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

The products of the reactions of 2-(4-methoxyphenyl)ethyl tosylate () and 2-(4-methyphenyl)ethyl tosylate () with nucleophilic anions were determined for reactions in 50/50 (v/v) trifluoroethanol/water at 25°C. In many cases the nucleophile selectivity / (M) 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 = -OCH, -Me) with nucleophilic anions in 50/50 (v/v) trifluoroethanol/water at 25°C shows the formation of , and (Nu = Br, Cl, CHCO, ClCHCO) from the trapping of symmetrical 4-substituted phenonium ion reaction intermediates . The observation of excess label in the α-position, > 1.0, for both the water and nucleophile adducts, shows that water and anionic nucleophiles also react by direct substitution at . The ratios of product yields, , and observed nucleophile selectivity (/) were used to dissect the contribution of direct nucleophile addition at and trapping of to the product yields. The product yields from partitioning of the intermediate gave the nucleophile selectivity / (M) for . Swain-Scott plots of log (/) are correlated by slopes of = 0.78 and = 0.73 for reactions of and , respectively. This shows that the sensitivity of bimolecular substitution at to changes in nucleophile reactivity is smaller than for nucleophilic substitution at the methyl iodide. Evidence is presented that nucleophile addition to proceeds through an "exploded" transition state.