Nucleophilic Substitution at Tetracoordinate Sulfur. Kinetics and Mechanism of the Chloride-Chloride Exchange Reaction in Arenesulfonyl Chlorides: Counterintuitive Acceleration of Substitution at Sulfonyl Sulfur by ortho-Alkyl Groups and Its Origin.

Marian Mikołajczyk,Monika Gajl,Marek Cypryk,Jarosław Błaszczyk,Bartłomiej Gostyński

doi:10.3390/molecules25061428

Abstract

The chloride-chloride exchange reaction in arenesulfonyl chlorides was investigated experimentally and theoretically by density functional theory (DFT) calculations. The second order rate constants and activation parameters of this identity reaction were determined for 22 variously substituted arenesulfonyl chlorides using radio-labeled Et4N36Cl. The chloride exchange rates of 11 sulfonyl chlorides bearing para-and meta-substituents (σ constants from −0.66 to +0.43) in the aromatic ring followed the Hammett equation with a ρ-value of +2.02. The mono- and di-ortho-alkyl substituted sulfonyl chlorides exhibit an enhanced reactivity although both inductive and steric effects lower the reaction rate. The DFT calculations of their structures together with X-ray data showed that an increased reactivity is mainly due to a peculiar, rigid, strongly compressed and sterically congested structure. The DFT studies of the title reaction revealed that it proceeds via a single transition state according to the SN2 mechanism. The analogous fluoride exchange reaction occurs according to the addition–elimination mechanism (A–E) and formation of a difluorosulfurandioxide intermediate. The reliability of the calculations performed was supported by the fact that the calculated relative rate constants and activation parameters correlate well with the experimental kinetic data.

Highlights

The mechanism and steric course of the bimolecular nucleophilic substitution reactions at the sulfur atom (SN -S) and their relationship are more complicated than those at the sp3 -carbon atom [1,2]
The progress of the reaction was followed by determining the radioactivity of the water phase containing Et4 N36 Cl which was obtained after interruption of the reaction and separation of the two reactants by rapid extraction with a water:carbon tetrachloride:methanol mixture in 1:5:0.4 ratio
We report the results of our extended kinetic studies on the reaction under discussion using a large number of substituted arenesulfonyl chlorides 1 (22 examples)

Summary

Introduction

The mechanism and steric course of the bimolecular nucleophilic substitution reactions at the sulfur atom (SN -S) and their relationship are more complicated than those at the sp3 -carbon atom [1,2]. As sulfur may form hypervalent tetra- and penta-coordinate compounds (commonly named sulfuranes), [3] the most important problem to solve is whether these SN -S reactions occur synchronously according to the classical Ingold’s SN 2 mechanism via a single transition state (TS) or stepwise by an addition–elimination mechanism (A–E) involving formation of unstable sulfurane as the reaction intermediate (trigonal bipyramidal intermediate, TBPI). They are formed by addition of a nucleophilic reagent (N) to the electropositive sulfur atom in reaction substrates and decompose in the second step by departure of the leaving group (L) to give substitution products (SP).

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Results

Conclusion