Flash vacuum thermolysis - Synthesis and characterization of unstabilized cycloalkenethiones

The flash vacuum thermolysis (FVT) of cycloalkenyl allyl (or propargyl) sul fides 8, 9, 12, and 13 led, as the main thermal pathway, to the conjugated cycloalkenethiones 1, 3, and 5, resulting from a retro-ene reaction, These reactive thioketones, thus obtained in ca. 70% yield, have been characteriz ed in the gas phase by mass spectrometry, and at low temperature by IR, UV/ Vis and NMR spectroscopy. The polymerization of 1, 3, and 5 upon warming is generally more rapid than the other attempted reactions and only the [2 3] dipolar cycloaddition with diazomethane led in low yield, besides polyme ric materials, to isomeric 1,3-dithiolanes 20 from cyclopentenethione 1. Th e obtention of the nonconjugated cycloalkenethiones 2 and 4, expected in th e FVT of sulfides 10 and 11, has been confirmed only by UV/Vis spectroscopy at -196 degrees C, due to their rapid enethiolization. In the case of the large-ring sulfide 14, the (E) geometry of the double bond favoured the com petitive retro-ene reaction yielding cyclododecene and propynethial; on the other hand, the FVT of the tricyclic gem-dithiol 15, a possible precursor of cyclobutenethione (7), led to vinyl thioketene (18), obtained by thermal ring opening of 7.