Key Intermediates : A Simple and Highly Selective Synthesis of 5-Amino-1-aryl-1 H-pyrazole-4-carbonitriles for Applications in the Crop Protection

A series of six pyrazoles was synthesized by Michael-type addition reaction. The molecules 5amino-1-aryl-1H-pyrazole-4-carbonitrile (3a-f) were synthesized from (ethoxymethylene)malononitrile (1) and fluorinated and non-fluorinated aryl hydrazines (2a-f) using ethanol and fluorinated ethanol as solvents at reflux. An excellent regio-selectivity was found when pyrazole derivatives were formed as an exclusive product. No other regioisomer or uncyclised hydrazide was observed. Their structures were confirmed by spectroscopy data (1H, 13C, 19F, COSY (correlation spectroscopy), HSQC (heteronuclear single-quantum correlation spectroscopy) and HMBC (heteronuclear multiple-bond correlation spectroscopy); MS (mass-spectrometry). The yields ranged from good to excellent (47% 93%) under mild reaction conditions. It would indicate a high selectivity in the one-step work procedure. These products (3a-f) and derivatives have a potential academic and industrial use as key intermediates, in special, for application in crop protection.


Introduction
Heterocyclic compounds containing nitrogen have played a major role in modern pesticide industry; it has been reported that more than 85% of pesticides with high activity and low toxicity contain nitrogen heterocyclic compounds [1].Pyrazoles are important heterocyclic compounds.They are also useful synthons and building blocks for many heterocyclic products and can act as a binucleophile [2] with a broad spectrum of remarkable biological activities.Many derivatives containing pyrazole nucleus have been commercialized as herbicides, insecticides, and fungicides for plant protection [3].
Several compounds of this type can be directly applied as pest-control agents, in particular as acaricides and nematicides [4]- [6], whereas others can be used for the treatment of bacterial, inflammatory diseases and some cancers [4].As they have proven to be useful intermediates for the preparation of new biological materials, the synthesis and applications of alkyl and aryl pyrazoles have been widely studied in the past few years [7]- [9].
Pyrazole compounds prepared by chemists and biologists have gained widespread attention as they have become fairly accessible and show diverse properties [10]- [12].These compounds, whose structure include pyrazole rings, have been used as herbicides to inhibit germination in some weeds and as fungicides agents, because combined with zymoproteins, they can activate the apoptosis proteins and cause the fungi to die [13] [14].
Experiments on aryl pyrazole insecticides began in 1985 [17] [18] and led to the development of Regent ® (BASF), a Fipronil-based insecticide widely used [12] [16] [19].Fipronil is the first phenyl pyrazole insecticide introduced in the market [20].This aryl pyrazole is highly effective against a significant range of economically important insect pests, and it has become a cornerstone in insect control programs in both non-crop and crop pest control situations in many areas of the world [21].This and other pyrazole derivatives have reached development status as crop insecticides (Figure 1).
In previous papers, our group reported the synthesis of a series of fluorinated pyrazole-5-carboxamide derivatives structurally related to the commercial acaricide Tebufenpyrad (Figure 1), in ethanol and fluorinated alcohols as solvents.Specifically, the use of trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) as solvents confirmed the influence of the fluorinated alcohols on the regioselectivity of the reaction [6].With these precedents, it was decided to explore the reaction between (ethoxymethylene)malononitrile (1) and fluorinated and non-fluorinated aryl hydrazines (2a-f) to produce a series of 5-amino-1-aryl-1H-pyrazole-4-carbonitriles to find compounds with high regioselectivity and without the presence of isomers in order to develop new pyrazole derivatives with potential use in crop protection.

General Methods
All the reagents were commercially available and used as received from the supplier.The solvents were analyt- iccal-grade and were purified according to standard methods.The purity and chemical structure of the synthesized compounds were checked by TLC, CG, UPLC-MS, HRMS (in case 2b and 2e), NMR spectra, and melting point.
Analytical TLC was performed on a Merck precoated TLC plate (Silica Gel 60 F 254 ).Gas chromathographic (GC) analyses were performed on a DANI Master GC chromatograph equipped with a 5% diphenyl, 95% dimethylpolysiloxane, low bleed capillary column (30 m × 0.53 mm, 0.5 µm film thickness) and a flame ionization detector.Column Chromatography was performed on silica gel (70 -230 mesh ASTM).High-purity grade, pore size 60 Å.Isolated and authentic compounds were used as internal standards to perform quantitative GC analyses.Ultra performance liquid chromatography and mass-spectrometry (UPLC-MS) analyses were performed on a H-CLASS SQD2 Detector (Waters).HRMS were obtained on a Bruker micro QTOF-Q11 mass spectrometer equipped with an electrospray ionization (ESI). 1 H NMR (300 MHz), 13 C NMR (75 MHz). 19F NMR (300 MHz), COSY (Correlation spectroscopy), HSQC (heteronuclear single-quantum correlation spectroscopy) and HMBC (heteronuclear multiple-bond correlation spectroscopy) experiments were recorded on a Bruker Avance 300 spectrometer in CDCl 3 using TMS as internal standard.Coupling constants are given in Hz and chemical shifts are reported in δ values in ppm.Data are reported as follows: chemical shift, multiplicity (s = singlet, s br = broad singlet, d = doublet, t = triplet, m = multiplet), coupling constants (Hz), and integration.Melting points (Mp) were recorded on a Büchi b-540 micro melting point apparatus and were uncorrected.
For aryl hydrazines hydrochlorides 2b, 2f (1.2 mmol), a previous step of neutralization with Et 3 N (1.0 mmol) at 0˚C in ethanol (2 mL) was needed.Then, (ethoxymethylene)malononitrile (1) was added as described in the methodology above.In this case, the reaction crude was diluted with ethyl acetate (50 mL) and washed with water (30 mL).The organic phase obtained was dried over Na 2 SO 4 , filtered, and the organic solvent was evaporated under reduced pressure.The workup and purification of the crude product was identical with that described above.

