“…13% of the experimental ratio, thus suggesting that the increase in reactivity on going from 2,4-dinitrophenyl 4‘-hydroxybenzenesulfonate to 5 should be ascribed mainly to an increased stability of the intermediate 4 with respect to 3 . As we have previously suggested in the case of the corresponding carboxylic acid esters, such increased stability could be the result of a more extended delocalization of π electrons due to the presence of an additional vinylene group.…”
Section: Resultsmentioning
confidence: 59%
“…We have previously shown 1 that the alkaline hydrolysis of 4-hydroxybenzoate esters of acidic phenols does not occur through the usual B Ac 2 route but proceeds instead via a E1cB mechanism with the participation of the p -oxoketene intermediate 1 . More recently, we have reported that also the alkaline hydrolysis of aryl 4-hydroxycinnamates having powerful leaving groups occurs through the E1cB mechanism via the “extended” p -oxoketene intermediate 2 . Our results indicated that the interposition of a vinylene group between the internal nucleophilic site (the 4-hydroxy group) and the carbonyl carbon atom is beneficial to the dissociative hydrolytic process.…”
Reactivity comparisons, activation parameters, and the substituent
effect in the leaving group
indicate that the hydrolysis of title esters in moderately to strongly
alkaline aqueous solution follows
a dissociative pathway with the probable participation of an extended
sulfoquinone (thioquinone
dioxide) intermediate. The interposition of a vinylene group
between the internal nucleophile and
the reaction site favors the elimination−addition process.
Present results further suggest the high
tendency of suitably substituted sulfonyl derivatives to hydrolyze
through dissociative mechanism.
“…13% of the experimental ratio, thus suggesting that the increase in reactivity on going from 2,4-dinitrophenyl 4‘-hydroxybenzenesulfonate to 5 should be ascribed mainly to an increased stability of the intermediate 4 with respect to 3 . As we have previously suggested in the case of the corresponding carboxylic acid esters, such increased stability could be the result of a more extended delocalization of π electrons due to the presence of an additional vinylene group.…”
Section: Resultsmentioning
confidence: 59%
“…We have previously shown 1 that the alkaline hydrolysis of 4-hydroxybenzoate esters of acidic phenols does not occur through the usual B Ac 2 route but proceeds instead via a E1cB mechanism with the participation of the p -oxoketene intermediate 1 . More recently, we have reported that also the alkaline hydrolysis of aryl 4-hydroxycinnamates having powerful leaving groups occurs through the E1cB mechanism via the “extended” p -oxoketene intermediate 2 . Our results indicated that the interposition of a vinylene group between the internal nucleophilic site (the 4-hydroxy group) and the carbonyl carbon atom is beneficial to the dissociative hydrolytic process.…”
Reactivity comparisons, activation parameters, and the substituent
effect in the leaving group
indicate that the hydrolysis of title esters in moderately to strongly
alkaline aqueous solution follows
a dissociative pathway with the probable participation of an extended
sulfoquinone (thioquinone
dioxide) intermediate. The interposition of a vinylene group
between the internal nucleophile and
the reaction site favors the elimination−addition process.
Present results further suggest the high
tendency of suitably substituted sulfonyl derivatives to hydrolyze
through dissociative mechanism.
“…We have previously found that the hydrolyses of aryl 4-hydroxybenzoates in moderately to strongly alkaline aqueous solution follow the usual associative route (B Ac 2) when the esters possess leaving groups having p K a higher than about 6.5, whereas esters having leaving groups with lower p K a hydrolyze through the E1cB mechanism with the participation of the unprecedented p -oxo ketene intermediate ( 1 ). We have subsequently demonstrated that the hydrolyses of aryl 4-hydroxycinnamates and 2-hydroxycinnamates in dioxane−water mixtures behave almost in the same way with the participation, in their dissociative paths, of the “extended” oxo ketene intermediates 2 and 3 , respectively. …”
To gain knowledge on the role played by the nature of the bridge interposed between hydroxyl and carbonyl groups in esters of the title type, in principle able to hydrolyze through dissociative pathways via the conjugate base of the substrate (E1cB mechanism), we have studied the alkaline hydrolyses of 2,4-dinitrophenyl esters in which the pi-system is a biphenyl, azobenzene, benzylideneaniline, or stilbene skeleton. Kinetic data, such as reactivity comparisons and Arrhenius parameters, show that these substrates react through the usual, associative, B(Ac)2 mechanism. This outcome is discussed and interpreted from both structural and energetic standpoints. The data suggest that a value of 0.0 is the most appropriate assignment of the sigma(p)() value for the benzylidenamino substituent (C(6)H(5)CH=N-).
“…Kinetic and other methods including the determination of pK a of the substrates were described in a previous paper. 10 The rate constants for the hydrolysis of 3-and 4-nitrophenyl esters were obtained by initial rates: they were measured for each run up to ca. 10% of the total reaction and were converted to pseudofirst-order rate constants using infinity values calculated from the known extinction coefficients of the products.…”
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