2979-53-5Relevant articles and documents
Rate and Equilibrium Studies of the Reaction of Oxyanions with 2-Phenyloxazol-5(4H)-one
Chrystiuk, Edwin,Jusoh, Adelina,Santafianos, Dino,Williams, Andrew
, p. 163 - 168 (2007/10/02)
Equilibrium constants for the reaction of phenoxide ions with 2-phenyloxazol-5(4H)-one at 25 deg C and 1M ionic strength obey a Broensted relationship (log kOH/kArO = log K' = -173pKArOH - 15.81) and are not subject to steric effects from ortho-substituents.Both forward and reverse rate parameters exhibit steric effects, and the Broensted equations for meta- and para-substituted species are log kOH = -0.81 pKArOH + 9.75, and log kArO = 0.95pKArOH - 6.40.There is no break in the Broensted line in the region of pKArOH 5-11, consistent with a single transition-state.An upward deviation exists for oxyanions with low basicity (pKXOH 5); one of these oxyanions, betaine, has a solvent deuterium oxide isotope effect for its reaction with the oxazolone greater than 2, consistent with a general base mechanism for attack for these species.The results for nucleophilic attack of phenolate anions are in agreement with a concerted displacement at the carbonyl group.
Structure-Reactivity Studies on the Equilibrium Reaction between Phenolate Ions and 2-Aryloxazolin-5-ones: Data Consistent with a Concerted Acyl-Group-Transfer Mechanism
Curran, Terence C,Farrar, Charles R.,Niazy, Omima,Williams, Andrew
, p. 6828 - 6837 (2007/10/02)
The rate and equilibrium constants for the reaction between phenolate anions and 2-aryloxazolin-5-ones have been measured as a function of the structures Ar and Ar'.The change in "effective" charge on both phenol-leaving oxygen and endocyclic oxygen from ground to transition state, as determined from the relevant Broensted parameters, is substantial and essentially additive consistent with a concerted displacement mechanism.The stepwise mechanism requires a small change in effective charge on the phenol oxygen because departure of phenolate ion from the tetrahedral intermediate cannot be rate limiting.Hydroxide ion attack on the C-5 atom of the oxazolinone to yield a benzoylglycine has a Hammett ?- dependence which can only arise from a concerted displacement; the rate-limiting step for the stepwise mechanism is the addition of hydroxide and the transition state of the rate-limiting step will therefore not involve much endocyclic C-O bond fission.An inverse deuterium oxide solvent isotope effect indicates that the observed general-acid catalysis has a specific-acid/nucleophilic mechanism; both hydroxide and oxonium ion catalysis are demonstrated by using 18O-labeling experiments to involve nucleophilic attack at the carbonyl (C-5) center.The equilibrium constant for reaction of azide ion with 2-phenyloxazolin-5-ones has been measured; it is suggested that the absence of racemization during azide coupling in peptide synthesis is related to the very unfavorable equilibrium constant for oxazolinone formation compared with that of activated oxygen esters.
