Swain-Scott Relationships for Nucleophile Addition to Ring-Substituted Phenonium Ions

The products of the reactions of 2-(4-methoxyphenyl)ethyl tosylate (MeO-1-Ts) and 2-(4-methyphenyl)ethyl tosylate (Me-1-Ts) with nucleophilic anions were determined for reactions in 50/50 (v/v) trifluoroethanol/water at 25°C. In many cases the nucleophile selectivity k_Nu/k_s (M^-1) for reactions of nucleophile and solvent, calculated from the ratio of product yields, depends upon [Nu^-]. This demonstrates the existence of competing reaction pathways, which show different selectivities for reactions with nucleophiles. A carbon-13 NMR analysis of the products of the reactions of substrate enriched with carbon-13 at the α-carbon, X-1-[α-, (X = -OCH₃, -Me) with nucleophilic anions in 50/50 (v/v) trifluoroethanol/water at 25°C shows the formation of X-1-[β-, X-1-[β- and X-1-[β- (Nu = Br, Cl, CH₃CO₂, Cl₂CHCO₂) from the trapping of symmetrical 4-substituted phenonium ion reaction intermediates X-2 . The observation of excess label in the α-position, [α- > 1.0, for both the water and nucleophile adducts, shows that water and anionic nucleophiles also react by direct substitution at X-1-[α-. The ratios of product yields, [α-, and observed nucleophile selectivity (k_Nu/k_s)_obs were used to dissect the contribution of direct nucleophile addition at Me-1-Ts and trapping of X-2 to the product yields. The product yields from partitioning of the intermediate gave the nucleophile selectivity k_Nu/k_s (M^-1) for X-2 . Swain-Scott plots of log (k_Nu/k_s) are correlated by slopes of s = 0.78 and s = 0.73 for reactions of MeO-2 and Me-2 , respectively. This shows that the sensitivity of bimolecular substitution at X-2 to changes in nucleophile reactivity is smaller than for nucleophilic substitution at the methyl iodide. Evidence is presented that nucleophile addition to X-2 proceeds through an "exploded" transition state.