SYNTHESISAND SPECTROPHOTOMETRIC STUDIES OF SOME NEW COUMARINDERIVATIVES

Fekria m.a. Soliman 1 , zeinab h. Ismail 1 , nahed f. Abd el-ghaffar 1 , nadia t.a. Dawood 1 and shaimaah. Abd el monem 2 . 1. Chemistry Department , Faculty of Science, Al-Azhar University (Girls’), Nasr City, Cairo, Egypt. 2. Department of Forgery and Counterfeiting Research, Forensic Medicine, Ministry of Justice, Cairo, Egypt. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 12 September 2018 Final Accepted: 14 October 2018 Published: November 2018

The current study was designed to investigate the reactivity of 3-acetyl-6-bromocoumarintowards a variety of electrophilic and nucleophilic reagents such as aromatic aldehydes to give 3-cinnamoyl cniramuos 2a,b. The reaction of2b with guanidine hydrochloride gave 6-bromo-3-(substituted pyrimidine)coumarin-3-yl 3. Other6-bromo-3-(substituted pyrimidine) coumarin-3-ylderivatives 5a and 5b were synthesized from treatment of 1 with thiosemicarbazide then treating the product 4 with active methylene compounds namely diethyl malonate and/or ethyl acetoacetate. Treating 4 with phenacyl bromide yielded the corresponding phenyl thiazole derivative 7. Alkylation of 4 with acetic anhydride and/or benzoyl chloride yielded the corresponding diacetyland/or di benzoyl derivatives6a,b. The reaction of 4 with ethyl chloroacetate and/or chloro acetic acid in different media yielded two isomers 8a and 8b. The reaction of 1 with nitriles such as malononitrile and/or ethyl cyanoacetatefollowed by treatment with sulfur gave corresponding coumarin -3-y-thiophene derivatives 10a,b. The structures of the newly synthesized derivatives were elucidated by means of microanalysis, IR, 1 H-NMR, 13 C-NMR and MS measurments. The UV-Vis absorption spectra of the various newly 3-substituted-6bromo coumarin derivatives 1-10were investigated in expectation of the shift to the longer wavelength region by the extension of the conjugated system. The absorption properties of the prepared compounds were investigated spectrophotometrically by dissolving in chloroform in the range 200-800 nm using 1cm quartz cells, showed good UV absorption properties and were discussed from the view point of the intermolecular charge transfer (ICT) between push-and pullsubstituents.

Reaction of 3-acetyl-6-bromocoumarin (1) with active nitrile compounds. Formation of 9a, 9b
To a dry solid of coumarin derivative1 (0.01mol.), either malononitrile (0.01mol.) or ethyl cyanoacetate (0.01mol.) was added followed by ammonium acetate (0.03mol.). The whole reaction mixture was heated on a boiling waterbath for 8h then left to cool. The formed product was triturated with diethyl ether and the solid that formed was collected by filtration, washed well with water and dilute alcohol and recrystallized from the proper solvents as 9a and 9b.

