Dilute mixtures of n-heptanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275–380 nm) at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were pent-1-ene, CO, vinyl alcohol and ethanal. The photolysis rates and the absolute quantum yields Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.36 ± 0.03, whereas at 700 Torr the total quantum yield was Φ700 = 0.31 ± 0.01. The results may be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified: the radical channel C6H13CHO → C6H13 + HCO, and the molecular channel C6H13CHO → C5H10 + CH2CHOH, having the absolute quantum yields of 0.031 and 0.118 at 700 Torr. The product CH2CHOH tautomerizes to ethanal.
{"title":"Photooxidation of n-heptanal in air: Norrish type I and II processes and quantum yield total pressure dependency","authors":"J. Tadić, I. Juranić, G. Moortgat","doi":"10.1039/B106476M","DOIUrl":"https://doi.org/10.1039/B106476M","url":null,"abstract":"Dilute mixtures of n-heptanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275–380 nm) at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were pent-1-ene, CO, vinyl alcohol and ethanal. The photolysis rates and the absolute quantum yields Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 \u0000= 0.36 ± 0.03, whereas at 700 Torr the total quantum yield was Φ700 \u0000= 0.31 ± 0.01. The results may be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified: the radical channel C6H13CHO → C6H13 \u0000+ HCO, and the molecular channel C6H13CHO → C5H10 \u0000+ CH2CHOH, having the absolute quantum yields of 0.031 \u0000and 0.118 at 700 Torr. The product CH2CHOH tautomerizes to ethanal.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"21 1","pages":"135-140"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79918993","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Formentin, M. Sabater, M. Chrétien, H. García, J. Scaiano
Photolysis of p-toluidides of methacrylic (1a) and cyclohex-2-enecarboxylic (1b) acids in nitrogen-saturated cyclopentane solution yields the corresponding 2-quinolones with over 90% chemoselectivity at almost complete conversion. In the presence of substoichiometric amounts (0.1 equivalents) of chiral inductor, low to moderate enantiomeric excesses (ee) are observed in the photo-product. Ephedrine gave the highest ee (37% ee for the photocyclization of 1a) in a series of 11 chiral inductors including alcohols, amines, aminoalcohols, α-amino and α-hydroxy acids. In the case of the irradiation of 1b in the presence of chiral inductors, both diastereo- and enantioselectivity were observed. A weakly absorbing transient species (λmax 400 nm) was detected following 308 nm laser excitation and was assigned to the zwitterionic enolate intermediate resulting immediately after the concerted electrocyclic ring closure. The lifetime of this intermediate is unaffected by oxygen but is quenched by trifluoroacetic acid (kq = 3.76 × 107 M−1 s−1) and ephedrine (kq = 1.19 × 107 M−1 s−1).
{"title":"Enantioselective photocyclization of p-toluidides of α,β-unsaturated carboxylic acids in solution. A mechanistic and preparative study","authors":"P. Formentin, M. Sabater, M. Chrétien, H. García, J. Scaiano","doi":"10.1039/B105122A","DOIUrl":"https://doi.org/10.1039/B105122A","url":null,"abstract":"Photolysis of p-toluidides of methacrylic (1a) and cyclohex-2-enecarboxylic (1b) acids in nitrogen-saturated cyclopentane solution yields the corresponding 2-quinolones with over 90% chemoselectivity at almost complete conversion. In the presence of substoichiometric amounts (0.1 equivalents) of chiral inductor, low to moderate enantiomeric excesses (ee) are observed in the photo-product. Ephedrine gave the highest ee (37% ee for the photocyclization of 1a) in a series of 11 chiral inductors including alcohols, amines, aminoalcohols, α-amino and α-hydroxy acids. In the case of the irradiation of 1b in the presence of chiral inductors, both diastereo- and enantioselectivity were observed. A weakly absorbing transient species (λmax 400 nm) was detected \u0000following 308 nm laser excitation and was assigned to the zwitterionic enolate intermediate resulting immediately after the concerted electrocyclic ring closure. The lifetime of this intermediate is unaffected by oxygen but is quenched by trifluoroacetic acid (kq \u0000= 3.76 × 107 M−1 s−1) and ephedrine (kq \u0000= 1.19 × 107 M−1 s−1).","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"61 1","pages":"164-167"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91098677","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Wenska, Katarzyna Taras-Goślińska, Katarzyna Lamparska-Kupsik, B. Skalski, M. Gdaniec, Z. Gdaniec
A series of 2′,3′,5′-tri-O-acetyl-5-halogeno-4-thiouridines, where halogen = Br, Cl, F, are synthesized and their photochemical transformations upon irradiation with near-UV light (λ > 300 nm) in aqueous acetonitrile solutions investigated. The main photochemical pathways in each case involve intermolecular additions leading to three main photoproducts with different relative distributions. The photoproducts are isolated and their structures determined based on MS, 1H and 13C NMR and UV spectral data.
