Homofugality: A new reactivity index describing the leaving group ability in homolytic substitution reactions

A mechanistic study on the SHi reaction at tin atoms in a radical cascade reaction

A kinetic study on the intramolecular direct radical substitution reaction on tin atoms was undertaken.

Nucleofugality in oxygen and nitrogen derived pseudohalides in Menshutkin reactions: the importance of the intrinsic barrier

In order to study the nucleofugality of polyatomic anionic leaving groups derived from oxygen and nitrogen, a contingent of 19 methylating agents consisting of amines or alcohols activated with carbonyl or sulfonyl substituents has been examined via ab initio calculations. We have calculated gas phase activation energies for alkylation of ammonia, and methyl cation affinities. We find that polyatomic anionic leaving groups derived from nitrogen will have higher activation energies for Menshutkin (SN2) alkylation even when they have similar methyl cation affinities. This inherent deficit in the nucleofugality of nitrogen-derived leaving groups appears to be a result of the way bond cleavage is synchronized with bond formation to the incoming ammonia nucleophile. The nitrogen leaving groups showed greater dissociation from the methyl fragment than oxygen leaving groups relative to the length of the forming carbon-nitrogen bond. Additionally the second sulfonyl group present in a sulfonimide appears to be less effective at activating nitrogen due to a preference for tetrahedral geometries at the departing nitrogen in the transition states involving leaving sulfonamide groups. Optimal delocalization of electron density is therefore frustrated due to the geometry of the leaving group.

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

We recently proposed a new reactivity indicator, termed the "general-purpose reactivity indicator", Xi, 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(-), NO(2)(-), SO(3)(2-). For electrophiles we consider dimethyl carbonate, N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS), and 1-chloro-2,4,6-trinitrobenzene (CNB).