General procedure for the synthesis of thiophene derivatives 10a,b
To a solution of either 9a (0.01mol.) or 9b (0.01mol.) in 1,4-dioxane (30ml) containing the catalytic amount of triethyl amine (1ml, 0.01mol.) was added sulfur (0.32g, 0.01mol.) and the reaction mixture was heated under reflux for 30 min. then left to cool at room temperature overnight. It was then poured onto ice/water, stirred well and the solid product formed was collected,washed well with water then dilute alcohol and recrystallized from the proper solvents as 10a and 10b.
The relevant absorption data in chloroform are summarized in Table 2 compounds (5a, 5b, 7, 8a,  10b). While for the values of the C=O at position-2 in the benzopyrone ring, its values were around 159.7 to 159.9 ppm, while the value of C=O for compound 10b showed a remarkable shift to the downfield with a value of 161.3 ppm. Similarly, for the substituent C-Br at the position -6 of the benzopyrone ring it showed a values for the ester (C=O)around 120.1-120.3. It is clear from the UV absorption results at Table 2 for compounds 2a, 3, 4, 5b, 6b, 7, 8b, 9a, 9b ,10a and 10b. We tried to correlate the UV absorption of each compound with the 3D-geometrical structure and the distribution of the electronegativity on theatoms for all the tested compounds. Thus, the absorption maxima (λmax), Fig. 1, its 3D-geometrical structure, Fig. 2the electronic charges of compound 1, Fig. 3and the atomic charges are listed in Table 3. Although the Hammett substituent constants are commonly used for the estimation of reactivity and there is no theoretical connection between σ p constants and absorbance, these parameters appear to be suitable and easily available to represent the total electronic effects in the ground state because of the absorption of coumarins based upon the typical ICT between push-and pull-substituted coumarins correlated to the Hammett constants. As shown in       The absorption spectra of the three different pyrimidine derivatives 3, 5a and 5bin chloroform are shown in Fig. 7; their 3D-geometrical structure are shown in Figs. 8a-c respectively; their atomic charge values are listed in Tables 5a-c and the distribution of the atomic charges on the atoms are shown in Fig. 9a-c respectively. As shown in Fig. 7, the differences in substituents in the 3-position of coumarin and in the pyrimidine itself affected the absorbance properties of these derivatives. The shape of the absorption bands of 5b around 300 and 370 nm is markedly different from that of 3 and 5a. Compound 5b with an oxo-, and a methyl group at 4-, and 6-position in the pyrimidine ring. Has also a small shoulder band at 280 nm and another shoulder at 365 nm, whereas compound3 has a small shoulder at 312 nm and two splitting absorption bands at 315 and 350 nm; and compound 5a had two splitting absorption bands at 299 and 360 nm respectively, as shown in Fig. 7. Such differences in the substituents wheather for the coumarin ring or for the pyrimidine ring were frequently observed in aromatic heterocyclic rings.

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The λmax values of the pyrimidine derivatives shifted to longer wavelengths in the order 5b>5a>3 with decreasing absorbance.    We examined in detail the absorption characteristic of thecoumarin derivative(4) from the viewpoint of the substituent effect in connection with the ICT in the coumarin skeleton and postulated that the ICT from an eletrondonating at 6-position to an electron-withdrawing lactone carbonyl group at the 2-position as well as the introduction of a conjugated system into the coumarin skeleton at position 3 would make it possible to absorb the UV in a different wavelength region. As shown in Table 2 three absorption peaks at λmax 275, 314 and 348 nm as shown in Fig. 10. The 3D-geometrical structure of 4, Fig. 11; the electronic charges on the atoms, Fig. 12 and the atomic charges in Table 6 explain the effect of the ICT to shift the absorption shift to the given values in Table 2. Fig.12:-The electronic charges of compound 4 It is clear from the above structure of the coumarin derivative (4) that the introduction of such conjugated system into the coumarin skeleton would make it possible to influence the absorption of such compound.  The absorption maxima (λmax) as listed in Table 2 agreed well with 13 C-NMR as the presence of long chain conjugated system in coumarin skeleton tends to shift the absorption wavelength region. Thus, the λmax value of 6a showe the values 298 and 360 nm as a result of the presence of an electron-withdrawing to acetyl groups in the side chain. As shown in the absorption spectrum of 6b, it showed a shift of the λmax to a lower value of 281 nm and abroad peak. As a result of the substituent effect in connection with the ICT in the coumarin skeleton with one from the electron-donating group at the 6-position and the long conjugated electron-withdrawing group at the 3-position which would make it possible to shift the absorption properties. It should be also noted that the annulation possibly affected the absorption properties of the coumarin derivative (6b).   Since the absorption of coumarinof the UV radiation is related to the ICT between push-and pull-substituents in the molecule. As the introduction of conjugated systems into the coumarin skeleton affects the absorption and would make it possible to absorb in a different wavelength region, it is clear that coumarin derivative 7 showed two λmax 275 and 350 nm which is quite different from the absorption of the coumarin derivative (1), this might be due to the introduction of a heterocyclic moiety with a phenyl ring to the conjugated chain at position-3 in the coumarin skeleton and would make it possible to shift the λmaxto a lower or higher value.

