Predicting the reactivity of ambidentate nucleophiles and electrophiles using a single, general-purpose, reactivity indicator

Abstract

We recently proposed a new reactivity indicator, termed the “general-purpose reactivity indicator”, Ξ, which describes not only the classical reactivity paradigms, but also describes reactions that are neither frontier-orbital nor electrostatically controlled. This indicator was proposed to be especially useful for reactants with multiple reactive sites, especially if the nature of the reactivity at those sites was different. This suggests that this reactivity indicator is especially appropriate for ambidentate molecules; this paper confirms this hypothesis. The general-purpose reactivity indicator not only identifies the most reactive sites, it also identifies which substrates prefer which reactive sites. In particular, the reactivity indicator allows one to clearly distinguish which sites of an ambidentate molecule are most reactive when electron transfer from the attacking reagent is large (a soft reagent) and which sites are most reactive when the attacking reagent is hard and highly charged (so that electron transfer is relatively insignificant). To illustrate the efficacy of the indicator for nucleophiles we consider SCN⁻, SeCN⁻, NO₂⁻, SO₃²⁻. For electrophiles we consider dimethyl carbonate, N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS), and 1-chloro-2,4,6-trinitrobenzene (CNB).