Study of the Mechanism for the Hydrolysis of Alkoxy(aryl)(phenyl)-λ6-sulfanenitriles, ArPhS(OR)(N)

Abstract

The hydrolysis of alkoxy(aryl)(phenyl)-λ6-sulfanenitriles in several buffer solutions was found to follow a good pseudo-first-order kinetic equation, giving the corresponding sulfoximides and alcohols (for the case of the hydrolysis of neopentyloxy-λ6-sulfanenitrile, giving a rearranged product, 2-methyl-2-butanol). The dependence of the rate of hydrolysis on the structure of the alkyl group showed the opposite trend to the usual SN2 character, i.e. Me < Pr < Bu ≈ Et < i–Pr, except for neopentyl. The pH-rate profiles indicated that the rate of hydrolysis is also first order in [H+] at pH more than 6.08, and trends to saturate at low pH. According to these kinetic results, a two-step reaction mechanism was proposed which involves a pre-equilibrium protonation on the nitrogen atom of the alkoxy-λ6-sulfanenitriles, followed by a rate-determining C-O bond cleavage via an SN2 or SN1 mechanism on the alkyl carbon atom depending on the structure of the alkyl group. From a double-reciprocal plot of 1/kobs vs. 1/[H+], the pKa value and the rate constant of the second reaction of neopentyloxy(diphenyl)-λ6-sulfanenitrile were estimated to be 5.02 and 7.02×10-3 s-1, respectively. The substituent effects on the phenyl group of neopentyloxy(diphenyl)-λ6-sulfanenitrile afforded a large negative ρ-value (-1.88) for pKa and positive one (+1.66) for the second reaction at 25.2 °C. The small negative ρ-values observed at pH 6.27 for diphenyl(propoxy)-λ6-sulfanenitrile (-0.42) and neopentyloxy(diphenyl)-λ6-sulfanenitrile (-0.26) were found to be the results of a cancellation of those for the opposite trend of the reactions of the pre-equilibrium and the second step. The activation parameters for both the pre-equilibrium and the subsequent reactions were also estimated based on the parameters for the hydrolysis of neopentyloxy(diphenyl)-λ6-sulfanenitrile at pH 6.22 and 2.99. The buffer effect is due to a nucleophilic attack of the buffer base to the alkyl carbon atom of the protonated alkoxy-λ6-sulfanenitriles. The sulfoximide moiety in the protonated λ6-sulfanenitrile is revealed to be a very good leaving group.

Application of alkoxy‐λ6‐sulfanenitriles as strong alkylating reagents

“…Only a few inorganic 6 -sulfanenitriles, such as F 3 SN, are known and their chemistry has been developed by Glemser et al [2] in the field of inorganic or fluorine chemistry. Meanwhile, we have explored the chemistry of organic 6 -sulfanenitriles since our first preparation of methoxydiphenyl- 6 -sulfanenitrile in 1989 [3] without use of trifluoro- 6 -sulfanenitrile as the starting material.…”

“…In recent years, we have prepared a series of organic 6 -sulfanenitriles [4,5], and have reported the X-ray structural analysis of some 6 -sulfanenitriles [4c,4d,5]. In our previous paper [5], it was reported that alkoxy- 6 -sulfanenitriles [5] exceptionally undergo a facile Ei reaction, although 6 -sulfanenitriles are tetracoordinated hexavalent sulfur compounds like sulfones and sulfoximides.…”

“…In our previous paper [5], it was reported that alkoxy- 6 -sulfanenitriles [5] exceptionally undergo a facile Ei reaction, although 6 -sulfanenitriles are tetracoordinated hexavalent sulfur compounds like sulfones and sulfoximides. Furthermore, during the synthesis and the assignment of the structure of these alkoxy- 6 -sulfanenitriles [5,6], it was found that they undergo acid-catalyzed hydrolysis readily, even under slightly alkaline conditions, to yield the corresponding sulfoximide [2] and alcohols.…”

“…In our previous paper [5], it was reported that alkoxy- 6 -sulfanenitriles [5] exceptionally undergo a facile Ei reaction, although 6 -sulfanenitriles are tetracoordinated hexavalent sulfur compounds like sulfones and sulfoximides. Furthermore, during the synthesis and the assignment of the structure of these alkoxy- 6 -sulfanenitriles [5,6], it was found that they undergo acid-catalyzed hydrolysis readily, even under slightly alkaline conditions, to yield the corresponding sulfoximide [2] and alcohols. Moreover, we reported that methoxydiphenyl- 6 -sulfanenitrile reacts with thiols to give methyl sulfides and diphenyl sulfoximide, and clarified the mechanism that involves an initial equilibrated protonation of the 6 -sulfanenitrile by thiols followed by nucleophilic attack of the thiolate anions on the methyl group in the rate-determining step [7].…”

“…Furthermore, during the synthesis and the assignment of the structure of these alkoxy- 6 -sulfanenitriles [5,6], it was found that they undergo acid-catalyzed hydrolysis readily, even under slightly alkaline conditions, to yield the corresponding sulfoximide [2] and alcohols. Moreover, we reported that methoxydiphenyl- 6 -sulfanenitrile reacts with thiols to give methyl sulfides and diphenyl sulfoximide, and clarified the mechanism that involves an initial equilibrated protonation of the 6 -sulfanenitrile by thiols followed by nucleophilic attack of the thiolate anions on the methyl group in the rate-determining step [7]. Both the acid-catalyzed hydrolysis and the reaction with thiols show that the sulfoximide moiety of protonated alkoxy-6 -sulfanenitriles is a very good leaving group and thus alkoxy- 6 -sulfanenitriles are very effective alkylating reagents.…”