5(4)-Chloro-1-(2,4-dinitrophenyl)pyrazoles from 2,4-Dinitrophenylhydrazones of Chlorovinyl Ketones and β,β-Dichloroacrolein

Abstract

A method has been developed for obtaining 3-alkyl(phenyl)-4(5)-chloro-1- (2,4-dinitrophenyl)pyrazoles from appropriate dinitrophenylhydrazones of 1-chloro-, 1,2-, and 2,2-dichlorovinyl ketones by heating the latter in polyphosphoric acid. The structure of the pyrazoles was studied by IR and 1 H NMR spectroscopy. pyrazoles are promising for making medicinal preparations, dyestuffs, insecticides, insectoacaricides, etc. [1-8], and are formed in reactions of the corresponding 1,2-dichloro-and 2,2-dihalovinyl ketones with alkylhydrazines in the presence of base [9][10][11][12]. An extensive series of 3-alkyl-1-methylpyrazoles and 5-chloro(bromo)-1-methylpyrazoles was obtained by us using new one-stage selective heterocyclization reactions of 2-chloro-and 2,2-dichloro(bromo)vinyl ketones with unsymmetrical dimethylhydrazine [13][14][15].It proved to be possible to synthesize 4(5)-chloro-1-phenylpyrazoles or 1-phenylpyrazoles unsubstituted in positions 4 and 5 by the intramolecular heterocyclization of previously obtained phenylhydrazones of 1-chlorovinylalkyl(aryl)-and 2,2-dichlorovinyl phenyl ketones [9,16]. At the same time, only in the one example of 5-chloro-1-(2,4-dinitrophenyl)-3-phenylpyrazole [17] has the preparation been reported of 1-nitrophenyl-substituted halopyrazoles by the thermal cyclization of the corresponding arylhydrazones of 2,2-dichlorovinyl phenyl ketone. It was shown simultaneously that 2,4-dinitrophenylhydrazones (DNPH) of aliphatic 2,2-dichlorovinyl ketones are not cyclized on thermolysis into the corresponding 5-chloropyrazoles. In a systematic study of the structure of DNPH of dichlorovinyl ketones [17,18] the determining role of the configuration of the hydrazone was established unequivocally. The DNPH of aliphatic ketones exist preferably in the s-cis-anti form, but the DNPH of aromatic ketones exist in the s-trans-syn form, capable of achieving intramolecular heterocyclization. The energy barrier for the transition of one form into the other is fairly high [17], but as was established in [18] the process of syn-anti conversion of geometric isomers of DNPH is catalyzed by acids.In view of the above we have developed a convenient method of obtaining 1-(2,4-dinitrophenyl)-pyrazoles by the thermal heterocyclization of the corresponding 2,4-dinitrophenylhydrazones of 2-chlorovinyl, 1,2-, and 2,2-dichlorovinyl ketones, and of 2,2-dichloroacrolein in an acidic medium. We have established that __________________________________________________________________________________________ A. E.

The Structure ofN‐phenyl‐pyrazoles and Indazoles: Mononitro, Dinitro, and Trinitro Derivatives

“…There are three reviews where 13 C NMR data of some compounds studied in this work are reported . A large series of 1 H chemical shifts of 1‐(2,4‐dinitrophenyl)‐1 H ‐pyrazoles was described by Larina et al while 1 H, 13 C, 15 N NMR in solution and in the solid state were recorded for 36 and 56 . Holzer used NOE difference spectroscopy for differentiating isomeric pairs of 1,3 and 1,5‐disubstituted pyrazoles, by means of through‐space connections between a pyrazole H‐5 proton and protons of the N‐1 substituent .…”

5-Chloropyrazole-4-carboxaldehydes as synthon in heterocyclic synthesis

“…Zhong et al [24] have prepared compound 4 via Vilsmeier reaction of 3a with bis(trichloromethyl)carbonate 8 in DMF instead of the traditional POCl 3 -DMF system (Scheme 1). Moreover, POCl 3 -DMF over silica gel has been used for the synthesis of 4 using solvent-free conditions and microwave irradiation [25][26][27][28]. Reduction of 1-phenyl-5-chloro-3-methyl-pyrazole-4-carboxylic acid chloride (9) with Pd-BaSO 4 in boiling xylene, afforded 4.…”

“…A direct method for the preparation of 1-methyl(aryl)-3-alkyl(aryl)-5-chloro(bromo)pyrazoles has been developed [26,27], but up to now it was impossible to obtain products of their formylation in isolated yield via Vilsmeier-Haack reaction [28]. The formation of 4 can be explained according to the reaction mechanism (Fig.…”