DECOMPOSITION OF 3,5-DIARYLOXATHIOLANE-2-OXIDES UNDER ELECTRON-IMPACT

1,2-diarylcyclopropanes are known to form the corresponding 3,5-diaryl -1,2-oxathiolane-oxides upon reaction with sulfur dioxide in the prese nce of trifluoroacetic acid. The reverse reaction, the elimination of sulfur dioxide from 3,5-diaryl-1,2-oxathiolane-2-oxides to form the co rresponding 1,2-diarylcyclopropanes, is also known in organic chemistr y. In the present work, the loss of sulfur dioxide from the molecular ion of 3,5-diphenyl-1,2-oxathiolane-2-oxide (1), phenyl-5-(p-methoxyph enyl)-1,2-oxathiolane-2-oxide (2), and 3,5-di-(p-methoxy phenyl)-1,2-o xathiolane-2-oxide (3) was observed in the ion source of a mass spectr ometer under electron impact ionization conditions (EI). The resulting (M-SO2)(+.) ions had the same collisional activation mass spectra as the cation-radicals of the authentic 1,2-diphenylcyclopropane (4), 1-( p-methoxyphenyl)-2-phenylcyclopropane (5), and 1,2-(p-methoxyphenyl) c yclopropane (6), respectively, indicating their common structure. Ther efore, the molecular ions of 1, 2-oxathiolane-2-oxides 1-3 lose SO2 to give rise to pseudomolecular ions of diarylcyclopropanes 4-6. The fra gmentation reactions of 1,2-diphenylcyclopropane (compound 4) under EI were studied in detail using tandem mass spectrometry and high resolu tion mass spectrometry in order to establish their ion structure and f ragmentation mechanisms. Our data suggests that the cation radicals of diphenylcyclopropane isomerize giving primarily cation-radicals of a methyldihydrophenanthrene structure. The latter undergoes dissociation to give rise to protonated phenanthrene (m/z = 179), fluorenyl-cation (m/z = 165) and indenyl-cation (m/z = 115). Since SO2 elimination fro m 3,5-diaryl-1,2-oxathiolane-2-oxides to form diarylcyclopropanes take s place both in solution and in the gas phase under EI, the compounds under study can serve as examples of a system in which a condensed pha se reactions parallel cation radical reactions within the ion source o f a mass spectrometer. Published by Elsevier Science B.V.