Pyrazolenines

Two double bonds. This is the most important class which includes the aromatic compounds pyrazole (3), indazole (4) and isoindazole (5), their non-aromatic isomers, pyrazolenines (or 3iL-pyrazoles 6), isopyrazoles (or 4JT-pyrazoles 7) and 3JT-indazoles (8), and the carbonyl derivatives of pyrazolines with the endocyclic double bond in positions 1, 2 or 3, i.e. (9), (10) and (11), respectively. The indazolones (12) and the pyrazolidinediones (13) and (14) also belong to this group. 

Sparse data on the pyrazole isomers, pyrazolenines and isopyrazoles, are presented in Table 12. Besides the obvious upheld effect on the chemical shift due to the suppression of the ring current, these compounds behave normally. Data on pyrazolidinones and their salts show the behaviour of cyclic hydrazides (66T2461,67BSF3502). 

Diazoalkanes and ynamines react with the electrophilic C(4)—C(5) double bond of pyrazolenine (359) to afford the cycloadducts (360) and (361), respectively, whereas diphenylketene yields the bicyclic diazetidinone (362) by reaction with the c/s-azo system of (359) (79CC568). 

Progress in the photochemistry of pyrazolenines and isopyrazoles is largely due to the studies of Diirr and Franck-Neumann. Since this subject has not previously been treated systematically, a relatively detailed account will be given here. 

Considerable effort has been devoted to studying the photolysis of pyrazolenines since it still constitutes the best way to obtain cyclopropenes. An important publication by Gloss... 

In contrast to pyrazolenines, there are only a few publications on the photochemistry of isopyrazoles and they concern exclusively their iV-oxides (390). Irradiation of (390) affords the iV-oxides of pyrazolenine (391) (70CC289). Bicyclic intermediates (392) and (393 Scheme 36) are believed to be implicated in this reaction (75MI40400). The final step is similar to that reported from studies of the valence bond isomerization of pyrazolenines (68JA173). 

Pyrazoles are formed when the diazo compounds react with alkynes or with functionalized alkenes, viz. the enols of /3-diketones. Pyrazolenines (353 Section 4.04.2.2.1) are isolated from disubstituted diazomethanes. Many pyrazoles, difficult to obtain by other methods, have been prepared by this procedure, for example 3-cyanopyrazole (616) is obtained from cyanoacetylene and diazomethane (7iJCS(C)2i47), 3,4,5-tris(trifiuoromethyl)pyrazole (617) from trifluorodiazoethane and hexafluoro-2-butyne (8lAHC(28)l), and 4-phenyl-3-triflylpyrazole (618 R =H) from phenyltriflylacetylene and diazomethane (82MI40402). An excess of diazomethane causes iV-methylation of the pyrazole (618 R = H) and the two isomers (618 R = Me) and (619) are formed in a ratio of 1 1. 

Using chloro- or bromoenines, one can immediately obtain 4-ethynylpyrazoles (omitting the stage of pyrazolenine isolation) which is accompanied by methylation and the formation of the mixture of 3- and 5-sulfonyl-l-methyl-4-acetylenylpyra-zoles in about 1 2 ratio (total yield 37%) (Scheme 14). 

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

Prostaglandins 624, 725, 960 Prostanoids 620 Protonation 565-567, 1049 photochemical 882 Pseudopotential methods 15, 16 Pummerer rearrangement 240, 243, 470, 843 Pyramidal inversion 602, 604 Pyrazolenines 749 Pyridazine oxides 640 Pyridine aldehydes, synthesis of 310 Pyridine oxides 640 Pyrolysis 102-105 of sulphones 110, 679-682, 962 of sulphoxides 739, 740 Pyrroles 265, 744... 

Finally photolysis of substituted pyrazolenines 140 in ether is reported (118) to yield allene 142 and diene 143 as products via vinyl cation 141. 

Analogous reactivity was observed by Jones303 with methylene cyclopropene 554, which on treatment with diazomethane yielded the pyrazole 556, thought to arise from thermal isomerization of pyrazolenine 555 formed initially ... 

The formation of pyrazolenines demands, as in the cyclopropenone series, the intermediacy of conjugated diazo compounds, e.g. 549, arising from valence tautomeriza-tion of diaza bicyclo(3,l,0)hexanes, e.g. 548. 

The irradiation of two isomeric triethylstannane-substituted pyrazolenines (23 and 24) results in the evolution of N2, and the formation of the same substituted cyclopropene40, as illustrated in reaction 28. 

The photolysis of pyrazolenine derivatives 113 yields mainly allene 114 and 1,3-diene derivatives 115. According to Day and Whiting... 

This important section deals with pyrazolenines, isopyrazoles and 4,4-disubstituted 2-pyrazolin-5-ones whose thermal and photochemical reactions will be discussed in this order. 

Pyrazolenine esters under similar conditions give allene and 1,3-diene esters by an ionic mechanism involving both inter and intramolecular processes, viz. 

---