As part of our continuing studies of the highly electron-deficient nature of nitrobenzofuroxans, nitrobenzofurazans and related heterocycles, we report here a kinetic and thermodynamic study of σ-complexation for a series of 2-aryl-4,6 -dinitrobenzotriazole 1-oxides (3a-e) over a large pH range in aqueous solution. The reaction series represents a modulation in electrophilic properties of the benzotriazole moiety in formation of the corresponding hydroxy σ-adducts (4a-e). Analysis of the data has allowed dissection of observed rates into forward (k) and reverse (k H ϩ Ϫ1 , k Ϫ2 ) rate constants as well as the obtention of pK a values for H 2 O addition to the benzotriazole moiety. Our results reveal that 3a-e are superelectrophilic compounds with respect to 1,3,5-trinitrobenzene (TNB) as a standard electron-deficient aromatic, but less superelectrophilic compared to 4,6-dinitrobenzofuroxan (DNBF). Some data pertaining to buffer catalysis of the formation and decomposition of the adducts together with solvent deuterium isotope effects for these pathways are also reported. From these results, it is concluded that adduct formation occurs via general base catalyzed water attack: the general bases include notably H 2 O, HCO 3 Ϫ , CO 3 2Ϫ as well as OH Ϫ . This contrasts with the situation for the σ-complexation of DNBF where HCO 3 Ϫ and CO 3 2Ϫ were found to act as nucleophilic catalysts whereas OH Ϫ functioned as a general base catalyst. This contrasting behaviour provides further evidence that the dinitro-activated carbocyclic ring of the benzotriazoles 3a-e ranks somewhat lower in electrophilic/ superelectrophilic properties compared to that in DNBF. Altogether, the results provide a basis for understanding the relationship between the superelectrophilic reactivities, as evidenced by the contrasting kinetic and thermodynamic properties of the systems at hand, and the varied abilities of these substrates to react in pericyclic Diels-Alder reactions.

A study of the reactions of oximate alpha-nucleophiles with diisopropylphosphorofluoridate (DFP) and two model phosphonates, has revealed either a levelling-off in reactivity or a bell-shaped behaviour in accordance with a critical decoupling of desolvation and bond formation (solvational imbalances); the relevance of these results to detoxification is emphasized.

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scribed previously.1 NaOH solutions were prepared from Titrisol.Kinetic determinations were made on a Durrum stopped-flow spectrophotometer, the cell compartment of which was maintained to ±0.5 °C. All runs were carried out under first-order conditions with a substrate concentration of about 5 X '6 M. Rate constants are considered accurate to ±3%.Acknowledgment. The authors thank the North Atlantic Treaty Organization for a grant (NATO Res. 1540) to F.T. and M.J.S. supporting this research.