Nucleophilic Substitution at Tricoordinate Sulfur-Alkaline Hydrolysis of Optically Active Dialkoxysulfonium Salts: Stereochemistry, Mechanism and Reaction Energetics

Optically active dialkoxyisopropylsulfonium salts were obtained by methylation (ethylation) of optically active alkyl isopropanesulfinates using methyl (ethyl) trifluoromethanesulfonate. Alkaline hydrolysis of a series of methoxy(alkoxy)sulfonium salts afforded the two sulfinate products methyl isopropanesulfinate and alkyl isopropanesulfinate, both formed with a slightly prevailing inversion of configuration at the sulfur atom. DFT calculations revealed that this substitution reaction proceeded stepwise according to an addition-elimination (A-E) mechanism involving the formation of high tetracoordinate S-IV sulfurane intermediates. In addition, the DFT calculations showed that recombination of the hydroxy anion with the methoxy(alkoxy)sulfonium cation-leading to the parallel formation of the two most stable primary sulfuranes, with the hydroxy and alkoxy groups in apical positions and their direct decomposition-is the most energetically favorable pathway.

Automated summary

Optically active dialkoxyisopropylsulfonium salts were synthesized by methylation (ethylation) of optically active alkyl isopropanesulfinates. The alkali hydrolysis of methoxy(alkoxy)sulfonium salts revealed the formation of methyl isopropanesulfinate and alkyl isopropanesulfinate, with a slightly predominant inversion of configuration at the sulfur atom. DFT calculations suggested a stepwise addition-elimination (A-E) mechanism with the formation of tetracoordinate S-IV sulfurane intermediates. Furthermore, the DFT calculations demonstrated that the recombination of hydroxy anion with the methoxy(alkoxy)sulfonium cation, followed by direct decomposition, is the most energetically favorable pathway leading to the parallel formation of the two most stable primary sulfuranes.