Synthesis and Structure of a New Stable Carbocation Stabilized by Two Neighboring Sulfur Atoms. Dimethyl-9,9-bis(methylthio)-1-fluorenylcarbenium Ion

Article abstract of DOI:10.1021/jo00209a024

9,9-Bis(methylthio)-1-isopropenylfluorene (3) and the corresponding alcohol 2 generate a carbocation 1 in trifluoroacetic acid solution at room temperature.This ion is sufficiently stable to be observed spectroscopically in this solution.A stable crystalline hexachloroantimonate of 1 was isolated.In trifluoroacetic acid-d at room temperature, the two C⁺-methyl groups of 1 undergo H-D isotopic exchange to give the corresponding hexadeuterio cation.On the basis of electronic and 1H NMR spectroscopic data at temperatures from 40 to -60 deg C, it is concluded that ion 1 is not a sulfonium species but exists as a carbocation stabilized by electrons from the two neighboring sulfur atoms.Carbon-13 NMR data show that the positive charge is extensively delocalized over the neighboring sulfur atoms, with small amount remaining on the carbon atom.The possibility is suggested that 1 is a pentacoordinated carbon species that resembles the transition states in S_N2 displacements.