engleski

Solvolytic Reactivity of Organic Phosphates

Heterolytic reactivities (nucleofugalities) of some organic phosphate leaving groups have been determined in terms of Nf parameters from solvolysis rate constants of corresponding benzhydryl derivatives according to equation log k (25 °C) = sf(Ef + Nf)(1).In this fashion, the nucleofugality of organic phosphates can be compared with nucleofugalities of numerous leaving groups in a wide range of reactivity in the existing nucleofugality scale. Nf values can further be used for estimating the heterolytic reactivity of various substrates according to equation (1). It was previously established that the negative hyperconjugation is operative in heterolytic transition states of carbonate diesters as well as in free aryl/alkyl carbonate anions, transferring electron density out of the carboxylate moiety. The difference in heterolytic reactivity between diphenyl phosphate and dimethyl phosphate leaving groups of even two orders of magnitude indicates that additional orbital effects exist in both heterolytic transition states and corresponding free phosphate anions since a direct transfer of the anionic charge to the aromatic rings of diphenyl phosphate is not possible. In order to investigate stabilizing effects that occur in leaving groups along the heterolytic reaction coordinate of phosphates, quantum chemical calculations have been employed.

organic phosphates ; nucleofugalities ; negative hyperconjugation

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