{"title":"Photochemical transformations of 5-halogeno-4-thiouridines","authors":"G. Wenska, Katarzyna Taras-Goślińska, Katarzyna Lamparska-Kupsik, B. Skalski, M. Gdaniec, Z. Gdaniec","doi":"10.1039/B108380E","DOIUrl":"https://doi.org/10.1039/B108380E","url":null,"abstract":"A series of 2′,3′,5′-tri-O-acetyl-5-halogeno-4-thiouridines, where halogen = Br, Cl, F, are synthesized and their photochemical transformations upon irradiation with near-UV light (λ > 300 nm) in aqueous acetonitrile solutions investigated. The main photochemical pathways in each case involve intermolecular additions leading to three main photoproducts with different relative distributions. The photoproducts are isolated and their structures determined based on MS, 1H and 13C NMR and UV spectral data.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"65 1","pages":"53-57"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88675291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Katsyuba, A. Chernova, R. Schmutzler, J. Grunenberg
The conformations and vibrational spectra of bis(2-hydroxyphenyl)methane have been analysed within the framework of density functional theory. The calculated force fields (B3LYP/6-31G*) of seven possible energy minima were transformed to internal coordinates, and a set of ten different scaling factors was applied. The scaled quantum mechanical (SQM) method reproduces the experimental IR and Raman spectra with high accuracy. The dependence of the CH2 stretching vibrations on the conformation and the possibility of its use as a probe for larger systems, e.g., calixarenes or polyphenols, are discussed.
{"title":"Vibrational spectra and conformational isomerism of calixarene building blocks. II. Bis(2-hydroxyphenyl)methane","authors":"S. Katsyuba, A. Chernova, R. Schmutzler, J. Grunenberg","doi":"10.1039/B108745B","DOIUrl":"https://doi.org/10.1039/B108745B","url":null,"abstract":"The conformations and vibrational spectra of bis(2-hydroxyphenyl)methane have been analysed within the framework of density functional theory. The calculated force fields (B3LYP/6-31G*) of seven possible energy minima were transformed to internal coordinates, and a set of ten different scaling factors was applied. The scaled quantum mechanical (SQM) method reproduces the experimental IR and Raman spectra with high accuracy. The dependence of the CH2 stretching vibrations on the conformation and the possibility of its use as a probe for larger systems, e.g., calixarenes or polyphenols, are discussed.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"89 1","pages":"67-71"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88265174","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xian‐Wen Wei, A. G. Avent, O. Boltalina, A. Darwish, P. Fowler, J. Sandall, J. M. Street, R. Taylor
Reaction of C60F18 with diethyl bromomalonate in the presence of DBU results in the nucleophilic replacement of either one, two, or three of the most accessible fluorine atoms by CBr(CO2Et)2 moieties, in preference to formation of a cyclopropanated derivative (the normal Bingel reaction). Substitution that takes place δ to the departing fluorine, is the first proven example of SN2′ substitution in a fullerene, and appears to be sterically driven. The ratio of mono-/poly-substitution products can be controlled by varying the rate of addition of the DBU and the molar ratio between C60F18 and the other reagents. The tri-substituted product is an [18]annulene, has an intense emerald-green colour ascribable to the electron delocalisation in the (equatorial) annulene belt (bond length variation 0.018 A), and has C3v symmetry. This is the first example on an annulenic fullerene (moreover of an all-trans annulene or trannulene). The extent of substitution in each compound is identified from the fluorinated fragments (C60F15, C60F16, and C60F17, respectively, for tri-, di-, and mono-substitution) in the EI mass spectra, and by their 1H and 19F NMR spectra. The structure of the tri-substituted [18]annulene was confirmed by single crystal X-ray diffraction. Normal Bingel cycloaddition also takes place between C60F18 and diethyl malonate–DBU in CBr4, to give C60F18C(CO2Et)2 and C60F16C(CO2Et)2 in relatively low yields. Calculations indicate a critical size of substituent required to produce δ-substitution, rather than ipso-substitution of the departing fluorine.
