- Experimental and theoretical study of the isomerisation of N-methylpyrrole
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The isomerisation of N-methylpyrrole to 2-methylpyrrole has been investigated experimentally by means of a single pulse shock tube over the temperature range 1000 to 1350 K and theoretically by ab initio calculations.Between 1000 and 1100 K the isomerization is found to occur via an intramolecular methyl shift.The experimental rate constant for this rearrangement was found to be kiso = 1E12.8(+/-0.4) exp-1/RT> s-1.Ab initio calculations gave Arrhenius parameters for the methyl shift in good agreement with the experimental values.Methyl shifting produces the intermediate 2-methyl-2H-pyrrole which undergoes a hydrogen shift of low activation energy to form the product of isomerisation, 2-methylpyrrole.Above about 1240 K there is a competing free-radical process which produces methane, pyrrole and traces of higher molecular weight products.
- Doughty, Alan,Mackie, John C.,Bacskay, George B.
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- Gas-Phase Heteroaromatic Substitution. 2. Electrophilic Methylation of Pyrrole and N-Methylpyrrole by CH3XCH3+ (X = F or Cl) Ions
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The gas-phase methylation of pyrrole (1) and N-methylpyrrole (2) by CH3XCH3+ (X = F or Cl) ions, from the γ radiolysis of CH3X, has been investigated at pressure ranging from 50 to 760 torr, in the presebce of a thermal radical scavenger (O2) and variable concentrations of an added base (NMe3).Both the reactivity of the selected pyrroles relative to benzene, used as the refernce substrate in competition experiments, and the isomeric distribution of their methylated derivatives depende markedly on the total pressure of the system and the concentration of NMe3.The apparent kP/kB ratios increase from 0.2 (1)-0.3 (2), in neat CH3F at 50 torr, to over 0.4 (1)-1.0 (2), at 760 torr containing 10 torr of NMe3.Concurently, the isomeric distribution of the methylated products changes from β:α:N = 80percent:13percent:7percent (from 1) and β:α = 65percent:35percent (from 2) to β:α:N = 50percent:15percent:35percent and β:α = 70percent:30percent.These results are consistent with a methylation mechanism involving kinetically predominant CH3FCH3+ attack on the β-carbons of the pyrrolic substrate and subsequent isomerization of the resulting excited intermediates to the thermodynamically most stable 3-methylpyrrole protonated on the 2-position.The substrate and positional selectivity of the gas-phase methylation and the mechanism of isomerization that appears of intramolecular nature are discussed in the light of the recent theoretical predictions on heteroaromatic reactivity and compared with the available data of related methylation reactions, occuring both in the gaseous and condensed phase.
- Angelini, Giancarlo,Sparapani, Cinzia,Speranza, Maurizio
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- Rhodium-Catalysed Reactions of Propargylamines with CO/H2. Formation of Pyrroles and Butenolides
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Rhodium-catalysed reactions of (arylpropargyl)amines with CO/H2 give β-arylpyrroles in good yields.Reactions of (alkylpropargyl)amines gave alkylpyrroles together with butenolides which are formed in an unusual reaction that probably involves double carbonylation, reduction of one carbonyl function and removal of the amine function by hydrogenolysis.The single-crystal X-ray structure of 5-methyl-N,3-diphenylpyrrole-2-carboxamide is recorded.
- Campi, Eva M.,Fallon, Gary D.,Jackson, W. Roy,Nilsson, Ylva
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p. 1167 - 1178
(2007/10/02)
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- Synthesis of 2-Alkylputrescines from 3-Alkylpyrroles
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Acylation of 2-(trichloroacetyl)pyrrole gave the 4-acyl derivatives (from 4-formyl to 4-hexanoyl) in good yields.Alkaline treatment gave corresponding 4-acyl-2-pyrrolecarboxylic acids, were decarboxylated to the 3-acylpyrroles by prior conversion to the 3-acyl-2,4,5-triiodopyrroles followed by hydrogenolysis.The 3-acylpyrroles were reduced by treatment with hydrazine in alkaline medium to the 3-alkylpyrroles.The latter were ring-opened by treatment with hydroxylamine in the presence of bicarbonate to give the dioximes of the corresponding 2-alkylsuccinaldehydes, which were then reduced to the 2-alkylpurescines (1,4-diaminobutanes).Ring-opening of 2,3-dimethylpyrrole followed by reduction of the dioxime gave 1,2-dimethylputrescine; the same sequence gave 1,3-dimethylputrescine from 2,4-dimethylpyrrole, while 3,4-dimethylpyrrole did not ring-open and gave the dioxime of 3,4-dimethylmaleimide.
- Garrido, Daniel O. A.,Buldain, Graciela,Ojea, Maria I.,Frydman, Benjamin
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p. 403 - 407
(2007/10/02)
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- Thermolysis of Polyazapentadienes. Part 9. Gas Phase Thermolysis of Some Dienamines, Enaminones, and Enaminothiones
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Thermolysis of the title compounds gives low yields of pyrroles by hydrogen-transfer-cyclisation-aromatisation sequence.The results are best explained by a dipolar mechanism for the hydrogen-transfer step: competitive aromatisation routes account for the range of substituted pyrroles which were obtained.
- Hickson, Clare L.,McNab, Hamish
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p. 339 - 342
(2007/10/02)
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- First Example of Predominant β-Orientation in Electrophilic Substitution of Pyrrole
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The isomeric distribution of the neutral products from the gas-phase electrophilic attack of 3He3H+, CH3FCH3+, and t-C4H9+ ions on pyrrole provides the first experimental evidence for predominant β-orientation in electrophilic substitution of pyrrole .
- Speranza, Maurizio
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p. 1177 - 1178
(2007/10/02)
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