Synthesis and tautomerism of 2-[3(5)-aryl(methyl)pyrazol-4-yl]-1-benzimidazoles

Abstract

An efficient method has been developed for the synthesis of 2-[3(5)-aryl(methyl)pyrazol-4-yl]-1H-benzimidazoles by cyclocondensation of 2-acylmethyl-1H-benzimidazoles benzoylhydrazones with DMFdimethylacetal. The tautomerism of the compounds obtained via migrations of a proton between the pyrazole nitrogen atoms has been studied by 1 H NMR. The more stable tautomers have electron acceptor aryl substituents placed at position 3 of the pyrazole ring and electron donor aryl substituents or a methyl at position 5.The tautomerism of pyrazole compounds due to the migrations of a proton between the nitrogen ring atoms has been systematically studied [1-3] since it depends unpredictable of the nature and position of the ring substituent. It influences the selectivity of a series of chemical reactions and has to be taken into account when determining the structure and chemical purity of the corresponding substrates by spectroscopic methods. The effect on it of the electronic effects of the two aryl substituents at positions 3 (or 5) and 4 of the heterocycle has not been studied up to this time. It is possible that such a situation is due to the absence of samples and methods suitable for carrying out a reliable investigation. Thus, starting from 2-phenacylbenzimidazole 1 via its formylation product 2 and subsequent reaction with hydrazine, we have obtained the pyrazole 3a which, according to 1 H NMR data exists in solution in equilibrium with its tautomeric form 3'a [4]. The signals of the pyrazole ring aromatic proton for both tautomers are seen separately and would seem suitable and unproblematic for qualitative and quantitative determination of the composition of the equilibrium. However, the hoped for assignment of signals to a specific tautomer is complicated by the lack of a sample for comparison. In addition, the method itself is limited for the preparation of a broad series of compounds for a number of reasons, in particular the poor selectivity in the final stage reaction. Hence we set ourselves the task of overcoming the problem by preparing a series of compounds of type 3 and studying their tautomeric behaviour.Routes for an efficient synthetic method were not obvious and we therefore chose a method of trial and error in the reactions of 2-phenacylbenzimidazole 1 and its hydrazones. Hence we tried a possible synthetic route analogous to the known method of preparing 2-(4-pyrazolyl)benzimidazoles [5] based on the cyclocondensation of compound 1 with aroylhydrazines. However, the corresponding reaction with formylhydrazine occurs to give a mixture of products, from which we isolated only 1-(3-pyrazolyl)-benzimidazole 4. The result of the reaction is unexpected. However, the structure of the product is not in doubt because we had obtained it before by refluxing 2-phenacylbenzimidazole hydrazone 5 in DMF [6].

Recyclization of 2-(3,6-diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles to 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles

“…In form 3'g the conjugated chain is two bonds longer and hence it is energetically less favored. We have previously observed similar prototropic effects in the 1 H NMR spectra of structural analogs of compounds 3a-g -2-(pyrazol-4-yl)-1H-benzi-midazoles which are substituted in the pyrazole ring by one or two aryl groups [3,4]. The 1 H NMR spectrum of salt 4 shows a phenylene fragment typical of benzimidazole salts (a symmetrical picture for the H-5,6 resonance signals and for H-4,7 at lower field, see [2]).…”

A Simple Phosphine–Diolefin‐Promoted Copper‐Catalysed N‐Arylation of Pyrazoles with (Hetero)aromatic Bromides: The Case of Chloroarenes Revisited

“…4‐(3‐Methyl‐1H‐pyrazol‐1‐yl)thiazole ( 25 a ) and 4‐(5‐methyl‐1H‐pyrazol‐1‐yl)thiazole ( 25 b ): The reaction of 3‐trifluoromethylpyrazole (490 mg, 3.6 mmol) and 4‐bromothiazole (0.32 mL, 3.6 mmol) gave two isomers in 1:1 ratio in 49 % yield (380 mg) 20. These products cannot be separated by chromatography.…”