Isotope Effects in the Solvolysis of Sterically Hindered Arenesulfonyl Chlorides

Abstract

Solvent isotope effects in the ethanolysis of sterically hindered arenesulfonyl chlorides ruled out a proton transfer in the rate-determining step and agreed with a S N 2 mechanism involving at least a second solvent molecule in the transition state (TS). The lack of a secondary kinetic isotope effect in the o-alkyl groups allows us to disregard the possible contribution of σ -π hyperconjugation. The measured activation parameters are consistent with a S N 2 mechanism involving the participation of solvent molecules in the TS, possibly forming a cyclic TS through a chain of solvent molecules. C 2015 Wiley Periodicals, Inc. Int J Chem Kinet 47: [744][745][746][747][748][749][750] 2015

Solvent network at the transition state in the solvolysis of hindered sulfonyl compounds

“…When a specific sulfonyl chloride is considered, the reaction rate varies in the order: methanolysis > ethanolysis > iso ‐propanolysis. The presence of ortho ‐methyl groups accelerates the process in agreement with earlier observations …”

“…The presence of ortho-methyl groups accelerates the process in agreement with earlier observations. [3][4][5]16,19,28,34] The slope, b, of the observed dependences gives the reaction order with respect to the nucleophile, that is, the corresponding reaction order, according to Eqn 3: [21]…”

“…[10,28] The observation of small kinetic solvent isotope effects is an evidence against the catalytic effect of a second nucleophile molecule present in the TS [4,5,11] and supports the participation of a network of alcohol molecules in the TS, even as a cyclic chain. [28] Here, we present a mechanistic study of the alcoholysis (Scheme 1) of different sterically hindered arenesulfonyl chlorides (X-ArSO 2 Cl) at 323 K (X = 2,4,6-Me 3 -3-NO 2 -; 2,6-Me 2 -4tBu-; 2,4,6-Me 3 -; 2,3,5,6-Me 4 -; 2,4,6-iPr 3 -; 2,4-Me 2 -; 2,4,6-(OMe) 3 -; H-; 4-Me-; 4-Br-) in methanol, ethanol, and iso-propanol to elucidate the structure of the TS and looking for a better understanding of the solvolytic process.…”

“…[10,13,15,[17][18][19] Based on the described observations, and others already available in the literature, deviations of the TS structure are expected for a number of hindered substrates. [1,2,4,5,17,18,[20][21][22][23][24][25][26][27] When studying the alcoholysis of deuterated 2,4,6-trimethylbenzenesulfonyl chloride, [28] we obtained kinetic data and activation parameters comparable with those for undeuterated 2,4,6-trimethylbenzenesulfonyl chloride, such evidence lets us neglect σ-π-hyperconjugation as a possible reason of the "positive steric effect" in this particular case. [10,28] The observation of small kinetic solvent isotope effects is an evidence against the catalytic effect of a second nucleophile molecule present in the TS [4,5,11] and supports the participation of a network of alcohol molecules in the TS, even as a cyclic chain.…”

“…[1,2,4,5,17,18,[20][21][22][23][24][25][26][27] When studying the alcoholysis of deuterated 2,4,6-trimethylbenzenesulfonyl chloride, [28] we obtained kinetic data and activation parameters comparable with those for undeuterated 2,4,6-trimethylbenzenesulfonyl chloride, such evidence lets us neglect σ-π-hyperconjugation as a possible reason of the "positive steric effect" in this particular case. [10,28] The observation of small kinetic solvent isotope effects is an evidence against the catalytic effect of a second nucleophile molecule present in the TS [4,5,11] and supports the participation of a network of alcohol molecules in the TS, even as a cyclic chain. [28] Here, we present a mechanistic study of the alcoholysis (Scheme 1) of different sterically hindered arenesulfonyl chlorides (X-ArSO 2 Cl) at 323 K (X = 2,4,6-Me 3 -3-NO 2 -; 2,6-Me 2 -4tBu-; 2,4,6-Me 3 -; 2,3,5,6-Me 4 -; 2,4,6-iPr 3 -; 2,4-Me 2 -; 2,4,6-(OMe) 3 -; H-; 4-Me-; 4-Br-) in methanol, ethanol, and iso-propanol to elucidate the structure of the TS and looking for a better understanding of the solvolytic process.…”