Synthesis of Some Novel Pyrazolo[1,5-a]pyrimidine Derivatives and Their Application as Disperse Dyes

A series of novel monoazo-disperse dyes containing pyrazolo[1,5-a]pyrimidine structures were synthesized starting with the coupling reaction between ethyl cyanoacetate and 4-hydroxybenzenediazonium chloride, followed by treatment of the resulting hydrazone product with hydrazine hydrate. The pyrazolohydrazone 6 is then treated with either 2,4-pentandione and enaminonitrile or aryl-substituted enaminoketones to give the target pyrazolo[1,5-a]pyrimidine dyes 7 and 15a-d. Structural assignments to the dyes were made using NMR spectroscopic methods. A new high temperature method, using microwave heating, was employed to apply these dyes to polyester fibers. Most of the dyed fabrics tested displayed moderate light fastness and excellent washing fastness properties.


Synthesis and Characteristics
Ethyl 2-arylhydrazonocyanoacetate 4 was formed by addition under mild conditions of ethyl cyanoacetate (1) to 4-hydroxybenzenediazonium chloride (2) that readily affords the corresponding hydrazone 3. The existence of this substance in the quininoid form 5 was ruled out based on the results of NOE difference experiments, which showed that irradiation of the two NH signals at 12.1 and 13.0 ppm caused an enhancement of the intensities of the aryl proton resonances at 6.3 and 6.8 ppm, while irradiation of the two OH signals at 9.68 and 9.57 ppm also resulted in an increase in the intensities of the aryl proton peak at 6.8 ppm. Moreover, the 13 C-NMR spectrum of 3 shows only two sets of two sp 3 carbon signals at 61.7 and 61.2, and 14.20 and 14.0 corresponding to the ethyl group. In addition, the data demonstrate that the hydrazone product exists as a 1:2 equilibrium mixture of syn- (3) and anti-forms (4) (Scheme 1). The major isomer is assumed to have the syn-stereochemistry 4 based on the expectation that its NH proton would be deshielded in the 1 H-NMR spectrum as a result of potential hydrogen bonding with the carbonyl ester moiety (cf. Scheme 1). Brecknell et al. [18] was able to isolate and characterize both isomers. The predominance of the anti-form of 2-phenylhydrazonoacetate was attributed to stereoelectronic effects. Recently Al-Awadi et al.
have shown that in similar systems stereoelectronic effects are more important than hydrogen bonding in governing syn-anti ratios [19].
The important features of the HMBC 2-D 1 H, and 13 C signals are shown in Figure 1. H 6 at 7.06 ppm correlates with C 5 and C 7 at 161. 2  Hydroxyphenylhydrazonopyrazolone 6 also reacts with 2-piperidinylacrylonitrile (9) to yield a product for which structures 10-13 are possible based on the absence of an amino-proton resonance in their 1 H-NMR spectra and a cyano band in the IR spectrum. The assignment of structure 10 was made by H-C correlations in the HMBC 2-D experiments ( Figure 2).     only is also observed. The results demonstrate that the phenyl moiety is located at C 7 and not at C 5 .

Dyeing and Fastness Properties
Arylazopyrazolopyrimidine derivatives 6, 7 and 15a-d were tested as dyes for polyester fibers using the high temperature dyeing method (HT) by employing microwave heating as an energy source. The physical and analytical data for the dyed fibers, given in Tables 1 and 2, reflect the efficiency of the microwave irradiation, leading to a large increase in dye uptake and dyeing rate along with a performance of dye leveling and color homogeneity.

Color strength
Data in Table 1 reveals that the color strengths (K/S) of dyed polyester fabrics are directly proportional to the amounts of the dyes applied (% o.m.f.). The hues of the fabrics treated with the azo dyes were found to vary from yellowish-orange to dark orange, based on the substitution patterns. Differences in the color strength observed depend on the substitution patterns, 'auxochromes', present in the arylazopyrazolopyrimidine disperse dyes referred to as 'chromogens' [14,23,24]. Data in Table 1 clearly shows that the magnitude of color strength obtained using dye 15d is much larger than that for 15c. Fastness data, obtained by measuring color intensity changes in the dyed fabrics, are given in Table  2. It shows that wash fastness varies with the substitution patterns in the dyes, where an increase occurs when stronger electron-attracting groups are present on the aryl moieties. This might be attributed to the fact that these disperse dyes are mainly dispersed within the pores in the polyester fabrics and are held in their places by different forces including Van der Waal forces. Electron-attracting groups enable stronger Van der Waal forces and also hydrogen bonding with the dyed fabrics that increases their stability to washing.

