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

Abstract

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-¹³C]Ts, (X = -OCH₃, -Me) with nucleophilic anions in 50/50 (v/v) trifluoroethanol/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, Cl₂CHCO₂) from the trapping of symmetrical 4-substituted 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 water and anionic nucleophiles also react by direct substitution at X-1-[α-¹³C]Ts. 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 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.

Figure 1

Swain-Scott correlations of nucleophile selectivities for reactions of X-2⁺ in 50/50 (v/v) TFE/H₂O (I = 0.5, NaClO₄) at 25 °C. The values of n were determined for substitution reactions of methyl iodide in methanol. Key: (●), MeO-2⁺; (■), Me-2⁺.

Figure 2

Logarithmic correlation between nucleophile selectivities for reactions of N-(methoxymethyl)-N,N-dimethyl(3-nitroanilinium) ion (log(k_Nu/k_w)_A, Scheme 4B) and MeO-2⁺ (log(k_Nu/k_w)_OMe, X = -OMe, Scheme 4A).

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Chart 1