Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 5. Selective extrusion of PH₃PO from β,γ,β′-trioxo ylides to give diacylalkynes

Pyrolytic decomposition of simple acetylenic esters

“…The preparation of 11 was most easily accomplished by pyrolytic extrusion of Ph 3 PO (FVP, 500 ˚C) from the stabilised ylide 12 formed from ethoxycarbonylmethylenetriphenylphosphorane and methyl oxalyl chloride in the presence of triethylamine. 2 As initially observed when the products 2 were formed by pyrolysis of ylides 1, either increasing the ylide pyrolysis temperature to 750 ˚C or isolating 2 from pyrolysis at 500 ˚C and subsequently subjecting it to FVP at 750 ˚C gave the alk-1-yne 3. Thus when 2 (R = Ph) was subjected to FVP at 750 ˚C, 3 (R = Ph) was formed in 70% isolated yield together with a low yield of ethene recognised by its 1 H NMR signal at 5.4 ppm which rapidly disappeared upon warming the solution to RT.…”

New Gas-Phase Domino Processes Leading to Benzopyranones and Benzofurans

“…Several approaches to the simple case of PhC(=S)-C(=PPh 3 )-C(=O)Ph failed but we were able to obtain the dioxo/thioxo ylide 40 using the method of Tebby and coworkers (Scheme 10), 25 involving addition of Ph 3 P and sulfur to dibenzoylacetylene (39), itself conveniently obtained by FVP of the trioxo ylide 38. 26 When this was subjected to FVP there was complete reaction at 700 or 750 °C but 31 P NMR showed almost no selectivity with a ratio of Ph 3 PS to Ph 3 PO of 1.2 to 1 in each case. This is doubly surprising since not only was Ph 3 PS expected to be eliminated much more readily than Ph 3 PO, but it is in stark contrast to the complete selectivity towards elimination of Ph 3 PO across the central position of most trioxo ylides as seen for 38 in Scheme 9.…”

Synthesis and Pyrolytic and Kinetic Behaviour of β,δ′‐Dioxo‐Stabilised Phosphorus Ylides: Convenient Preparation of γ,δ‐Alkynyl Ketones and 2,3‐Functionalised Butadienes

“…Scheme 2 Previous studies on the pyrolysis of ylides where there is a choice of different carbonyl oxygen atoms available for elimination have shown that while the β,βЈ-dioxo ylides 10 undergo non-selective elimination to give a mixture of alkynes, [7] the β,γ,βЈ-trioxo ylides 11 undergo selective elimination across the ''central'' position to give diacylalkynes in most cases. [8] On the other hand, ylides such as 12 and 13 lose Ph 3 PO exclusively between the ylide function and the more remote carbonyl group to give cyclopentenones and cyclohexenones as shown, with no trace of the acetylenic ketones expected from 1,2-elimination. [9] When the ylide 3a was subjected to FVP at 10 Ϫ2 Torr reaction was found to be incomplete at 500°C, with some unchanged ylide being recovered.…”

“…We have also found previously that a high value of 2 J P,CO (Ն 10 Hz) is a reliable indicator for failure of Ph 3 PO elimination. [8] With values of 12Ϫ20 Hz for 2 J P,CO compounds 5aϪd therefore appear unlikely to undergo 1,2-elimination. When the ylides 5 were subjected to FVP under the same conditions as for 3 there was again complete extrusion of Ph 3 PO at 650°C to give products which were readily identified as the substituted 1,3-dienes 20 ( Table 4).…”