No abstract

The second-order rate constants have been measured for the reaction of substituted phenolate ions with 2,4 -di nitrophenyl acetate, 2,4 -dinitrophenyl 4 -met hoxy -2,6-dimet hyi benzoate and acetic anhydride in aqueous solution at 25 "C. The data are over a wide range of phenolate ion basicity and obey good Brsnsted equations which have p,,, values of, respectively, 0.57 0.03, 0.15 & 0.07 and0.59 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 an open transition state, namely one with very weak bonds to entering and departing ligands. The transition state possesses a Kreevoy tightness parameter (7) 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 (k,) for the reaction of 2,4-dinitrophenolate ion with the ester. This data may be fitted to a theoretical Lewis-Kreevoy plot (log k, vs. pK,) possessing both positive and negative values of pji (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.Recent work has provided evidence that the transfer of the carbonyl acyl group (RCO-) between weakly basic oxyanions is concerted in aqueous solution.'q2 Other weakly basic nucleophiles have been shown to react with carbonyl groups via a concerted t pathway, namely a single-step reacti0n.j~~

No abstract

Syntheses of 2,2-dimethylchromene-5,8-quinone, 2,2-dimethylchromene-6,7-quinone, and 6,7dimethoxy-2,2-dimethylc hromene-5,8 -quinone are described. Being easily bioreduci ble to electronrich chromenes these compounds, and their oxirane derivatives, are of interest as possible anti-tumour alkylating agents.

A Bronsted exponent (fJnuc) of 0.12 has been obtained for reaction of seventeen substituted phenolate ions with Zr4-dinitrophenyl diphenyl phosphate in methanol/water solution. The Leffler a value for P-0 bond formation (0.09) indicates that the transition-state of the symmetrical concerted reaction of 2,4-dinitro phenolate ion with the ester has 91 % of the character of the putative phosphorylium ion intermediate state.