The alpha-effect in the SNAr reaction of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with anionic nucleophiles: effects of solvation and polarizability on the alpha-effect

Abstract

A kinetic study on SNAr reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with various anionic nucleophiles in 80 mol% water - 20 mol% DMSO at 25.0 degrees C is reported. The Bronsted-type plot for the reaction of 1a with a series of substituted phenoxides and HOO- results in an excellent linear correlation with beta(nuc) = 1.17. However, OH- exhibits dramatic negative deviation from the Bronsted-type plot, while N-3(-), C6H5S-, and butane-2,3-dione monoximate (Ox(-)) deviate positively from linearity. HOO- is 680-fold more reactive than OH- but does not exhibit the alpha-effect. In contrast, Ox(-) is 166-fold more reactive than isobasic 4-Cl-C6H4O- and exhibits the alpha-effect. Differential solvation effects have been suggested to be responsible for the alpha-effect in this study, i.e., Ox(-) exhibits the alpha-effect, since it is 5.7 kcal/mol less strongly solvated than 4-Cl-C6H4O- in the reaction medium, while HOO- does not show the alpha-effect due to a strong requirement for partial desolvation before nucleophilic attack. The highly enhanced reactivity of polarizable N-3(-) and C6H5S- and extremely decreased reactivity of nonpolarizable OH- are in accord with the hard-soft acid and base principle.