110079-43-1Relevant articles and documents
Concertedness in Acyl Group Transfer in Solution: A Single Transition State in Acetyl Group Transfer between Phenolate Ion Nucleophiles
Ba-Saif, Salem,Luthra, Ajay K.,Williams, Andrew
, p. 6362 - 6368 (1987)
Rate constants have been measured for nucleophilic substitution of 4-nitrophenol from 4-nitrophenyl acetate by a series of phenolate anions.The Bronsted type plot is linear for unhindered phenolate ions with pKa values significantly above and below that of the displaced 4-nitrophenol: (log kArO = 0.75pKArOH - 7.28; n = 17, r = 0.984); this is consistent with a mechanism involving a single transition state or a mechanism with an intermediate that has a very low barrier to decomposition.A small change in effective charge on the carbonyl group from reactant to transition state (measured from βnuc and the known βeq for the overall reaction) points to an almost coupled concerted mechanism for the transfer of acetyl function between phenolate ion nucleophiles.The conclusions of this work are consistent with previous results that indicate relatively stable tetrahedral intermedates in reactions at reactive acyl centers; a spectrum of mechanisms exists for substitution reactions of acyl functions in solution that ranges from SN1 (or ElcB for an ester with an α-carbanion) through concerted to BAc2.
An Open Transition State in Carbonyl Acyl Group Transfer in Aqueous Solution
Ba-Saif, Salem A.,Colthurst, Matthew,Waring, Mark A.,Williams, Andrew
, p. 1901 - 1908 (2007/10/02)
The second-order rate constants have been measured for the reaction of substituted phenolate ions with 2,4-dinitrophenyl acetate, 2,4-dinitrophenyl 4-methoxy-2,6-dimethylbenzoate and acetic anhydride in aqueous solution at 25 deg C.The data are over a wide range of phenolate ion basicity and obey good Broensted equations which have βnuc values of, respectively, 0.57 +/- 0.03, 0.15 +/- 0.07 and 0.59 +/- 0.05.The principal conclusion of this work is that the identity reaction of 2,4-dinitrophenolate ion with 2,4-dinitrophenyl 4-methoxy-2,6-dimethylbenzoate has anopen transition state, namely one with very weak bonds to entering and departing ligands.The transition state possesses a Kreevoy tightness parameter (τ) of 0.18.The open transition state arises from the stabilising effect of the acyl group substituents on the benzoylium ion and their destabilising effect on the putative tetrahedral intermediate as well as the weak basicities of the nucleophile and nucleofuge.This is the first example of an open transition state in an acyl group transfer which does not require the assistance of a negatively charged internal nucleophile.The data for 2,4-dinitrophenyl acetate may be employed to calculate an identity rate constant (kii) for the reaction of 2,4-dinitrophenolate ion with the ester.This data may be fitted to a theoretical Lewis-Kreevoy plot (log kii vs. pKi) possessing both positive and negative values of βii (slope of the line).Microscopic medium effects place a limit to the accuracy of predictions of rate constants, including kii, from linear free energy relationships.
