- Fluorescent sensing system for palladium(II) based on the Heck reaction
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In this paper, we report a fluorescent sensing system based on the palladium-catalyzed Heck reaction between N-methyl vinylpyridinium and 4-bromo-N,N′-dimethylaniline. Generation of a new fluorophore as a product enhances fluorescence and permits selective detection of palladium(II) among other metal species.
- Yu, Suh Young,Rhee, Hyun-Woo,Hong, Jong-In
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supporting information; experimental part
p. 1512 - 1514
(2011/05/16)
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- Catalysis of the β-elimination of HF from isomeric 2-fluoroethylpyridines and 1-methyl-2-fluoroethylpyridinium salts. Proton-activating factors and methyl-activating factors as a mechanistic test to distinguish between concerted E2 and E1cb irreversible mechanisms
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Second-order rate constants, kOHN, M-1 s-1, for the β-elimination reactions of HF with 2-(2-fluoroethyl)pyridine (2), 3-(2-fluoroethyl)pyridine (3), and 4-(2-fluoroethyl)pyridine (4) in OH-/H2O, at 50°C and μ = 1 M KCl, are kOHN = 0.646 × 10-4 M-1 s-1, kOHN = 2.97 × 10-6 M-1 s-1, and kOHN = 5.28 × 10-4 M-1 s-1, respectively. When compared with the second-order rate constants for the same processes with the nitrogen-methylated substrates 1-methyl-2-(2-fluoroethyl)pyridinium iodide (5), 1-methyl-3-(2-fluoroethyl)pyridinium iodide (6), and 1-methyl-4-(2-fluoroethyl)pyridinium iodide (7), the methyl-activating factor (MethylAF) can be calculated from the ratio kOHNCH3/kOHN, and a value of 8.7 × 105 is obtained with substrates 5/2, a value of 1.6 × 103 with 6/3, and a value of 2.1 × 104 with 7/4. The high values of MethylAF are in agreement with an irreversible E1cb mechanism (ANDE* + DN) for substrates 5 and 7 and with the high stability of the intermediate carbanion related to its enamine-type structure. In acetohydroxamate/acetohydroxamic acid buffers (pH 8.45-9.42) and acetate/acetic acid buffers (pH 4.13-5.13), the β-elimination reactions of HF, with substrates 2 and 4, occur at NH+, the substrates protonated at the nitrogen atom of the pyridine ring, even when the [NH+] is much lower than the [N], the unprotonated substrate, due to the high proton-activating factor (PAF) value observed: 3.6 × 105 for 2 and 6.5 × 104 for 4 with acetohydroxamate base. These high PAF values are indicative of an irreversible E1cb mechanism rather than a concerted E2 (ANDEDN) mechanism. Finally, the rate constant for carbanion formation from NH+ with 2 is kBNH+ = 0.35 M-1 s-1, which is lower than when chlorine is the leaving group (kBNH+ = 1.05 M-1 s-1; Alunni, S.; Busti, A. J. Chem. Soc., Perkin Trans. 2 2001, 778). This is direct experimental evidence that some lengthening of the carbon-leaving group bond can occur in the intermediate carbanion. This is a point of interest for interpreting a heavy-atom isotope effect.
- Alunni, Sergio,Laureti, Valeria,Ottavi, Laura,Ruzziconi, Renzo
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p. 718 - 725
(2007/10/03)
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- Nucleophilicity towards a Vinylic Carbon Atom: Rate Constants for the Addition of Amines to the 1-Methyl-4-vinylpyridinium Cation in Aqueous Solution
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Second-order rate constants (kNu) have been measured for the addition of 44 primary amines (including five α-effect amines), 28 secondary amines, 19 tertiary amines, ammonia and hydroxide ion to the vinyl group of the 1-methyl-4-vinylpyridinium cation (1) in aqueous solution at 25 deg C (ionic strength 0.1 mol dm-3).Nucleophilic attack is shown to be rate-determining for primary and secondary amines being generally more reactive than primary amines, with secondary amines of the same basicity.After classification of these species in terms of structure, they describe a number of Broensted-type correlations having βnuc in the range 0.35-0.54 for six structural classes of primary amine, βnuc = 0.48 for α-effect amines, and βnuc in the range 0.23-0.34 for four structural classes of secondary amine.Substitution upon the α-carbon atom reduces amine nucleophilicity of both primary and secondary amines.The presence of an unsaturated carbon atom (either sp2- or sp-hybridized) as the β-carbon atom leads to an enhanced reactivity relative to the corresponding β-sp3 species in all cases.Tertiary amines are in general less reactive than other amines of the same basicity.Broensted-type plots for tertiary amines present the appearance of random scatter which is not readily decipherable in terms of structure. β-Hydroxy and β-amino tertiary amines are unusually reactive relative to their basicity.All of these phenomena suggest that protonation of the carbanionic intermediate by a molecule of water is the rate-determining step for the addition of tertiary amines to 1.Rate constants for the attack of primary and secondary amines on 1 are shown to correlate with literature data for a variety of other reactions involving rate-determining nucleophilic attack of amines upon electrophilic carbon.These kNu for primary and secondary amines reacting with 1 are also shown to correlate with Ritchie's N+ parameters for nucleophilic attack at electrophilic sp2-carbon.N+ parameters for amine nucleophiles have not been widely available previously; the parameters that have been available for selected amines are known to be sensitive to the nature of the defining electrophile.The minimal steric hindrance at the electrophilic centre in nucleophilic attack upon 1 suggests that this species is an appropriate electrophile for the definition of N+ parameters for amine nucleophiles; these parameters are evaluated for 70 primary and secondary amines and ammonia and are suggested to provide an appropriate data base for future investigations of the reactivity and selectivity of amine attack upon sp2-carbon electrophiles in aqueous solution.
- Heo, Christina K. M.,Bunting, John W.
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p. 2279 - 2290
(2007/10/02)
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- The Protection of Heterocyclic N-H Groups
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Pyrrole, indole, carbazole and imidazole have been protected as N- derivatives.Deprotection occured under mild conditions after quaternisation. 3-Bromoindole was prepared by bromination of the protected parent indole.
- Katritzky, Alan R.,Khan, Ghulam R.,Marson, Charles M.
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p. 641 - 644
(2007/10/02)
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- PYRIDYLETHYLATION - A NEW PROTECTION METHOD FOR ACTIVE HYDROGEN COMPOUNDS
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2-(2- or 4-Pyridyl)ethyl group is readily introduced by pyridylethylation and easily removed after activation by quaternisation with methyl iodide.
- Katritzky, Alan R.,Khan, Ghulam R.,Schwarz, Otto A.
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p. 1223 - 1226
(2007/10/02)
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