Theoretical studies on the methylsulfenyl chloride addition to propene

Abstract

AbstractThiiranium heterocycles play an important role in biocatalytic processes of cells. Usually formation of thiiranium ions is known to proceed by the electrophilic additions of sulfenylhalides to substituted olefins, subsequently undergoing the regioselective and stereoselective nucleophilic attack of the halide atom on either C‐1 or C‐2 carbon atom of the thiiranium intermediate to gave two isomeric adducts. The detailed sequence of the reaction mechanism, the nature of intermediates, and transition states that occur in this electrophilic addition reaction are not well understood. In our work, this reaction has been modeled using Ab initio methods at the MP2/6‐31+G(d,f) level of theory to look into the mechanism of the reaction and to explain how the regioselectivity of the reaction is controlled. We focused on the electrophilic addition reaction of the methylsulfenyl chloride to propene. Our calculations show that the reaction is predicted to proceed via two distinct directions. The first direction proceeds when the starting reacting molecules formed the cis‐methyl‐oriented thiiranium intermediate, and the second direction is when the starting reactants resulted in the trans‐methyl‐oriented thiiranium intermediate. The calculated reaction potential energy surface profile suggests that the minimum energy pathway via the first direction is energetically more preferred than that via trans one. Moreover, calculation of the intrinsic reaction coordinate on the minimum energy pathway revealed the stepwise mechanism for the addition reaction. Thus the energetically preferred first reaction direction consists of the addition of methylsulfenyl chloride to the double bond of propene undergoing synchronous concerted transition state leading to the thiiranium intermediate formation (the rate‐limiting step in the electrophilic addition reaction); regioselective thiiranium intermediate ring‐opening process by the chloride anion attack on the C‐2 carbon of the thiiranium intermediate forming 2‐chloro adduct of kinetically controlled addition reaction; the isomerization reaction of 2‐chloro adduct to more energetically favorable thermodynamically stable 1‐chloro product. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:1–13, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20571