The 5-alkylidene-4,5-dihydro-3H-1,2,4(h3)-diazaphospholes (4-phosphapyrazolines) are thermally much more stable than related compounds without the exocyclic double bond. Thermolysis reactions typically occur in the range 110-150°C in toluene solution, and different, mostly competing, reaction pathways are observed. Thermal extrusion of nitrogen from 6a-g gives rise to b-phosphanylsiloxyalkenes 10, benzo[c]-phosphole derivatives 11, 14, and 15, (P-siloxyalky1idene)-phosphiranes 12, and dihydro-1,3-oxaphospholes 13. The thermolysis of 5-alkylidene-4,5-dihydro-4-trimethylsilyl-3-trimethylsilyloxy-3H-1,2,4-diazaphospholes 17 afforded three products, including the highly substituted and stable 2-phosphabutadienes 18 formed by nitrogen extrusion and rearrangement. Finally, the 4-chloro-3-trimethylsilyloxy-substituted heterocycle 21 was transformed at 170°C into 4H-1,2,4-diazaphosphole 23. The structures of 13c and 18a were determined by single-crystal X-ray diffraction.A convenient route to bis(alky1idene)phosphoranes 2 and phosphiranes 3 is provided by the thermal or, less frequently, the photochemical. extrusion of dinitrogen from 4,5-dihydr0-3H-l,2,4(3~~)-diazaphospholes 1 [']. So far, the heterocycles 1 have been prepared exclusively by [3 + 21 cycloaddition of diazo compounds to phosphaalkenes ['-5]; quite often, the nitrogen elimination already occurs during the cycloaddition piocedure at or below room temperature["-q. The resulting bis(alky1idene)phosphoranes 2 (as well as those prepared by other methods and which are substituted differentlyL5I) typically undergo a thermally induced, conrotatory 4n-electrocyclic isomerization to the thermodynamically more stable phosphiranes 3. Depending on the substituent pattern, this rearrangement occurs in a temperature range between -SO and +120"C, i.e. compounds 2 can have a fleeting existence (e.g. 2, R3 = Cl[l'.fl) and can be detected spectroscopically at room temperature (e.g. R' = R2 = %Mei, R' = R4 = R' = Ph[ld]), or a stable enough to be isolated (SiMe? groups at one or both C atoms, NR2 at P ['d,b,51). Thcrmal reactions of 2 which do not lead to phosphiranes are rare [j].Several 4,5-dihydro-3 H-diazaphospholes do not lend themselves as precursors to 2 and 3. As in other cases (see above), the dediazonation of P-trimethylsilyl- We have found recently that 5-alkylidene-4,5-dihydro-3H-1,2,4(h')-diazaphospholes can be synthesized from phosphaalkenes, and 1 -diazo-2-siloxyethenes generated in situ from a-diazo-a-silyl ketone^ [^,^]. These novel phoshole derivatives are thermally much more stable than their related compounds lacking the exocyclic C-C double bond, and can therefore be isolated and handled con~eniently [~]. Nevertheless. we expected them to lose nitrogen at higher
