Stabilization of electron-deficient sulfur by neighboring sulfur and oxygen groups.

Abstract

The mesocyclic trithioether 1,4,7-trithiacyclononane shows neighboring group participation upon oxidation as evidenced by the peak potential for oxidation in cyclic voltammetry, 0.9 V versus Ag/0.1 M AgNO₃, and by the formation of its monosulfoxide by controlled potential electrolysis in an overall two electron process. Pulse radiolysis of 1,4,7-trithiacyclononane leads to the formation of a two-center three-electron bond of moderate stability. Naptho [1,8-b,c]-1,5-dithiocin shows neighboring group participation upon electrochemical oxidation. A low peak potential is observed, 0.47 V versus Ag/0.1 M AgNO₃, in the cyclic voltammetry. Controlled potential electrolysis leads to formation of the monosulfoxide in a two electron process. The rigid methionine analog (±)-2-exo-amino-6-endo methylthiobicyclo [2.2.1] heptane-2-endo carboxylic acid exhibits neighboring group participation upon electrochemical oxidation. The degree of participation depends on the acid/base chemistry of the α-amino acid. The zwitterion shows the strongest neighboring group participation. Controlled potential electrolysis led to the formation of two diastereomeric sulfoxides in a two electron process. The diastereomer ratio was suggestive of participation by the carboxylate in the oxidation. The structures of the sulfoxides were confirmed by comparison with chemically prepared sulfoxides and sulfoxide amino acid derivatives. The stereochemistry was assigned by comparison with related compounds upon which x-ray crystallographic analysis had been performed. Single-electron pulse radiolytic oxidation of the amino acid led to the formation of an absorbing transient which was assigned to a two-center three-electron sulfur-carboxylate oxygen species. The yield of the species formed was pH dependent. At a pH above 3.5 decarboxylation becomes a major decomposition pathway in contrast to the electrochemical experiments where no decarboxylation was seen. The alcohol (±)-endo-2-hydroxy-6-endo-methylthiobicyclo [2.2.1] heptane shows significant neighboring group participation upon electrochemical oxidation in comparison with its exo-hydroxy isomer as indicated by a 350 mV difference in the peak potentials. Controlled potential electrolysis in a two electron process leads to formation of a mixture of diastereomeric alkoxysulfonium salts.