Swain-scott relationships for nucleophile addition to ring-substituted phenonium ions

The products of the reactions of 2-(4-methoxyphenyl)ethyl tosylate (MeO-1-OTs) and 2-(4-methyphenyl)ethyl tosylate (Me-1-OTs) with nucleophilic anions were determined for reactions in 50:50 (v/v) trifluoroethanol (TFE) / water at 25 °C. In many cases, the nucleophile selectivity k_Nu/k_s ((mol/L)^-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 ¹³C NMR analysis of the products of the reactions of substrate enriched with ¹³C at the α-carbon, X-1-[α-¹³C]OTs (X = -OCH₃ or -Me), with nucleophilic anions in 50:50 (v/v) TFE/water at 25 °C shows the formation of X-1-[β-¹³C]OH, X-1-[β-¹³C]OCH₂CF₃, and X-1-[β-¹³C]Nu (Nu = Br, Cl, CH₃CO₂, or Cl₂CHCO₂) from the trapping of symmetrical phenonium ion reaction intermediates X-2⁺. The observation of excess label in the α-position, [α-¹³C]/[β-¹³C] > 1.0, for both the water and nucleophile adducts, shows that these nucleophiles also react by direct substitution at X-1-[α-¹³C]OTs. The ratios of product yields, [α-¹³C]/[β-¹³C], and observed nucleophile selectivity (k_Nu/k_s)_obs were used to dissect the contribution of nucleophile addition at Me-1-OTs 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 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.