Pyrazole chlorination

Nitration of 4-(2-thienyl)- (301) and 4-(3-thienyl)-pyrazoles (302) mainly occurs on the thiophene ring, but when acetyl nitrate is used as the nitration agent small quantities of products nitrated on the pyrazole ring are isolated (position of the nitro group uncertain) (80CS( 15)102). Pyrazol-l -ylpyridines (303) undergo electrophilic reactions (bromination, chlorination and nitration) preferentially in the pyrazole ring. Thus, the nitration of (303 R = R = = H) either with a mixture of nitric acid and sulfuric acid at 10-15 °C or with... 

Halogenation is one of the most studied electrophilic substitutions in the pyrazole series (67HC(22)1, B-76MI40402). The results concern chlorination, bromination and iodination since there is no report on direct fiuorination of pyrazoles (fiuoropyrazoles are prepared by other... 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

Bromine in chloroform and bromine in acetic acid are the reagents used most often to brominate pyrazole. When nitric acid is used as a solvent, both bromine and chlorine transform pyrazoles into pyrazolones (Scheme 24). Thus 3-methyl-l-(2,4-dinitrophe-nyOpyrazole is brominated at the 4-position (309). The product reacts with chlorine and nitric acid to give the pyrazolone (310). The same product results from the action of bromine and nitric acid on (311). The electrophilic attack of halogen at C-4 is followed by the nucleophilic attack of water at C-5 and subsequent oxidation by nitric acid. 

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

However, addition of bromine is accompanied by bromination of 3,5-dimethyl-pyrazolate and pyrazolate ligands at position 4, which is blocked in 4-methyl-pyrazolate. The product of electrochemical oxidation of 175 (R -- R = Me, R = Br) is 177 (R = R = Me, R = Br, X = CIO4). Chlorination proceeds similarly to bromination. 

In addition, it was clear that the presented scheme does not exhaust all the possible paths of anomalous chlorination. Thus, 4-acetyl-l,3,5-trimethylpyrazole with 4-5 moles of phosphorus pentachloride gave mainly 4-Q ,/3-dichlorovinyl-5-chloromethyl-l,3-dimethylpyrazole, and its precursor (according to GLC data) was the dichloride. It could be possible that in the case of 4-chlorovinyl derivatives of pyrazole the chlorination at the p position is facilitated by the electron-donating characteristics of the 4-pyrazolyl radical (86TH1). 

The halogen migration is completely suppressed by halogen-metal exchange when the chloroethynyl group is in position 5 of the pyrazole ring. The concentrations of 3-pyrazolyl and 4-pyrazolyl anions are probably small, and they cannot compete with NH2 anions for chlorine bonded to the acetylenic carbon. 

Pyrazolo[l,2-a]pyrazoles give 3-bromo products unless the 3-position is blocked by a methyl when bromomethyl products arise (80JA4983 81JOC1666, 87JOCI673). When all of the pyrazole ring positions are filled by phenyl groups, chlorination and bromination occur in thepara-positions (74BCJ946) (see also B, 1,2). 

Extension of the 1,3-DC approach to the synthesis of novel pyrazoline-fused chlorin 78 by the reaction of P-nitro-meso-tetraphenylporphyrin le with diazomethane has also been explored by Cavaleiro and co-workers (Scheme 27) <02S 1155>. The resulting chlorin 78 could be further converted into the pyrazole-fused porphyrin 79 by treatment with DBU or into the methanochlorin 80 by refluxing in toluene. 

Pyrazolo[l,5-A benzisothiazoles 252 were prepared from 3(5)-[2 -methyl-thiophenyl]pyrazoles 251 and AT-chlorosuc-cinimide. The ring closure may involve a sequential nucleophilic heteroaromatic nitrogen displacement of a sulfonium chlorine atom followed by dealkylation of the resulting sulfonium salt to form the sulfenylimine moiety (Equation 110) <1996H(43)221>. 

With chlorinated quinones. New heterocycles containing 1,2-dihydro-imidazo [l,2- ]imidazol-3-one 405 or 1/7-imi-dazo[l,2- ]pyrazole moieties were obtained via charge-transfer interaction of creatinine or 3-aminopyrazole with some 7i-deficient compounds such as 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, 2,3,5,6-tetrachloro-l,4-benzoquinone, 2,3-dichloro- or 2,3-dicyano-1,4-naphthoquinone, and 3,4,5,6-tetrachloro-l,2-benzoquinone (Equation 183) C1996BSB159, 2001HC0541, 2000PS1>. 

The chlorinated intermediate 255 is eliminated and cycloadds to Cjq, yielding pyrazo-linofullerenes of the structure 257 (Scheme 4.42). The 4-nitrophenyl-group can be replaced by a 4-methoxyphenyl- or a phenyl substituent. In this reaction various aromatics and substituted aromatics are tolerated as residues R (e.g. furan, ferrocene, pyrazole or benzene and substituted benzene). The nitro group of the nitrophenyl residue can be reduced with Sn-HCl to the aniline derivative, which can be further functionalized by amide coupling with acid chlorides [311]. 

Chlorination of 6-(4-chloro-3-methylphenyl) -(3,5-dimethyl-l//-pyrazol-l-yl), 5-dihydropyridazin-3(2//)-one 116 with a mixture of phosphorus pentachloride and phosphorus oxychloride is followed by an elimination of 3,5-dimethyl-177-pyrazole giving the aromatized 3-chloro-6-(4-chloro-3-methylphenyl)pyridazine 117 (Scheme 25) <2005CJC251>. 

The other possible isomer, 1,4-dimethylpyrazole (and its other N-alkyl and -aryl analogues), reacted with chlorine in dichloroethane at 25-35°C to produce 5-chloro derivatives in around 70% yields (90EUP366329). 5-Aryl-3-methylpyrazoles were chlorinated by NCS at C-4 (86JHC459), as were a range of pyrazoles by chloroperoxidase in the presence of hydrogen peroxide and potassium chloride at pH 2.9. Yields of 68-83% make this latter process an improvement over some traditional chemical methods (87JHC1313). 

Synthesis The pyrazole-4-carbaldehyde synthesized according to Vilsmeier is reduced to the alcohol, which is chlorinated. The chloro derivative is reacted with sodium cyanide to give the nitrile, which is hydrolyzed to Lonazolac. The calcium salt, slightly soluble in water, is formed by adding calcium chloride to the free acid (Rainer et al., 1981 Unterhalt, 1982 Rainer et al. (Byk Gulden), 1969 1982 Kleemann et al., 1999). 

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