Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

Abstract

Following a potentiometric determination of the relevant pK_a values of the (R₁R₂)CNOH functionality, the second order rate constants (k^Ox) for reaction of a large set of oximate bases with two model organophosphorus esters, i.e. bis-(4-nitrophenyl)phenylphosphonate (BNPPP) and bis-(4-nitrophenyl)methylphosphonate (BNPMP), and three toxic compounds, i.e., sarin (GB), soman (GD) and diisopropylphosphorofluoridate (DFP), in aqueous as well as a 30 : 70 (v/v) H₂O–Me₂SO mixture have been measured. The corresponding Brønsted-type nucleophilicity plots of log k^Oxvs. pK_a^Ox reveal a clear tendency of the reactivity of the oximates to suffer a saturation effect with increasing basicity in aqueous solution. In the case of BNPMP and the three toxic esters, this behaviour is reflected in a levelling off at pK_a ≈ 9 but a more dramatic situation prevails in the BNPPP system where the attainment of maximum reactivity at pK_a ≈ 9 is followed by a clear decrease in rate at higher pK_a's. Interestingly, a number of data reported previously by different authors for the sarin, soman and DFP systems are found to conform rather well to the curvilinear Brønsted correlations built with our data. Based on this and previous results obtained for reactions at carbon centers, it can be concluded that the observed saturation effect is the reflection of an intrinsic property of the oximate functionality. An explanation of this behavior in terms of an especially strong requirement for desolvation of the oximates prior to nucleophilic attack which becomes more and more difficult with increasing basicity is suggested. This proposal is supported by the observed changes in pK_a^Ox and k^Ox brought about by a transfer from H₂O to a 30 : 70 H₂O–Me₂SO mixture. The implications of the saturation effect on the efficiency of oximates as nucleophilic catalysts for smooth decontamination are emphasized. Also discussed is the effect of basicity on the exalted (α-effect) reactivity of these bases.