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

Abstract

Alcoholysis rates of unhindered benzenesulfonyl chlorides (X‐ArSO2Cl, X = H‐; 4‐Br‐; 4‐Me‐) are similar in methanol; the same behavior is also observed in ethanol, whereas the reactivity order in iso‐propanol is 4 Me‐ < H‐ < 4‐Br‐. On the other hand, alcoholysis of sterically hindered arenesulfonyl chlorides (X‐ArSO2Cl) (X = 2,4,6‐Me3‐3‐NO2‐; 2,6‐Me2‐4‐tBu‐; 2,4,6‐Me3‐; 2,3,5,6‐Me4‐; 2,4,6‐iPr3‐; 2,4‐Me2‐; 2,4,6‐(OMe)3‐) in all studied alcohols show a significant increase in reactivity, the so‐called positive steric effect. Most of the substrates showed a reaction order b ~ 2 with respect to the nucleophile in methanol and ethanol, and b ~ 3 in iso‐propanol. The correlation between reactivity and the Kirkwood function (1/ξ) gives negative sensitivity (U) for all systems. All substrates showed high sensitivity to media nucleophilicity that depends on ΣσX. Obtained results suggest the alcoholysis of benzenesulfonyl chlorides proceeds through SN2 mechanism where the transition state (TS) involves the participation of 2–3 alcohol molecules; such a TS can be cyclic, in the case of unbranched alcohols, or linear, for alcohols with bulkier hydrocarbon groups like iso‐propanol. To include the number of alcohol molecules playing such a role in the TS, the following terminology is proposed: cSN2sn for SN2 reactions involving n solvent molecules in a cyclic (c) TS, where “s” stands for the solvent and “n” is either the closest integer or half‐integer to the reaction order relative to the solvent or, in computational studies, the proposed number of solvent molecules taking part in the TS, whereas SN2sn is proposed when the TS is not cyclic. Copyright © 2016 John Wiley & Sons, Ltd.

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

“…Later on, the increased reactivity of ortho-alkyl substituted benzenesulfonyl chlorides, defined as "steric acceleration", was claimed to be due to relief of strain in the transition state having trigonal bipyramidal structure [40]. More recently, it was proposed that the structure of reactants and molecularity of the transition state with nucleophilic assistance of solvent may be responsible for the increased reactivity of hindered sulfonyl chlorides [13].…”

“…With regard to the sulfonyl transfer reactions, The results of very extensive kinetic studies on the nucleophilic substitution reactions at the sulfonyl center and mechanistic conclusions drawn from them have been summarized in an excellent review paper entitled "Sulfonyl Transfer Reactions" published in 1989 [12]. This direction of kinetic investigations is continued until recently with a various intensity [13,14]. Generally, the substitution reactions at the sulfonyl sulfur are discussed in term of the concerted S N 2-type displacement involving a single transition state which may be tighter or looser depending on the substituents present.…”

Propanolysis of arenesulfonyl chlorides: Nucleophilic substitution at sulfonyl sulfur

“…Some of the rate constants used in this correlation are taken from ref . The full set of data is available as a Table S8 in the Supporting Information.…”

“…From Tables and , and published research, it follows that not only electronic effects play a role in the alcoholysis of arenesulfonyl chlorides . Although no Hammett correlations have been found, there is a tendency for the reactivity to increase with the number of alkyl substituents (Tables and Figures and ), but at the same time, the reaction rate of non‐ ortho ‐alkylated substrates is reduced by 30% when H‐ or Me‐ is replaced by t ‐butyl in para‐ position (Table ).…”

Mechanistic studies of the solvolysis of alkanesulfonyl and arenesulfonyl halides

“…It has been proposed that, in addition to having an alcohol or water as the attacking nucleophile with a second alcohol of water as a general base, one can proceed in an alcoholysis reaction through a cyclic structure involving three molecules of the alcohol and the sulfur of the sulfonyl chloride present in the ring [ 52 ]. One would predict that such an extremely ordered pathway would have a very negative entropy of activation, but the required studies with temperature variation to investigate this aspect were not reported.…”