{"title":"Reaction of C60F18 with diethyl bromomalonate: diversion of the Bingel reaction and formation of the first 18π annulenic fullerene","authors":"Xian‐Wen Wei, A. G. Avent, O. Boltalina, A. Darwish, P. Fowler, J. Sandall, J. M. Street, R. Taylor","doi":"10.1039/B105921C","DOIUrl":"https://doi.org/10.1039/B105921C","url":null,"abstract":"Reaction of C60F18 with diethyl bromomalonate in the presence of DBU results in the nucleophilic replacement of either one, two, or three of the most accessible fluorine atoms by CBr(CO2Et)2 moieties, in preference to formation of a cyclopropanated derivative (the normal Bingel reaction). Substitution that takes place δ to the departing fluorine, is the first proven example of SN2′ substitution in a fullerene, and appears to be sterically driven. The ratio of mono-/poly-substitution products can be controlled by varying the rate of addition of the DBU and the molar ratio between C60F18 and the other reagents. The tri-substituted product is an [18]annulene, has an intense emerald-green colour ascribable to the electron delocalisation in the (equatorial) annulene belt (bond length variation 0.018 A), and has C3v symmetry. This \u0000is the first example on an annulenic fullerene (moreover of an all-trans annulene or trannulene). The extent of substitution in each compound is identified from the fluorinated fragments (C60F15, C60F16, and C60F17, respectively, for tri-, di-, and mono-substitution) in the EI mass spectra, and by their 1H and 19F NMR spectra. The structure of the tri-substituted [18]annulene was confirmed by single crystal X-ray diffraction. Normal Bingel cycloaddition also takes place between C60F18 and diethyl malonate–DBU in CBr4, to give C60F18C(CO2Et)2 and C60F16C(CO2Et)2 in relatively low yields. Calculations indicate a critical size of substituent required to produce δ-substitution, rather than ipso-substitution \u0000of the departing fluorine.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"68 1","pages":"41-46"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82450828","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this paper absorption and fluorescence spectra and emission quantum yields and lifetimes of the red colorants, carminic acid and its metal complex, carminic lake, were studied in solution and on painted surfaces. Accelerated photo-aging of carminic acid and lake was investigated in solution, in the presence and absence of a binder (arabic gum) commonly used in water-colour painting, while natural ageing was followed for several months on water-colour painted paper. The study of carminic acid in water as a function of pH showed that the absorption spectrum changes with pH. Four acid–base dissociation steps were detected and the corresponding pKs were determined from spectrophotometric and fluorimetric titrations. The fluorescence quantum yields (in the 10−2–10−4 range) and the lifetimes (on the sub-nanosecond timescale, 90–1000 ps) were markedly dependent on the pH of the medium. Excited state pK*s were calculated by means of the Forster cycle. The acidity decreased upon excitation for the first deprotonation step involving the –COOH group (ΔpK* = −1.9), but increased slightly for the successive deprotonation steps involving three phenolic hydroxy groups (ΔpK* = 0.6, 2.0 and 0.2, respectively). The results obtained from the aging experiments indicate that both carminic acid and lake are bleached during irradiation. While the binder prevents lake from fading, it destabilises the carminic acid. These findings are discussed in the light of the interactions of the dye with the solvent and matrix.
