Process for the preparation of pure 1-phenyl-3-carbalkoxy-5-hydroxypyrazoles

Process for the preparation of pure 1-phenyl-3-carbalkoxy-5-hydroxypyrazoles of the general formula ##STR1## in which R denotes methyl or ethyl, by reaction of acetylsuccinic esters with phenyldiazonium chloride and a base with pK.sub.B value of 3.6 to -1.5 at a pH of 4 to 9 in an aqueous medium to give the corresponding phenylhydrazone, which is cyclized at a pH of 7 to 9.5, with warming and with the addition of ammonia, to give the ammonium salt of the hydroxypyrazole. Compound I is formed by acidification.

The invention relates to a process for the preparation of pure 
1-phenyl-3-carbalkoxy-5-hydroxypyrazoles. 
Various hydroxypyrazoles designated in their tautomeric form as pyrazolones 
are described in US-PS No. 2,457,823. They are used as azo dyestuffs and 
they are prepared in accordance with the following equation: 
##STR2## 
the intermediates stages III and IA not being isolated. However, only 
those azo dyestuffs of the general formula IV in which the substituent 
R.sub.1 has the same meaning both in the 1-position and in the 4-position 
of the ring can be prepared by this method. If azo dyestuffs which have 
different radicals R.sub.1 in the 1- and 4-position are to be prepared, it 
is necessary to isolate the compounds of the general formula IA. 
This is described in GB-PS No. 585,780. This specification discloses a 
process for the preparation of hydroxypyrazoles of the general formula IA, 
and amongst other things also the preparation of 
1-phenyl-3-carbalkoxy-5-hydroxy-pyrazole, by reaction of acetylsuccinic 
esters with diazonium salts in a basic medium. 
When reworking these examples, however, it was found that the yields were 
only about 20%, and in the specification itself no yields are stated. In 
GB-PS No. 585,780 itself, it is stated that by-products, such as, for 
example, the azo dyestuff of the general formula IV, are formed which must 
be filtered off. A comparison of the melting points given therein with the 
melting points of the pure compounds also shows that in spite of 
recrystallization, the product is impure. GB-PS No. 585,780 thus gives the 
melting point of 1-phenyl-3-carbethoxy-5-hydroxypyrazole as 180.degree. 
C., whereas the melting point of the pure compound is 
183.degree.-184.degree. C. 
As can be seen from GB-PS No. 585,780, there are two possibilities for 
carrying out the reaction. It is carried out either (a) in the presence of 
weak bases, such as sodium acetate or pyridine, the reaction being brought 
to completion with a stronger base, such as, for example, sodium carbonate 
or sodium hydroxide solution, in which case, however, an organic solvent 
must be used, or (b) in the presence of a strong base, such as, for 
example, sodium hydroxide solution, from the beginning. 
It has now been found, unexpectedly, that the reaction must not be carried 
out with either a strong base or a weak and then a stronger base and an 
organic solvent if the product is to be obtained in a yield suitable for 
an industrial process in combination with a high purity. Rather, pure 
1-phenyl-3-carbalkoxy-5-hydroxypyrazole can be obtained in a high yield 
only if the reaction is carried out in two part steps, the reaction to 
give the hydrazone of the general formula III being carried out in the 
presence of an aqueous solution of a base with a pK.sub.B in the range 
from +3.6 to -1.5 and the cyclization to the hydroxypyrazole of the 
general formula IA being carried out in the presence of a somewhat weaker 
base, in particular in the presence of ammonia, the pH being kept 
substantially constant. No organic solvent is used here. 
The invention accordingly relates to a process for the preparation of pure 
1-phenyl-3-carbalkoxy-5-hydroxypyrazole of the general formula 
##STR3## 
in which R denotes methyl or ethyl, which comprises converting 
acetylsuccinic esters of the general formula 
EQU ROOC--CH.sub.2 --CH(COCH.sub.3)--COOR (II) 
in which R has the abovementioned meaning, with simultaneous addition of 
phenyldiazonium chloride and an aqueous solution of a base with a pK.sub.B 
value of +3.6 to -1.5 at a pH of 4 to 9, to the phenylhydrazone of the 
general formula 
##STR4## 
in which R has the abovementioned meaning, and subsequently cyclizing this 
compound, with warming and with the addition of ammonia at a pH of 7 to 
9.5, to give the ammonium salt of the hydroxypyrazole of the general 
formula I, from which the hydroxypyrazole of the general formula I is 
liberated by addition of acid. 
The process according to the invention is carried out in two part steps in 
one operation. The nature of the base is adapted to suit the particular 
part step and the pH is controlled exactly, which means that side 
reactions are avoided. 
In the first part step, an acetylsuccinic ester of the general formula II 
is coupled with phenyldiazonium chloride, the acetyl radical being split 
off and the corresponding phenylhydrazone of the general formula III being 
obtained. Coupling is carried out in the presence of a base with a 
pK.sub.B of +3.6 to -1.5. Such bases are, for example, sodium carbonate or 
potassium carbonate with a pK.sub.B of +3.6 or sodium hydroxide solution 
or potassium hydroxide solution with a pK.sub.B which can also reach 
negative values down to -1.5, depending on the particular activity. 
The reaction is carried out at a pH from about 4 to 9, the range from 7.0 
to 8.5 being particularly preferred. At this pH, the coupling takes place 
so rapidly that the base added at the same time as the diazonium chloride 
is consumed immediately and, in practice, virtually no excess of base is 
present. Decomposition of the diazonium salt and contamination of the 
phenylhydrazone formed are thereby avoided. At a higher pH, the side 
reactions get out of control, and at a lower pH the reaction proceeds too 
slowly. The temperature here is preferably kept at about 
10.degree.-30.degree. C., particularly preferably at about 
20.degree.-25.degree. C. 
According to a preferred embodiment, the acetylsuccinic ester of the 
general formula II is emulsified in water and an aqueous diazonium 
chloride solution and an aqueous sodium carbonate solution, which is 
preferably 10-30% strength, are simultaneously added in the course of 1 to 
2 hours such that a pH of about 7.9 to 8.1 is maintained. 
Without the phenylhydrazone of the general formula III formed in the first 
step being isolated, the cyclization to the hydroxypyrazole is carried out 
in the second part step by warming and adding ammonia, the pH being kept 
at about between 7.0 and 9.5, preferably between 8.0 and 8.5. The ammonium 
salt of the hydroxypyrazole of the general formula I is thereby formed, 
without side reactions taking place. The ammonium salt can be obtained by 
adding aqueous ammonia in various concentrations, for example 20-30% 
strength, or also by passing in gaseous ammonia. The addition of the 
ammonia is carried out continuously at the rate at which the ammonium salt 
of the hydroxypyrazole forms. The end of the reaction is indicated by an 
increase in the pH. The cyclization is carried out at a temperature of 
about 40.degree.-80.degree. C., preferably at about 55.degree.-65.degree. 
C. 
The hydroxypyrazole of the general formula I is liberated from the ammonium 
salt. The liberation is effected by addition of an acid, in particular by 
adding mineral acids, such as, for example, hydrochloric acid. 
The product is thereby obtained in a yield of at least 94% and has a purity 
of 99.5%. 
It can be used to prepare pure azo dyestuffs, which can be built up either 
symmetrically (the substituents R.sub.1 of the ring in positions 1 and 4 
in the general formula IV are identical) or unsymmetrically (the 
substituents R.sub.1 of the ring in positions 1 and 4 in the general 
formula IV are not identical).

