Isotope effects in nucleophilic substitution reactions. VIII. The effect of the form of the reacting nucleophile on the transition state structure of an S_N2 reaction

Abstract

YAO-REN FANG and KENNETH CHARLES WESTAWAY. Can. J. Chem. 69, 1017Chem. 69, (1991. A spectroscopic investigation indicated that lithium thiophenoxide exists as a contact ion pair complex in dry diglyme whereas the other alkali metal thiophenoxides exist as a solvent-separated ion pair complex in diglyme. The addition of small amounts of water converts the lithium thiophenoxide contact ion pair complex into a solvent-separated ion pair complex. A smaller secondary a-deuterium kinetic isotope effect and a larger Hammett p value are observed when the nucleophile is the contact ion pair complex in the SN2 reaction between n-butyl chloride and thiophenoxide ion in diglyme. This indicates that the transition state for the contact ion pair complex reaction is tighter with a shorter nucleophile-a-carbon bond than the transition state for the solvent-separated ion pair complex reaction. The secondary a-deuterium kinetic isotope effects for the free ion and the solvent-separated ion pair complex reactions between sodium thiophenoxide and n-butyl chloride in DMF suggest that the loosest transition state is found when the nucleophile is the free ion.

Reaction of Lithium Diethylamide with an Alkyl Bromide and Alkyl Benzenesulfonate: Origins of Alkylation, Elimination, and Sulfonation

“…The product ratios are sensitive to isotopic substitution. The measured isotope effects using 1,1- 1 - d 2 and 2,2- 1 - d 2 (Table ) are consistent with an S N 2 substitution and E2 elimination . GC-MS analyses also confirmed β- rather than α-eliminations .…”

Using 11C/14C Incoming Group and Secondary α-Deuterium KIEs To Determine How a Change in Leaving Group Alters the Structure of the Transition State of the SN2 Reactions between m-Chlorobenzyl para-Substituted Benzenesulfonates and Cyanide Ion

“…Finally, it was of interest to try to determine the form of the reacting nucleophile in these S N 2 reactions. Westaway and co-workers − have shown that ion pairing can affect the structure of an S N 2 transition state markedly. Since Jobe and Westaway had examined the infrared spectrum of cyanide salts in dipolar aprotic solvents, the form of the reacting nucleophile could be determined in these S N 2 reactions.…”

Nucleophilic Reactivities of Thiophenolates

“…To characterize substituent effects on ArS – reactivity, Bordwell and Hughes determined the rate constants for the reactions of several thiophenolates with n -butyl chloride in DMSO . The influence of ion pairing on the reactivity of alkali metal thiophenolates toward n -butyl chloride in aq diglyme solutions was investigated by Fang and Westaway …”

“…18 The influence of ion pairing on the reactivity of alkali metal thiophenolates toward n-butyl chloride in aq diglyme solutions was investigated by Fang and Westaway. 19 Despite the fact that such quantitative data on the nucleophilic reactivity of ArS − are at hand, the comparison of ArS − with other types of nucleophiles, even with analogously substituted phenolates (ArO − ), 20 is hampered by a lack of kinetic data about ArS − reactivity toward common reference electrophiles.…”