title: 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 selectivitykNu/ks(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 ofX-1-[β-13C]OH,X-1-[β-13C]OCH2CF3andX-1-[β-13C]Nu(Nu = Br, Cl, CH3CO2, Cl2CHCO2) from the trapping of symmetrical 4-substituted phenonium ion reaction intermediatesX-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 atX-1-[α-13C]Ts. The ratios of product yields,[α-13C]/[β-13C], and observed nucleophile selectivity (kNu/ks)obswere used to dissect the contribution of direct nucleophile addition atMe-1-Tsand trapping ofX-2+to the product yields. The product yields from partitioning of the intermediate gave the nucleophile selectivitykNu/ks(M-1) forX-2+. Swain-Scott plots of log (kNu/ks) are correlated by slopes ofs= 0.78 ands= 0.73 for reactions ofMeO-2+andMe-2+, respectively. This shows that the sensitivity of bimolecular substitution atX-2+to changes in nucleophile reactivity is smaller than for nucleophilic substitution at the methyl iodide. Evidence is presented that nucleophile addition toX-2+proceeds through an "exploded" transition state.
authors: Department of Biochemistry and Applied Chemistry, Kurume National College of Technology, Komorinomachi, Kurume 830-8555, Japan.
authors: Yutaka
authors: Y
authors: Tsuji
authors: Department of Chemistry, University at Buffalo, SUNY, Buffalo, NY 14260, USA.
authors: John P
authors: JP
authors: Richard