{"title":"Role of protolytic interactions in photo-aging processes of carminic acid and carminic lake in solution and painted layers","authors":"G. Favaro, C. Miliani, A. Romani, M. Vagnini","doi":"10.1039/B104595B","DOIUrl":"https://doi.org/10.1039/B104595B","url":null,"abstract":"In this paper absorption and fluorescence spectra and emission quantum yields and lifetimes of the red colorants, carminic acid and its metal complex, carminic lake, were studied in solution and on painted surfaces. Accelerated photo-aging of carminic acid and lake was investigated in solution, in the presence and absence of a binder (arabic gum) commonly used in water-colour painting, while natural ageing was followed for several months on water-colour painted paper. \u0000 The study of carminic acid in water as a function of pH showed that the absorption spectrum changes with pH. Four acid–base dissociation steps were detected and the corresponding pKs were determined from spectrophotometric and fluorimetric titrations. The fluorescence quantum yields (in the 10−2–10−4 range) and the lifetimes (on the sub-nanosecond timescale, 90–1000 ps) were markedly dependent on the pH of the medium. Excited state pK*s \u0000were calculated by means of the Forster cycle. The acidity decreased upon excitation for the first deprotonation step involving the –COOH group (ΔpK* \u0000= \u0000−1.9), but increased slightly for the successive deprotonation steps involving three phenolic hydroxy groups (ΔpK* \u0000= 0.6, 2.0 and 0.2, respectively). \u0000 The results obtained from the aging experiments indicate that both carminic acid and lake are bleached during irradiation. While the binder prevents lake from fading, it destabilises the carminic acid. These findings are discussed in the light of the interactions of the dye with the solvent and matrix.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"15 1","pages":"192-197"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75383439","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Al-Matar, A. Abdul-Sada, A. G. Avent, P. Fowler, P. Hitchcock, K. M. Rogers, R. Taylor
Reaction of freshly prepared C60Cl6 (from chlorination of [60]fullerene by ICl in benzene) with methyllithium followed by hydrolysis and work-up including HPLC separation yields Cs symmetry C60Me6 (isostructural with C60Br6 and C60Cl6), together with unsymmetrical C60Me12 which is comprised of two of the motifs present in C60Me6 and must arise from the presence of a small amount of C60Cl12 in the C60Cl6. From the same reaction mixture we have also obtained C60Me5Cl [isostructural with C60Ar5Cl and C60(OR)5Cl], hydroxyepoxides [C60Me5O2OH (symmetrical), C60Me5OOH and C60Me4PhO2OH (both unsymmetrical)] and unsymmetrical C60Me5O3H (a cage-opened ketone). The results provide further information concerning the addition patterns and mechanistic features of fullerene chemistry, show that methylated, arylated, alkoxylated and halogenated [60]fullerenes are isostructural, and that C60Cl6 also contains traces of C60PhCl5. Some of the compounds give exceptionally high intensities of the C58+ fragmentation ion during EI mass spectrometry.
{"title":"Isolation and characterisation of symmetrical C60Me6, C60Me5Cl and C60Me5O2OH, together with unsymmetrical C60Me5O3H, C60Me5OOH, C60Me4PhO2OH, and C60Me12; fragmentation of methylfullerenols to C58","authors":"H. Al-Matar, A. Abdul-Sada, A. G. Avent, P. Fowler, P. Hitchcock, K. M. Rogers, R. Taylor","doi":"10.1039/B108292M","DOIUrl":"https://doi.org/10.1039/B108292M","url":null,"abstract":"Reaction of freshly prepared C60Cl6 (from chlorination of [60]fullerene by ICl in benzene) with methyllithium followed by hydrolysis and work-up including HPLC separation yields Cs symmetry C60Me6 (isostructural with C60Br6 and C60Cl6), together with unsymmetrical C60Me12 which is comprised of two of the motifs present in C60Me6 and must arise from the presence of a small amount of C60Cl12 in the C60Cl6. From the same reaction mixture we have also obtained C60Me5Cl [isostructural with C60Ar5Cl and C60(OR)5Cl], hydroxyepoxides [C60Me5O2OH (symmetrical), C60Me5OOH and C60Me4PhO2OH (both unsymmetrical)] and unsymmetrical C60Me5O3H \u0000(a cage-opened ketone). The results provide further information concerning the addition patterns and mechanistic features of fullerene chemistry, show that methylated, arylated, alkoxylated and halogenated [60]fullerenes are isostructural, and that C60Cl6 also contains traces of C60PhCl5. Some of the compounds give exceptionally high intensities of the C58+ fragmentation ion during EI mass spectrometry.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"202 1","pages":"53-58"},"PeriodicalIF":0.0,"publicationDate":"2002-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83553912","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Arnaud, A. Milet, C. Adamo, C. Einhorn, J. Einhorn
In this paper, we report a theoretical study of the hydrogen abstraction reactions from ethylbenzene by a series of imide-N-oxyl radicals. Geometry optimizations and vibrational frequencies were performed using density functional theory at the B3LYP/6-31G(d,p) level. Single-point energy calculations were carried out at the PMP2/6-31G(d,p) and B3LYP/6-311+G(2df,2p) levels. Calculations reproduce experimental trends. In the absence of dioxygen, calculated barriers are not too high to prevent H-abstraction but the process is endothermic. The factors governing the reactivity of nitroxide radicals have been discussed in the scope of the state correlation diagram approach. Moreover, the influence of dioxygen on the mechanism of these reactions has also been studied. Thus, the addition of dioxygen occurs after the H-abstraction by nitroxide radicals and no clear evidence for an energetic barrier to O2 addition was found. However, in the presence of dioxygen the whole process is exothermic and thus H-abstraction becomes irreversible.
