Conjugated azoalkenes. Part XI. Direct synthesis of substituted pyrazoles and 4-triphenylphosphoranylidene-4,5-dihydropyrazol-5-ones by heterocyclization of some zwitterionic 1,4-adducts between conjugated azoalkenes and triphenylphosphine

1,2-Diaza-1,3-butadienes: just a nice class of compounds, or powerful tools in organic chemistry? Reviewing an experience

“…Phosphine- [86][87][88] and nitroso-89 derivatives add to the conjugate azo-ene moiety of DBD 1 to initially produce the corresponding 1,5-zwitterionic intermediates, 9 and 11, respectively, that then tautomerize into the respective hydrazones, 10 and 12, respectively (Scheme 5). …”

“…16 28,86,88 or nitroso 89 derivatives, respectively (Scheme 5). In these cases, the attacking nucleophiles represent the functionalities in position 4 (Scheme 10).…”

“…94 The pyrazolopyridazinones, 28, were prepared from the pyrazoles, 27 (Scheme 11). 41 Pyrazoles were also reached by treatment of DBD with triphenylphosphine, 16,86,87 thiocarboxylic acids, 75 and β-tricarbonyl compounds. 17 …”

Phospha‐Michael‐Type Reactions between 1,2‐Diaza‐1,3‐dienes and Bidentate Nucleophiles: Formation of New Mono‐ and Diylides and their Elaboration to Heterocycles

“…The structure of 8 was ascertained by comparison of its spectral properties with those of an authentic sample prepared as already described. [8] The new heterocycles 2,3-dihydro-1-aza-4-phosphinine (7a) and 5,6-dihydro-1-aza-4-phosphepine (7b) proved stable and might be of interest in both synthetic and applied fields. [25] The spontaneous formation of these new heterocycles 7a and 7b from the precursors 6a and 6b is probably due to an internal nucleophilic attack of the amine nitrogen atom of 6 on the iminic carbon atom [26] with displacement of a molecule of semicarbazide through a pathway analogous to that recently reported.…”

“…[6] Previously, some of us examined phospha-Michael reactions between DDs and different phosphorus nucleophiles. In particular, new pyrazoles or pyrazol-5-ones were obtained from the reactions between DDs and triphenylphosphane [8] or trialkylphosphanes, [9] whereas with trialkyl phosphites [10] or dialkylphenylphosphonites [11] under solvent-free conditions the reactions gave new 1,2,3-diazaphospholes or (E)-hydrazono-phosphonates in high yields. Furthermore, the use of phosphoramidites, phosphorodiamidites or tris(dimethylamino)phosphanes gave stable α-phosphoranylidene-hydrazones, which were in turn transformed into the corresponding phosphoranylidene pyrazole derivatives.…”