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 degrees 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 C-13 NMR analysis of the products of the reactions of substrate enriched with C-13 at the alpha-carbon, X-1-[alpha-C-13]OTs (X = -OCH3 or -Me), with nucleophilic anions in 50: 50 (v/v) TFE/water at 25 degrees C shows the formation of X-1-[beta-C-13]OH, X-1-[beta-C-13]OCH2CF3, and X-1-[beta-C-13]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 alpha-position, [alpha-C-13]/[beta-C-13] > 1.0, for both the water and nucleophile adducts, shows that these nucleophiles also react by direct substitution at X-1-[alpha-C-13]OTs. The ratios of product yields, [alpha-C-13]/[beta-C-13], 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.