New Mass Spectra
Mass Spectra of Some Secondary 2-Furancatrbothioamides Thioamides are generally known to rearrange under electron impact (EI) more readily than their oxygen analogues. This may be attributed to the bulkiness of the sulphur atom and its capacity to localize the charge and to use the vacant d-orbitals in the formation of bonds.
The mass spectra of thioamides have been analysed in several papers. '-' ' Their fragmentation pathways involve an interesting four-centred skeletal rearrangement of the molecular ions with migration of the alkyl or aryl group from the nitrogen towards the sulphur atom with subsequent eliminatim of a thioalkoxy or thioaryloxy radical. The [M -SH] + ion peaks observed in the spectra give evidence of a thione-thiol tautomeric equilibrium of the molecular ions. Some authors explain their formation, however, by assuming an interaction of the sulphur atom with the hydrogen atoms of the neighbouring alkyl groups.10,12J3.16
Since not much attention has yet been given to thioamides derived from heterocyclic systems, we present here the results of a mass spectrometric investigation of 2furancarbothioamides. We were prompted to undertake this study by our earlier observations concerning 2-thiazole-and 2-benzothiazolecarbothioamides15 and the thioacyl derivatives of 2-aminothiazole and 2-aminobenzothia~ole.~ In the EI-induced fragmentation of those compounds we observed an interesting rearrangement with the formation of nonaromatic spirocyclic intermediates, from which 2thiazolethione and 2-benzothiazolethione cation radicals, respectively, have been formed by a loss of an isonitrile.
The mass spectra of the investigated secondary 2furancarbothioamides (1-13) were recorded on a JOEL JMS D-100 instrument (ionizing voltage 75 eV, ionizing current 300 pA and accelerating voltage 3 kV). The high-resolution mass spectra were obtained by the peak-matching method with a resolution of loo00 at a 10% valley. Metastable ion transitions in the first field-free region were analysed with an MS-MT-01 metastable ion detector by scanning the accelerating voltage.
The undeuterated compounds (1-13) were synthesized as described earlier.'8*'9 The N-deuterated analogues of 1-13 were prepared by dissolving the thioamides in CH30D and refluxing for 15 min; the solution was evaporated in vacuo and the operation repeated three times.