abaca, and silk textiles lost tensile strength faster and more significantly than undyed equivalents during accelerated thermal ageing and all of the dyed model textiles, most notably the cotton, discoloured more than the undyed equivalents on ageing. The abaca, cotton, and silk model textiles are judged to be suitable for use as substitutes for cultural heritage materials in the testing of stabilisation treatments.
Hassan H Elsentriecy - One of the best experts on this subject based on the ideXlab platform.
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Effects of pH and temperature on the deposition properties of Stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy
Electrochimica Acta, 2008Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:Abstract The effects of pH and temperature of a Stannate bath on the quality of Stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the Stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.
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effect of surface pretreatment by acid pickling on the density of Stannate conversion coatings formed on az91 d magnesium alloy
Surface & Coatings Technology, 2007Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:Abstract Hydrofluoric and hydrochloric acid solutions and a mixture of them were tested as pickling solutions for AZ91 D Mg alloy before application of Stannate coatings. Optical microscopy and energy dispersive X-ray spectroscopy (EDX) of the alloy surface after the pickling process showed that the Mg-rich α phase dissolved preferentially rather than the Al-rich β phase in hydrochloric acid solution. On the other hand, in hydrofluoric acid solution, Mg dissolved in a form of pitting corrosion. Pickling pretreatment with a mixture of these acids at an optimal concentration and an optimal pickling time resulted in relatively uniform dissolution of the alloy surface. The potentiodynamic polarization technique was used to investigate the anodic behavior of the uncoated and coated magnesium alloy in borate buffer solution. The morphology of the coatings was observed using a scanning electron microscope (SEM) before and after corrosion tests. The experimental results showed that coating film density and corrosion resistance of Stannate-coated samples prepared with pickling pretreatment were improved compared with those of the coated sample without pickling pretreatment.
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improvement in Stannate chemical conversion coatings on az91 d magnesium alloy using the potentiostatic technique
Electrochimica Acta, 2007Co-Authors: Hassan H Elsentriecy, Kazuhisa Azumi, Hidetaka KonnoAbstract:The deposition of Stannate chemical conversion coatings on AZ91 D magnesium alloy by using the potentiostatic technique was investigated to improve uniformity and corrosion protection performance of the coating film. It was found that anodic polarization during the Stannate conversion process accelerated dissolution of magnesium ions and promoted deposition of the coating film. Optimal polarization potential was explored as a function of corrosion protection performance evaluated by potentiodynamic polarization curves in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the Stannate-coated samples confirmed significant improvement in uniformity and corrosion resistance of coatings deposited by the potentiostatic technique compared to those of coatings deposited by the simple immersion method.