Wavelength (nm)
The ICT from the electron-donating group at position-6 to the electron-withdrawing at position-3 and the electronwithdrawing lactone carbonyl group at the 2-position contributed to the absorbance intensity.  0.593585 The structural isomerism in compounds8a and 8b causes a slight difference in the ICT which in turn causes a slight shift in the absorption values and intensely. Thus, the absorption maxima of 8a (λmax) showed values of 298 and at 355 nm, while the λmax of 8b showed slight shift to a higher value 298 and shoulder at 355 nm respectively. The 13 C-NMR chemical shifts of 8a and 8b (Table 1)correlated to this shift in the absorption values. The decrease in the value of λmax for compound (8a) from (8b) may be attributed to the structure -N-CH 2 -CONH in 8a whereasthe structure at (8b)is -N-CO-CH 2 NH, thus this difference in the arrangement might disturb the push-pull π-electron system.
The absorption spectra of the two isomers 8a and 8b in chloroform are shown in Fig. 19 and their values are summarized in Table 2. As predicated the arrangement of the atoms in the imidazolinone-2-thioxo affected the value and shape of absorption of 8a from 8b. The shape of absorption band of 8b from around 298 to 355 nm is markedly different from that of 8a. Compound 8b with arrangement of the atoms is -N-CO-CH 2 -NHC=S had three peaks and two small shoulder bands at around 250-310 nm, whereas compound 8a had two splitting absorption bands at 296 and 355 nm as shown in Fig. 19 and in which the arrangement of the atoms is N-CH 2 -CO-NH-C=S. This difference in the shape and value of absorption may be due to this quite different arrangement.   We examined in detail the 3D-geometrical structures of each of compounds 8a and 8b (Fig.20a, 20b)  We found that for each corresponding atom in these two isomers, the value of the charge atom (Hückel) is quite different. This difference in that parameterappears to be suitable to explain the difference in the ICT as well as the push-pull in each of the two isomers, which is in agreement with differences in the 13 C-chemical shifts for each isomer (Table 1). The absorption spectrum of 3-substituted coumarin9a differed markedly from the absorption spectrum of 1 .A drastic decrease in the value of λmax of the former from that of 1. The comparison of the two substituents, which contain an electron-withdrawing groups at the 3-position i.e. cyanoethylidene for 9a and an acetyl for 1.The prominent decrease in 9a of λmax value 248 nm is 51 nm may be attributed to the presence of two cyano groups at the substituent in position-3, where the push-pull π-electron system and the bromine at 6-position must cause such distinct difference in the absorption spectrum.

Wavelength (nm)
(9a) (9b) Fig. 23aFig. 23b Figs.23a,b: The 3D-geometrical structure of compounds 9a and 9b There is also a quiet difference in the value of λmax for 9a which showed a value of 248 nm in the shape of a boarded band, while the absorption maxima for 9b showed λmax at 288 and 355 nm. This difference in the value of the shape and absorption is in agreement with the electronic charges (Figs. 24a, 24b) and the atomic charges (Hückel) (Tables 10a, 10b).  The absorption spectrum of 10a differed markedly as compared to 3-substituted coumarin1. The λmax value of 10a was shifted to a shorter wavelength with a value of250 nm, i.e. it shifted to a shorter wavelength region by a value of 49 nm. This prominent decrease may be attributed to the increase in the electron-withdrawing ability at the 3position which confirms that the ICT from the bromine at the 6-position to a substituent such as thiophene moiety containing a cyano group at the 3-position of the coumarin skeleton greatly contributes to the absorption wavelength. In contrast, the absorption spectrum of coumarin derivative10bdiffered markedly with a drastic decrease in itsλmax for it was 260 and 338nm. This decrease in absorption by 49 nm for 10aand of 39, 16 nm for 10b may be attributed to the non-equivalent push-pull π-electron systems between a bromo atom at the 6-position and a thiophene moiety containing a cyano/or ethoxy carbonyl substituents, attached to position-3 of the coumarin skeleton.