本文报道了一系列亚胺- n -氧自由基对乙苯的吸氢反应的理论研究。利用密度泛函理论在B3LYP/6-31G(d,p)水平上进行几何优化和振动频率。在PMP2/6-31G(d,p)和B3LYP/6-311+G(2df,2p)能级进行单点能量计算。计算再现了实验趋势。在没有二氧的情况下，计算出的势垒不会太高，无法阻止氢的提取，但这个过程是吸热的。本文在状态相关图方法的范围内讨论了影响氮氧化物自由基反应性的因素。此外，还研究了双氧对这些反应机理的影响。因此，双氧的加入发生在h被氮氧自由基抽离之后，没有明确的证据表明氧的加入存在能量障碍。然而，在双氧的存在下，整个过程是放热的，因此h的提取是不可逆的。
{"title":"Hydrogen abstraction from ethylbenzene by imide-N-oxyl radicals with and without O2: a DFT theoretical study","authors":"R. Arnaud, A. Milet, C. Adamo, C. Einhorn, J. Einhorn","doi":"10.1039/B205485J","DOIUrl":"https://doi.org/10.1039/B205485J","url":null,"abstract":"In this paper, we report a theoretical study of the hydrogen abstraction reactions from ethylbenzene by a series of imide-N-oxyl radicals. Geometry optimizations and vibrational frequencies were performed using density functional theory at the B3LYP/6-31G(d,p) level. Single-point energy calculations were carried out at the PMP2/6-31G(d,p) and B3LYP/6-311+G(2df,2p) levels. Calculations reproduce experimental trends. In the absence of dioxygen, calculated barriers are not too high to prevent H-abstraction but the process is endothermic. The factors governing the reactivity of nitroxide radicals have been discussed in the scope of the state correlation diagram approach. Moreover, the influence of dioxygen on the mechanism of these reactions has also been studied. Thus, the addition of dioxygen occurs after the H-abstraction by nitroxide radicals and no clear evidence for an energetic barrier to O2 addition was found. However, in the presence of dioxygen the whole process is exothermic and thus H-abstraction becomes irreversible.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"444 1","pages":"1967-1972"},"PeriodicalIF":0.0,"publicationDate":"2002-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76502426","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Darbeau, R. S. Pease, E. V. Perez, R. Gibble, Faith A. Ayo, A. Sweeney
Benzyl cations were generated via thermolysis of N-benzyl-N-nitrosopivalamide in molten 2-R-substituted benzonitriles (R = MeO, Me, H, F, Cl, and Br). The corresponding N-2-R-benzonitrilium species, in contrast to their 4-R-benzonitrilium counterparts, underwent limited reaction with pivalate ion to form unsymmetrical diacylamines via rearrangement of their initial imidic anhydrides. The yield of diacylamines, though small, varied systematically with the nature of the R group in a manner suggesting the operation of interesting steric and/or electronic effects on the pivalate ion-nitrilium ion collapse. The ortho-substituent, though present on only one side of the benzonitrilium ion inhibits reaction at both sides via steric hindrance in the near-ground state and steric crowding in the transition state (a “persistent steric” effect). The proposed electronic effect involves a π*-acceptor agostic-type interaction between n or σ electrons (HOMO) and the π* system (LUMO) of the nitrilium ion. Additionally, in many cases, attack by water on the nitrilium ion occurred to a significantly larger extent than attack by the much more nucleophilic and positionally favored pivalate ion on the same species. This observation is interpreted in terms of the differences in the sizes and docking trajectories of both species with the nitrilium ion due to charge and charge distribution on both nucleophiles.
