The thermal fragmentation of 1,6-dioxa-6a lambda(4)-thiapentalenes: formation of acylthioketenes and thioacylketenes

Formylthioketene (4a) was observed in the flash vacuum pyrolysis of 1,6-dio xa-6a gimel (4)-thiapentalene (2a) double dagger and identified by a combin ation of Ar matrix isolation IR spectroscopy and on-line collisional activa tion 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 degreesC. s-Z-Benzoylthioketene (4b) as well as thiobe nzoylketene (8b) could be observed in the pyrolysis of 2-phenyl-1,6-dioxa-6 a gimel (4)-thiapentalene (2b) in this case only the s-Z thioketene could b e observed directly in the IR of the pyrolysis products. The final pyrolysi s products SCO and phenylacetylene could also be observed. In the MS experi ment, both benzoylthioketene and its rearrangement product thiobenzoylketen e could be unambiguously identified. In both cases, a competing fragmentati on 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 sp ectra of 4a and 8a were supported by ab initio calculations.