Light fastness
The light fastness properties of substituted of fabrics treated with the disperse dyes were determined ( Table 2). The low light fastness properties observed are most likely result the photochemical reactivity of the arylazopyrazolopyrimidine dyes. Indeed, the results of LCMS monitoring of the photolysate, obtained by irradiation of 3-(4-hydroxyphenylazo)-7-phenylpyrazolo[1,5-a]pyrimidin-2-one (14a), under 16 W low pressure mercurey arc-lamp (254 nm) in acetonitrile for 48 h, supported this finding. Elnagdi et al. [25] observed that 3,4-diarylaminopyrazoles are reactive when treated under reflux with AcOH-H 2 SO 4 mixture. Table 2. Fastness properties of azo disperse dyes on polyester fabrics, prepared by using high temperature dyeing method.

GC/MS/MS) and by LC-MS using an Agilent 1100 series LC/MSD with API-ES/APCI ionization mode. Microanalyses were performed on a LECO CH NS-932 Elemental
Analyzer. The microwave oven used is a single mode cavity Explorer Microwave (CEM Corporation, Matthews, NC, USA) and irradiate in heavy-walled Pyrex tube (capacity 10 mL and 80 mL for dyeing). The color strengths (K/S) of the dyed polyester fabrics and the color fastness to light were evaluated at the Dyeing, Printing and Textile Auxiliaries Department, Textile Research Division, National Research Centre, Giza, Egypt.

General Procedure for the Synthesis of azo Disperse Dyes
Ethyl cyano(4-hydroxyphenylhydrazono)acetate (3). p-Aminophenol (10.9 g, 0.1 M) was dissolved in concentrated HCl (30 m) and water cooled in ice (20 mL) and then NaNO 2 (7 g) in water (50 mL) was added in portions. A mixture of ethyl cyanoacetate (10 g, 0.1 M), NaOAc (20 g), ethanol (15 mL) and water (50 mL) was prepared separately and cooled in ice. The diazonium salt solution was added slowly to the second solution, with ice cooling. The cooled mixture was stirred for 0.5 h and filtered to give brown crystals, which were crystallized from alcohol/water to yield 2.  (6). A mixture of 3 (2.33 g, 10 mmol), hydrazine hydrate (2.5 mL) in ethanol (20 mol) was stirred at reflux for 3-4 h. The solvent was removed under vacuum and the formed solid was collected and crystallized from ethanol/water to give 6. Red brown crystals from alcohol, yield 2. General procedure for the synthesis of pyrazolo[1,5-a]pyrimidines 7, 10 and 15a-d A mixture containing 6 (0.22 g, 10 mmol), and acetylacetone, 2-piperidinylacrylonitrile or enaminones 5a-d (1 mmol) in acetic acid (5 mmol) was irradiated in a microwave oven at 140 °C for 2 min. The mixture was then poured into ice water (50 mL). The formed was collected and crystallized from the appropriate solvent (see below).  Scoured and bleached polyester 100% (150 130 g/m 2 , 70/2 denier) was obtained from El-Shourbagy Co., Egypt. The fabric was treated before dyeing with a solution containing non-ionic detergent (Hostapal CV, Clariant-Egypt, 5 g/L) and sodium carbonate (2 g/L) in a ratio of 50:1 at 60 °C for 30 min, then thoroughly washed with water and air dried at room temperature.

Dyeing
Dyeing of polyester fabrics was carried out at 130 °C for 60 min, under pressure in a microwave oven in a 20:1 liquor ratio and pH 5.5 in the presence of a 1:1 ratio of the dispersing agent sodium lignin sulphonate and the with a 1-4% shade. After dyeing, the fabrics were thoroughly washed and then subjected to a surface reduction cleaning [(5 g NaOH + 6 g sodium hydrosulphite)/L]. The samples were heated in this solution for 10 min. at 60 °C and then thoroughly washed and air-dried. where R = decimal fraction of the reflectance of the dyed fabric; R o = decimal fraction of the reflectance of the undyed fabric; K = absorption coefficient; S = scattering coefficient.

Fastness testing
After washing using 2 g/L of the non-ionic detergent Hostapal CV at 80 °C for 15 min, the dyed fabrics were tested, employing ISO standard methods [26]. Wash fastness tests were carried out in accordance with ISO 105-C04 (1989), in which 5 g/L soap and 2 g/L soda ash solution were used at 95 °C for 30 min in the presence of 10 steel balls (liquor ratio 50:1) and color fastness to light (carbon arc), ISO 105-B02 (1988).

Conclusions
In summary, a series of novel monoazo disperse pyrazolopyrimidine dyes were synthesized via a sequence involving initial coupling of ethyl cyanoacetate with 4-hydroxybenzenediazonium chlorides. Subsequent treatment with hydrazine hydrate gave the corresponding pyrazolohydrazone that was then treated with either 2,5-pentandione or arylenaminoketones to give the target pyrazolo[1,5-a]pyrimidine dyes. The dyes produced in this manner were then applied to polyester fibers by using HT dyeing conditions and microwave heating. The dyed fabrics, which displayed yellow to yellow brown hues on polyester fibers, have low fastness levels to light and excellent wash fastness.