Changes of Selectivity in the Reactions of Substituted 4-Nitrobenzyl Sulfonates with Nucleophilic Reagents

Article abstract of DOI:10.1021/ja00197a057

The selectivities for anionic nucleophilic reagents are larger for reaction with 4-nitrobenzyl mesylate than with the corresponding triflate.Selectivities were determined over a range of rate constants of ca. 1E4 for nucleophiles and ca. 1E5 for 4-nitrobenzyl sulfonates in 20:80 CH3CN/HOH (v:v) at 25 deg C, ionic strength 0.5 M (NaClO4).The slope of a correlation of log (k_N/k_HOH) for the two compounds is 1.12.Larger increases in selectivity of up to 11-fold were observed with more basic nucleophiles.The size of the additional increase is roughly proportional tothe basicity of the nucleophile, and it is suggested that reaction of the partially desolvated species may contribute to these differences.The results are described by a positive interaction coefficient p_xy = <*>s'/<*> log k_HOH = <*>s_1g/<*> log k_nuc = 0.026.No significant change in selectivity of p-nitrobenzyl tosylate, nosylate, and triflate toward substituted anilines was observed over a 5-6-fold range of amine reactivity.The values of β_nuc = 0.31 and 0.26 for the reactions of substituted anilines and aliphatic amines, respectively, with 4-nitrobenzyl tosylate do not differ significantly (70:30 EtOH/HOH (v:v), 25 deg C).The tosylate anion is a stronger nucleophile than water toward 4-nitrobenzyl triflate.It is concluded that S_N2 substitution reactions do follow the Hammond postulate, or "Bema Hapothle", but that the small magnitude of the changes in selectivity requires the examination of a large range of reactivity for both the nucleophile and the substrate.