Thermal rearrangement of .alpha.-oxo-.alpha.,.beta.-unsaturated azines to N-substituted pyrazoles

Palladium-catalyzed C–C bond cleavage of N-cyclopropyl acylhydrazones

“…26,27 Finally, cycloisomerization proceeds under heating conditions to produce pyrazole 2 . 28 The possibility of C–C bond cleavage via C–H activation is ruled out due to the high energy level of 5-membered chelate palladium complex A′ and the subsequent transition state in A′ to palladacycle D . 11…”

Five‐Membered Heterocycles: 1,2‐Azoles. Part 1. Pyrazoles

“…Cinnamaldehyde azine and derivatives 61 are converted into pyrazoles 62 in high yields when heated at about 200 C (with or without solvent) (Scheme 8.14) [107]. Other a,b-unsaturated azines behave similarly [108,109]. The thermal decomposition of polyhalogenated propenal azines can be used for the synthesis of mono-and dihalogenated N-unsubstituted pyrazoles [110].…”

Cyclization of conjugated azines. Synthesis and thermal rearrangements of 1-oxo-3,4-diaza-2,4,6,7-octatetraenes (allenyl azines)

“…When R is something other than hydrogen, this proton transfer is not possible and the dipolar intermediate (4) decomposes by a second ring opening reaction to yield the acyclic azines 7. An analogous intermediate (i.e., 9) is presumably involved in the thermal rearrangement of certain conjugated azines (8) to N-substituted pyrazoles (10 and 11). Symmetrical azines derived from ,ß-unsaturated aldehydes and ketones10 (i.e., 8a) and unsymmetrical azines formed from -diketone monohydrazones and ,ß-unsaturated aldehydes and ketones11 (i.e., 8b) rearrange to N-propenylpyrazoles (10a) and a-pyrazolyl ketones (11), respectively (Scheme I, eq 3).…”