2-Pyrazolin-5-ones, elimination

Pyrazolines substituted at position 4 or 5 with hydroxy or amino groups readily eliminate a molecule of water or amine yielding pyrazoles. The 4-substituted derivatives are relatively more stable than the 5-substituted ones, because for the last group the lone pair at N-1 assists the elimination (407) -> (408) -> (409). The sulfonyl group at position 1 is also easily eliminated and this property is taken advantage of in Dorn s elegant synthesis of 3-aminopyrazole (Section 4.04.3.3.1). 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

A number of halobenzylidenemalonate derivatives give unstable pyrazolines with diazomethane. One pyrazoline (43, Scheme 14) eliminated HF with... 

The thermal ring opening of the pyrazoline (351) occurs with elimination of nitrogen and the formation of a keto ester. The latter cyclizes with loss of ethanol to the pyran-2-one (352 Scheme 110) (02CB782). Since pyrazolines may be obtained from a 1,3-dipolar cycloaddition of diazoacetic ester and an unsaturated ketone, this route is in effect a further example of a type (i) synthesis (Scheme 85). 

In (39), we have an example of a stereospecific formation of a pyrazo-line. When pyrazolines are photolyzed, the elimination of nitrogen, if concerted, should be syn. This stereospecific product has been observed (McGreer and Wu, 1967). The pyrolysis of pyrazolines, if concerted, should be anti. It is not clear why this process turns out to be complex in (39), one alkene is formed stereospecifically by an anti hydrogen migration, but two isomeric cyclopropanes are formed. Crawford and Ali (1967), find rather different product patterns in the pyrolysis of 3-methyl and 3,4-dimethylpyrazolines, and give evidence for diradical intermediates. The general problem of unstable intermediates in a step-wise process will be discussed in a later section. 

The reaction has proceeded satisfactorily with one or two benzoyl groups present,153,507 but a benzoyl group in position 1 has also been eliminated.153 The oxidation proceeds similarly when position 4 is substituted.608 Pyrazoline itself and monoalkyl- and monoaryl-pyrazolines give the corresponding pyrazoles only in small yield on oxidation with bromine.509,510 In the majority of cases bromination occurs together with oxidation, and often brominated products are the only ones isolated. Thus 1,3,5-triphenylpyrazoline yields a tribromide of unknown constitution.49 The hydro bromide of 4-bromopyrazole (43) was obtained almost quantitatively from 3-butoxy-J1-pyrazoline (42).611A mixture of 1,5-dimethylpyrazole and the 4-bromo compound... 

Addition reactions of diazoalkanes, especially diazomethane, diazopropane, diphenyl-diazomethane and ethyl diazoacetate, onto olefins, substituted with one or two electron-withdrawing groups take place very smoothly affording pyrazolines which upon heating or photolysis can give rise to the corresponding substituted cyclopropanes by elimination of nitrogen. 

The alkylation of 4-nitro-2-pyrazolin-5-ones at C-4 with diazomethane478 has already been mentioned. This hydrogen atom is also replaceable with bromine.1642 Reduction of the nitro group with stannous chloride forms the corresponding amine.478,480 Picrolonic acid condenses with itself to form a bis(2-pyrazolin-5-one) connected by a double bond at the 4,4 -position, with elimination of two moles of nitrous acid.690 Condensation also occurs in the presence of phenyl-hydrazine, but 4,4 -bis[3-methyl-l-(4-nitrophenyl)-2-pyrazolin-5-one] is formed.690... 

Pyrazolin-5-ones react with salts of various metals to form compounds in which the pyrazolinone has reacted in its enolic form with replacement of the enolic hydrogen to give a salt and having semipolar bonds formed by donation of electrons to the metal by the nitrogen atoms393 394 Usually these compounds contain the number of pyrazolinone residues corresponding to the valence of the metal atom. Such salts as cuprous iodide, ferric iodide, cobaltous iodide, silver iodide and silver diiodide participate in such reactions.393,394 In addition, complexes may be formed in which there has been no elimination of a small molecule between the reactants and no formation of ionic bonds.432... 

In the presence of sulfuric acid 1-nitroso-3-pyrazolidinones lose nitrous acid by elimination, giving 2-pyrazolin-5-ones which are then nitrosated at C-4.335,1028 The amino group in 4-amino-3-pyrazol-idinones is diazotizable and the resulting diazonium salt undergoes coupling.12... 

Although by far the majority of 2-pyrazolin-5-ones used as color couplers have no substituent at C-4, this is not a necessary condition. 4,4 -Bis(2-pyrazolin-5-ones) can be used for this purpose since the 4-substituents can be eliminated and an azomethine dye formed. Such couplers may offer certain advantages. It is claimed that they are more stable and also they make it possible to form two azomethine dyes if the two portions of the molecule are different. Weissberger, Vittum and Porter1540,1603 have patented a number of 4,4 -arylidenebis(2-pyra-zolin-5-ones) for use as color couplers. Gluck580 has suggested 4,4 -bis-(2-pyrazolin-5-ones) linked by sulfur which he prepared. 

---