Thermal Reactions of 5‐Alkylidene‐4,5‐dihydro‐3 H ‐1,2,4(λ 3 )‐diazaphospholes (4‐Phosphapyrazolines) – A Route to Various PHeterocycles and to 2‐Phosphabutadienes

The 5‐alkylidene‐4,5‐dihydro‐3H‐1,2,4(λ 3 )‐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 8a‐g gives rise to β‐phosphanaylsiloxyalkenes 10 , benzo[c]‐phosphole derivatives 11, 14 and 1 15 , (β‐siloxyalkylidene)‐Phosphiranes 12 , and dihydro‐1,3‐oxaphospholes 13 . The thermolysis of 5‐alkylidene‐4,5‐dihydro‐4‐trimethylsilyl‐3‐trimethylsilyloxy‐3H‐1,2,4‐dizaphospholes 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.