EXAMPLE 1 
(a) Phenyldiazonium chloride 
93.1 g (1 mole) of aniline were initially introduced into 250 ml of water 
and 557 ml of 35% strength hydrochloric acid and a solution of 69 g (1 
mole) of sodium nitrite in 250 ml of water was added dropwise at 0.degree. 
C., with stirring. 
(b) Diethyl phenylhydrazonesuccinate 
216 g (1 mole) of diethyl acetylsuccinate were emulsified in 300 ml of 
water and the phenyldiazonium chloride solution described under (a) was 
added at 20.degree. to 25.degree. C. in the course of one and a half 
hours. An aqueous 20% strength sodium carbonate solution was added at the 
same time such that a pH of 8.0 was maintained. 
(c) 1-Phenyl-3-carbethoxy-5-hydroxypyrazole 
The emulsion obtained under (b) was heated to 60.degree. C. and 25% 
strength aqueous ammonia was added continuously so that a pH of 8.2 to 8.4 
was maintained. After filtration from traces of undissolved components, 
the resulting ammonium salt of 1-phenyl-3-carbethoxy-5-hydroxypyrazole was 
precipitated by addition of hydrochloric acid, filtered off, washed with 
water and dried. Yield: 220 g (95%); melting point: 
183.degree.-184.degree. C.; content: 99.5% (potentiometric bromate 
titration). 
EXAMPLE 2 
Starting from 188 g (1 mole) of dimethyl acetylsuccinate, 
1-phenyl-3-carbomethoxy-5-hydroxypyrazole was prepared in the same manner 
as in Example 1. Yield: 205 g (94%); melting point: 
197.degree.-198.degree. C.; content: 99.5% (potentiometric bromate 
titration).