Mass spectrometry in isomer differentiation: 4,5‐bis(alkylthio)‐1,3‐dithiol‐2‐ones and 1,2‐dithiol‐3‐ones

Abstract

A mass spectrometric study of compounds 1 seemed worthwhile for two reasons: (i) to our knowledge, perdeutero-4,5-ethylenedithio-1,3-dithiol-2-one is the only compound of type 1 whose mass spectrum has been described in some detail," (although some alkyland aryl-substituted 1,3-dithioI-2-ones have been studied), and (ii) we have recently reported on the differentiation and electron ionization-induced isomerization of 4,5-bis(alkylthio)-l,3-dithiole-2-thiones and their 1,2-dithioIe-3-thione isomer^.^ Thus, we reasoned that a comparative study of the mass spectra behaviour of compounds 1 and their 1,2-dithiol-3-one isomers 2 could not only reveal possible isomerizations but also show similarities and differences between the corresponding thioxo-and 0x0-analogues. Moreover, no reference to the preparation of compounds 2 has been found in the Literature, although to our surprise, an X-ray diffraction study of compound 2a has been reported.6 Thus, in this paper we report on the positiveion electron ionization (EI') mass spectra of compounds 1 and 2, along with the synthesis of compounds 2. EXPERIMENTAL ChemicalsCompounds la,' lb'" and 1c3' were prepared as previously described. Compounds 2a-c were prepared as follows. The corresponding 4,5-bis(alkylthio)-l,2-(1 mmol) was dissolved in CHCI3 (25 mL) and to this solution mercury(I1) acetate (5 mmol) and acetic acid (20mL) were added. The mixture was stirred at room temperature for 5 h and the yellow precipitate was filtered off through a celite pad and washed with dichloromethane (3 X 10 mL). The combined filtrates were washed with 1N aqueous NaHCO, and then with water. After drying (MgSO,) and evaporation of solvent under vacuum, the crude product was purified by silica column chromatography (eluent CH2CI2+hexane, 2 : l for 2a and 2c, CH2CI2 + hexane, 1 : 1 for 2b). Mass spectrometryThe mass spectra were recorded on a VG-Autospec EBE spectrometer (VG Analytical, Manchester, UK). The samples were introduced via the solids probe. The conditions for electron ionization were: electron energy 70 eV, trap current 200 PA, source temperature 200 "C. Accurate mass measurements were obtained by narrow-range voltage scanning at 10 000 resolution (measured as peak width at 5% height) using perfluorokerosene as reference compound and were correct to an accuracy of k10ppm of the theoretical value. Metastable transitions in the first field-free region were detected by linked scans'' at constant BIE and B'IE (El = E2, V = constant). Collisional experiments" were performed by colliding 8 keV ions with air in such a way that the main beam intensity was reduced to ca. 25% of its initial value. In order to ensure identical experimental conditions, the spectra of ions with the same mass value, but generated from different compounds, were recorded one after the other without adjustment of instrumental parameters. RESULTS AND DISCUSSIONThe 70eV mass spectra of compounds 1 and 2 are gathered in Table 1 and the fragmentation pattern is depicted in Scheme 1. Metastable analysis indicates that t...