Selection of the Solvent
Initially, it was showen the effectiveness of TFE as a solvent in the preparation of 5-amino-1-phenyl-1H-pyrazole-4-carbonitrile 3a.(Ethoxymethylene)malononitrile 1 was treated with phenylhydrazine 2a at reflux in various solvents (TFE, EtOH, MeOH and THF) (Table 1, entries 1-4).As it is clear from this table, the highest yields of the desired pyrazole were obtained with TFE and ethanol as solvents (Table 1, entries 1 and 2).The reaction in an aprotic solvent, such as THF (Table 1, entry 3), occured quite slowly after 30 min and a low yield of the product was obtained.When methanol was used as a protic solvent (Table 1, entry 4) the yield of the product increased but the reaction did not improve as significantly as when EtOH or TFE were used.
In order to show the generalities of this reaction, different substituted pyrazoles were synthesized using a variety of aryl hydrazines in TFE as a solvent, chosen for its great yield in the first assays (Table 2).The substituted aryl hydrazines reacted smoothly under the same reaction conditions to give of moderate to low yield of the desired pyrazoles (Table 2, entries 2 -5).The highest yield was achieved for 3a in TFE after 30 min (Table 2, entry 1), butit was not possible to reproduce this performance for the other aryl hydrazines employed.
The low to moderate yields obtained from TFE in the synthesis of different aryl pyrazoles were compared with the results obtained from the synthesis carried out under the same reaction conditions with EtOH as a solvent (Table 3).It was then decided to continue the synthesis with EtOH as a solvent, as it showed yields ranging from good to excellent.Besides, EtOH is cheaper than TFE, is commercially available, and it is also eco-friendly because it is obtained from a renewable source.

Synthesis of Aryl Pyrazoles
The synthesis of the compounds 3a-f was accomplished in a one-step reaction.Yields ranged from good to excellent (46% -93%), though no attempt was made to optimize the conditions for each reaction.
A detailed study of different aryl hidrazines was performed toward the formation of aryl pyrazole derivatives (Table 3, entries 1-6).To study the influence of stereo and electronic effects between the hydrazines 2a-f and the substrate 1 toward the formation of 3a-f, fluorinated and non-fluorinated phenyl groups were chosen as Ar substituents.At the same time, the fully unsubstituted hydrazine ring, phenylhydrazine 2a, was also considered as the model substrate to identify the method of choice for the synthesis of aryl pyrazoles.

Mechanism of the Reaction and Selectivity in the Addition
After reviewing the literature, a plausible mechanism for the formation of the 5-amino-1-aryl-1H-pyrazole-4carbonitriles (3a-f) may be explained by the initial Michael type addition [22].First, the most nucleophilic amino group is added selectively to the β-carbon of 1, resulting in the intermediate hydrazide 4. The latter, by elimination, and rearranges to give alkylidene hydrazide 5 and ethanol 6 as a leaving group.The intermediate 5 undergoes intramolecular cycloaddition by the nucleophilic attack of the second amine on the nitrile carbon and gives the pyrazole imine 7. This, consequent upon aromatization leads to the 5-amino-1-aryl-1H-pyrazole-4carbonitriles (3a-f), as depicted in Scheme 2.

Conclusion
This study has clearly demonstrated that 5-amino-1-aryl-1H-pyrazole-4-carbonitriles can be synthesized and used as predecessors-key intermediates-of a wide variety of heterocycles for academic interest and agrochemical applications.We have shown that it is possible to synthesize a series of six pyrazoles by a one-step reaction involving the refluxing of the reactants in ethanol as the most suitable solvent.A plausible mechanism was proposed to explain the regioselectivity observed for the reaction.In general, the yields of pyrazoles obtained under the experimental conditions with aromatic hydrazines and (ethoxymethylene)malononitrile ranged from good to excellent, with a simple and efficient methodology.The aryl pyrazoles 3a-f were obtained and characterized by NMR and MS spectroscopy.It is worth mentioning that the functional groups (i.e.cyano, amine and fluor) in the pyrazole moiety could have interesting applications in the crop protection.Currently, we are working with these key intermediaries to the development of new agrochemicals.

Scheme 2 .
Scheme 2.A plausible mechanism for the formation of 3a-f.
a. Isolated yields.b.Quantified by GC (internal standard method).