Decomposition of 3,5-diaryloxathiolane-2-oxides under electron impact

Abstract

1,2-diarylcyclopropanes are known to form the corresponding 3,5-diaryl-1,2-oxathiolane-oxides upon reaction with sulfur dioxide in the presence of trifluoroacetic acid. The reverse reaction, the elimination of sulfur dioxide from 3,5-diaryl-1,2-oxathiolane-2-oxides to form the corresponding 1,2-diarylcyclopropanes, is also known in organic chemistry. In the present work, the loss of sulfur dioxide from the molecular ion of 3,5-diphenyl-1,2-oxathiolane-2-oxide ( 1), 3-phenyl-5-(p-methoxyphenyl)-1,2-oxathiolane-2-oxide ( 2), and 3,5-di-(p-methoxy phenyl)-1,2-oxathiolane-2-oxide ( 3) was observed in the ion source of a mass spectrometer under electron impact ionization conditions (EI). The resulting (M-SO 2) + 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) cyclopropane ( 6), respectively, indicating their common structure. Therefore, the molecular ions of 1,2-oxathiolane-2-oxides 1–3 lose SO 2 to give rise to pseudomolecular ions of diarylcyclopropanes 4–6. The fragmentation reactions of 1,2-diphenylcyclopropane (compound 4) under EI were studied in detail using tandem mass spectrometry and high resolution mass spectrometry in order to establish their ion structure and fragmentation 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 SO 2 elimination from 3,5-diaryl-1,2-oxathiolane-2-oxides to form diarylcyclopropanes takes 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 phase reactions parallel cation radical reactions within the ion source of a mass spectrometer.