在熔融的2-R取代苯甲腈(R = MeO, Me, H, F, Cl和Br)中，n -苄基-n -亚硝基戊酰胺热裂解生成苄基阳离子。与4- r -苯并硝基相比，n -2- r -苯并硝基通过初始亚胺酸酐的重排，与戊酸盐离子发生有限反应，生成不对称的二酰基胺。二酰基胺的产率虽然很小，但随着R基团的性质有系统地变化，这表明在私人离子-腈离子崩溃过程中存在有趣的空间和/或电子效应。邻位取代基虽然只存在于苯氮离子的一侧，但通过近基态的位阻和过渡态的位挤来抑制两侧的反应(一种“持续的位压”效应)。提出的电子效应涉及n或σ电子(HOMO)与硝离子的π*系统(LUMO)之间的π*受体自发型相互作用。此外，在许多情况下，水对腈离子的攻击比亲核性更强、位置更有利的私人离子对同一物种的攻击要大得多。由于两种亲核试剂上的电荷和电荷分布，这两种物质与硝态氮离子在大小和对接轨迹上的差异解释了这一观察结果。
{"title":"N-Nitrosamide-mediated Ritter-type reactions.","authors":"R. Darbeau, R. S. Pease, E. V. Perez, R. Gibble, Faith A. Ayo, A. Sweeney","doi":"10.1039/B204255J","DOIUrl":"https://doi.org/10.1039/B204255J","url":null,"abstract":"Benzyl cations were generated via thermolysis of N-benzyl-N-nitrosopivalamide in molten 2-R-substituted benzonitriles (R = MeO, Me, H, F, Cl, and Br). The corresponding N-2-R-benzonitrilium species, in contrast to their 4-R-benzonitrilium counterparts, underwent limited reaction with pivalate ion to form unsymmetrical diacylamines via rearrangement of their initial imidic anhydrides. The yield of diacylamines, though small, varied systematically with the nature of the R group in a manner suggesting the operation of interesting steric and/or electronic effects on the pivalate ion-nitrilium ion collapse. The ortho-substituent, though present on only one side of the benzonitrilium ion inhibits reaction at both sides via steric hindrance in the near-ground state and steric crowding in the transition state (a “persistent steric” effect). The proposed electronic effect involves a π*-acceptor agostic-type interaction between n or σ electrons (HOMO) and the π* system (LUMO) of the nitrilium ion. Additionally, in many cases, attack by water on the nitrilium ion occurred to a significantly larger extent than attack by the much more nucleophilic and positionally favored pivalate ion on the same species. This observation is interpreted in terms of the differences in the sizes and docking trajectories of both species with the nitrilium ion due to charge and charge distribution on both nucleophiles.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"174 1","pages":"2146-2153"},"PeriodicalIF":0.0,"publicationDate":"2002-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79349461","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The EPR spin-trapping technique has been applied extensively to the detection of organic peroxyl radicals in biological systems. The most widely used spin trap is 5,5-dimethyl-1-pyrroline N-oxide (DMPO), of which adducts displaying EPR signals with a(N) ∼1.43 mT, a(β-H) ∼1.17 mT and a(γ-H) ∼0.12 mT have been routinely assigned to trapped peroxyl radicals. Recently, however, it has been shown that such signals are from alkoxyl radical adducts, generated during the decomposition of peroxyl radical adducts. In the present investigation, we have used the CeIV–tert-butyl hydroperoxide redox couple as an efficient means of generating peroxyl radicals (tBuOO˙) in a fast-flow, dielectric mixing-resonator. This allowed the direct, EPR observation of tBuOO˙ radicals, as well as a short-lived radical adduct upon the inclusion of DMPO. Although the hyperfine coupling constants for this adduct were essentially indistinguishable from those of the more stable methoxyl radical adduct (DMPO–˙OMe), it is reasoned on kinetic and chemical grounds why this species is believed to be the tert-butylperoxyl radical adduct (DMPO–˙OOtBu). The rate constant for tBuOO˙ spin trapping was estimated to be ca. 30 M−1 s−1, which is considerably lower than the value of > 103 M−1 s−1 proposed recently by Honeywill and Mile (J. Chem. Soc., Perkin Trans. 