Structure-reactivity correlation in the pyridinolysis of substituted phenyl acetates

Yoh, Soo-Dong; Kang, Joong-Kyu; Kim, Sung-Hong

doi:10.1016/s0040-4020(01)81723-3

Cited by 6 publications

(8 citation statements)

References 9 publications

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“…This is consistent with the fact that the pyridinolyses of aryl acetates, ,3c,4a, aryl benzoates, ethyl S -aryl thiolcarbonates, and acetic anhydride are stepwise, whereas the phenolyses of the same substrates are concerted. 5b,d,f,, …”

Section: Resultssupporting

confidence: 82%

“…The substrates were synthesized as previously reported . Methyl phenyl carbonate (one of the products from the reactions of the substrates with phenoxide anion) was prepared as described previously 4a…”

Section: Methodsmentioning

confidence: 99%

“…The kinetics and mechanisms of the aminolysis and pyridinolysis of carboxylic acid derivatives, such as esters and carbonates, have been extensively investigated. − Nevertheless, the mechanisms of the reactions of phenoxide anions with these substrates have been much less studied …”

Section: Introductionmentioning

confidence: 99%

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Concerted Mechanisms of the Reactions of Methyl Aryl Carbonates with Substituted Phenoxide Ions

2001

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The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 degrees C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (k(obsd)) are found throughout. Plots of k(obsd) vs substituted phenoxide concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Brönsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes beta = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitudes of these Brönsted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Brönsted plot for a stepwise mechanism should be pK(a)(0) = 7.1 (the pK(a) of 4-nitrophenol). This curvature does not appear within the pK(a) range of the substituted phenols studied (5.3--10.3), indicating that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates that would be formed if the reactions were stepwise. The reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence. The k(N) values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The k(N) values found in the present reactions are subjected to a dual regression analysis as a function of the pK(a), of both the nucleophile and leaving group, the coefficients being beta(N) = 0.5 and beta(lg) = -0.3, respectively. These coefficients are consistent with a concerted mechanism.

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Section: Resultssupporting

confidence: 82%

Section: Methodsmentioning

confidence: 99%

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Concerted Mechanisms of the Reactions of Methyl Aryl Carbonates with Substituted Phenoxide Ions

2001

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“…The values of these slopes are consistent with a stepwise mechanism, where a zwitterionic tetrahedral addition intermediate ( T ± ) is found on the reaction pathway. Generally, for most of the reactions of acyl compounds, a high magnitude in ρ and β values has always been attributed to the rate‐determining breakdown of T ± to products 10,35–46 for simple unassisted nucleophilic attack and a low magnitude to rate‐determining formation of T ± 5–7, 29,40,45–54 for assisted nucleophilic attack whereby T ± is stabilized by a second molecule of nucleophile either in free form or in protonated form and proceeds through rapid formation of anionic tetrahedral intermediate ( T − ) and cationic tetrahedral intermediate ( T + ) respectively to products.…”

Section: Resultsmentioning

confidence: 99%

Mechanistic change in the reactivity of substituted phenyl acetates over phenyl thiolacetates toward imidazole in aqueous phase

Rajarathnam

Jeyakumar

Nadar

2005

Int J of Chemical Kinetics

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The kinetics of aminolysis of several substituted phenyl acetates by imidazole is studied in aqueous medium at 20 • C and an ionic strength of 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 272-401 nm), under excess free imidazole, pseudofirst-order rate constants (k obs ) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics and the plots of (k obs − k H ) against free imidazole concentration are linear at constant pH. The macroscopic nucleophilic substitution rate constants (k N ) are obtained as the slopes of these plots and found to be pH independent. For the esters with poor nucleofuges, a rate dependence on more than first power of the free imidazole and a linear dependence of k 2 on free imidazole is observed. Accordingly, the microscopic rate constants for the assisted paths viz. k ga and k gb have been disseminated besides for simple bimolecular attack. The Brönsted-type plots and Hammett plots were constructed whose slope values are consistent with a stepwise mechanism through a bipolar tetrahedral addition intermediate whose formation or breakdown being rate determining for various paths. Comparison of this reaction of oxyesters with the earlier reported works on similar reaction of analogue thiolesters under identical reaction conditions showed remarkable mechanistic differences which are discussed in detail. The discussion is extended to include the details on previously studied ammonolysis of these two types of esters wherein thiolesters showed differed reactivity than that reported in the present study. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [211][212][213][214][215][216][217][218][219][220][221] 2005

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“…Contrarily, larger ρ lg values are obtained when rate‐limiting expulsion of the leaving group is involved 46–48. Generally, for most of the reactions of acyl compounds, a low magnitude in β and ρ values have always been attributed to the rate‐limiting attack of the nucleophiles 2, 15, 49–57.…”

Section: Resultsmentioning

confidence: 99%

Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

Rajarathnam

Babu

Nadar

2001

Int J of Chemical Kinetics

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The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18 • C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 260-410 nm), under excess free ammonia, pseudo-first-order rate coefficients (k obs ) are obtained. The plots of (k obs − k H ) against free ammonia concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k N ) are obtained as the slopes of these plots and found to be pH-independent for all the thiolesters. The Brönsted-type plot (log k N against pK a of leaving groups) and the Hammett plot (log k N against σ values of substituents) obtained for the title reactions of thiolesters are linear with slope values of β lg = −0.34 and ρ = 0.74 respectively. From the magnitude of these values, experimental data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thiolesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwitterionic tetrahedral addition intermediate (T ± ), whereby its formation is rate-determining (k 1 step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger k N values. The change in mechanism and enhanced reactivity observed by substitution of the oxygen atom by sulphur atom on the phenyl moiety are discussed in detail.

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Structure-reactivity correlation in the pyridinolysis of substituted phenyl acetates

Cited by 6 publications

References 9 publications

Concerted Mechanisms of the Reactions of Methyl Aryl Carbonates with Substituted Phenoxide Ions

Concerted Mechanisms of the Reactions of Methyl Aryl Carbonates with Substituted Phenoxide Ions

Mechanistic change in the reactivity of substituted phenyl acetates over phenyl thiolacetates toward imidazole in aqueous phase

Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

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