The thermal fragmentation of 1,6-dioxa-6aλ4-thiapentalenes: formation of acylthioketenes and thioacylketenes

Abstract

Formylthioketene (4a) was observed in the flash vacuum pyrolysis of 1,6-dioxa-6aλ⁴-thiapentalene (2a) and identified by a combination of Ar matrix isolation IR spectroscopy and on-line collisional activation mass spectrometry. Bands due to both the s-Z and s-E conformers could be observed in the IR spectra. Rearrangement of 4a to thioformylketene (8a) was observed at 900 °C. s-Z-Benzoylthioketene (4b) as well as thiobenzoylketene (8b) could be observed in the pyrolysis of 2-phenyl-1,6-dioxa-6aλ⁴-thiapentalene (2b); in this case only the s-Z thioketene could be observed directly in the IR of the pyrolysis products. The final pyrolysis products SCO and phenylacetylene could also be observed. In the MS experiment, both benzoylthioketene and its rearrangement product thiobenzoylketene could be unambiguously identified. In both cases, a competing fragmentation leading to furan and thiophene derivatives was observed, and in the case of 2b, 5-phenylfuran-3-thione was identified. The observed interconversion of acylthioketenes and thioacylketenes and the identification of the IR spectra of 4a and 8a were supported by ab initio calculations.