2, 2002, 569), who concluded that alkylperoxyl radicals form only diamagnetic adducts, via their multiple addition to DMPO. Complementary spin-trapping experiments in a static system resulted in detection of the methoxyl and tert-butoxyl radical adducts of DMPO (generated via DMPO–˙OOtBu decomposition), as well as the three-electron oxidation product 5,5-dimethyl-1-pyrrolidone-2-oxyl. These findings demonstrate that the chemistry underlying the generation of the radical adducts detected by spin trapping in peroxyl-radical generating systems must be interpreted with extreme caution. Furthermore, through the direct observation of the DMPO–˙OOtBu adduct under continuous-flow conditions, this work gives support to earlier suggestions that DMPO peroxyl radical adducts are formed, but are too unstable to be detected under the conditions employed in typical spin-trapping studies.
{"title":"EPR detection of the unstable tert-butylperoxyl radical adduct of the spin trap 5,5-dimethyl-1-pyrroline N-oxide: a combined spin-trapping and continuous-flow investigation","authors":"Claire M. Jones, M. Burkitt","doi":"10.1039/B207709B","DOIUrl":"https://doi.org/10.1039/B207709B","url":null,"abstract":"The EPR spin-trapping technique has been applied extensively to the detection of organic peroxyl radicals in biological systems. The most widely used spin trap is 5,5-dimethyl-1-pyrroline N-oxide (DMPO), of which adducts displaying EPR signals with a(N) \u0000∼1.43 mT, a(β-H) \u0000∼1.17 mT and a(γ-H) \u0000∼0.12 mT have been routinely assigned to trapped peroxyl radicals. Recently, however, it has been shown that such signals are from alkoxyl radical adducts, generated during the decomposition of peroxyl radical adducts. In the present investigation, we have used the CeIV–tert-butyl hydroperoxide redox couple as an efficient means of generating peroxyl radicals (tBuOO˙) in a fast-flow, dielectric mixing-resonator. This allowed the direct, EPR observation of tBuOO˙ radicals, as well as a short-lived radical adduct upon the inclusion of DMPO. Although the hyperfine coupling constants for this adduct were essentially indistinguishable from those of the more stable methoxyl radical adduct (DMPO–˙OMe), it is reasoned on kinetic and chemical grounds why this species is believed to be the tert-butylperoxyl radical adduct (DMPO–˙OOtBu). The rate constant for tBuOO˙ spin trapping was estimated to be ca. 30 M−1 s−1, which is considerably lower than the value of > 103 M−1 s−1 proposed recently by Honeywill and Mile (J. Chem. Soc., Perkin Trans. 2, 2002, 569), who concluded that alkylperoxyl radicals form only diamagnetic adducts, via their multiple addition to DMPO. Complementary spin-trapping experiments in a static system resulted in detection of the methoxyl and tert-butoxyl radical adducts of DMPO (generated via DMPO–˙OOtBu decomposition), as well as the three-electron oxidation product 5,5-dimethyl-1-pyrrolidone-2-oxyl. These findings demonstrate that the chemistry underlying the generation of the radical adducts detected by spin trapping in peroxyl-radical generating systems must be interpreted with extreme caution. Furthermore, through the direct observation of the DMPO–˙OOtBu adduct under continuous-flow conditions, this work gives support to earlier suggestions that DMPO peroxyl radical adducts are formed, but are too unstable to be detected under the conditions employed in typical spin-trapping studies.","PeriodicalId":17267,"journal":{"name":"Journal of The Chemical Society-perkin Transactions 1","volume":"92 1","pages":"2044-2051"},"PeriodicalIF":0.0,"publicationDate":"2002-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84108927","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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