Indi an Journal of Chemistry Vol. 428, February 2003 , pp. 363-368

Synthesis, characterization and electrochemical behaviour of some substituted 3-arylazo-8-aldehydo-4-methylcoumarins at dropping mercury and

glassy carbon electrodes

Ashok Kumar*, Pratibha Sharma & Ravi Sharma

School of Chemical Sciences, Devi Ahilya University, Indore (M.P.) 452 017, Indi a Email: drashokshanna200 I @yahoo.com

Received 20 March 2001; accepted (revised) 8 March 2002

A series of 3-arylazo-8-aldehydo-4-methylcoumarins have been sy nthesized in excellent yields (80 - 90%) and their structures establi shed on the basis of IR , IH NMR and e lemental analysis. Their purity has been ascertained by chroma­tographic resolution using acetic ac id-to luene (6:4, v/v) as eluent. The electrochemical reduction of 3-arylazo-8-aldehydo-4-methylcoumarins have been studied over a wide pH range at dropping mercury and glassy carbon e lectrodes. All the com­pounds found to ex hibit wel l-defined, diffusion-controlled irreversible wave. They give two-electron wave corresponding to the reduction of azo group . On the basis of cyclic vol!ammetry. coulometry, spectrophotometry , number of protons invol ved in the rate-determining step, the number of electrons in the reduction and product identification, a plausible reduction mechanism is suggested. Kinetic para mete rs, i.e. charge-transfer coeffic ient (an) and forward rate constant (K\h) have also been calculated. The pK values are calculated by pol arographic and spectrophotometric method.

Coumarin deri vatives have been found to ex hibit re­markable acti vit ies such as fluorescent dyes I, CNS depressants2

, antitumor agents3, HIV proli ferator4 and as powerful anticoagulants5

. A systematic perusal of earlier literature, reveals that inspite of pharmaceuti­cal importance associated with ary lazo compounds, relatively few reports6

.9 ex ist on the electrochemical

behaviour of azo compounds having heterocyclic moiety attached at one of the ends of the azo group. Hence, it was thought worthwhile to undertake sys­tematic and comprehensive electrochemical studies on some synthesized 3-arylazo-8-aldehydo-4-methylcou­marins with a view (i) to decide about the rate of elec­tl'o-reduction process, (ii ) to elucidate the mechanism of electro-reduction, and (iii ) to find out the effect of various experimental conditions.

Experimental Section All the chemicals used were of the AR grade. Stock

solutions( 1 x 1 0'3 M) of all the substituted 3-ary lazo-8-aldehydo-4-methyl coumarins were prepared in OMF. Tetrabutyl ammonium bromide (TBAB) (0.01 M) was used as the supporting electrolyte. Britton-Robinson 10

buffers in the pH range 2.5 - 11 .5 were used to per­form studies at various pH values. Test aliquots were prepared by taking 1 mL stock solution mixed with 2 mL of OMF, 1 mL of TBAB and 6 mL of BR buffer solution of desired pH value. The solutions were

deaerated by purging with a stream of nitrogen gas for 15 minutes before recording the current-voltage curve. Cyclic voltammetric (CV) studies were carried out on computerized VSM/EC/30-S potentiostat and polarograms were recorded on Toshniwal digital pola-

h Th '11 h .. (2/3 1/6 ) rograp . e capl ary c aractenstlcs m twas 1.96 m g2/3s1/2 in 1.0 M TBAB . Saturated calomel and sil ver/sliver chloride electrodes were used as refer­ence electrodes, whereas, dropping mercury and glassy carbon electrodes served the purpose of work­ing electrode for DC polarography (OCP) and cyclic voltammetric (CV) studies, respectively .

Melting points were recorded on an electrically heated block and are uncorrected . IR spectra (VITIa< in cm'l) were recorded on a Perkin-Elmer Ff-IR paragon 1000 spectrophotometer in KBr di sc. I H NMR spectra were recorded in OMSO - d6 on a ORX 300 MHz spectrometer using TMS as an inter­nal standard (chemical shifts in 8, ppm).

Preparation of 3-arylazo-8-aldehydo-4-methyl­coumarins A series of various hydrazono product coupled with an appropriate active methylene com­pound have been synthesized using literature procedure". In a one liter three-necked bolt head R.B. fl ask, fitted with a thermometer, mechanical stirrer and dropping funnel, 10.0 mL of conc. H2S04 was taken and placed in an ice bath . After the temperature, had fallen to 10°C, 9.73 mL (0.091 mole) of salicylal-

364 INDIAN J. CHEM., SEC B, FEBRUARY 2003

dehyde and 2-arylhydrazono-l-ethoxy-l,3-butanone (9- 16) (0.0 1 mole) were added maintaining the tem­perature below lODe. After the addition is complete, the reaction vessel was kept at room temperature for 18 hr. To this approximately 50 g of crushed ice was added with vigorous stirring. The precipitates thus formed, were collected by filtration under suction and washed thrice with 5% sodium hydroxide solution (3 x 5 mL). To the filtrate 2M H2S04 (10 mL) was added slowly with vigorous stirring until the solution was just acidic to litmus, filtered at the pump, and thor­oughly washed with ice-cold water (4xlO mL). Both the crops of crude coumarin were collected and re­crystallized from a mixture of acetic ac id and toluene (6:4, v/v). Physical characteristics of all the synthe­sized products (Scheme I) are given in Table I .

Structures of all the compounds were established on the basis of elemental analysis (Table I) , IR and 'H NMR spectral data and are reported in Table II.

Electrochemical studies In a typical polarographic experiments, e.g., when

studying the influence of pH on the polarographic behaviour, a 0.5xlO,3 M solution of the depolari zer was prepared by mixing appropriate amounts of di­methyl formam ide (4.0 mL) which was necessary to prevent precipitation, the appropriate buffer, tetrabu­tyl ammonium bromide (Bu4N+Br) (2 mL) as the sup­porting electrolyte. The temperature coefficient was determined by the method of the Meites ' 2 and found below 1.6% deg". To determine the number of elec­trons involved in the reduction it was compared with those for azobenzene in B.R. buffer by the method of DeVries and Kroon' 3 using mercury pool cathode. The formula used was -

_ [t1id aZObenzene] [ id SUbstance] VIC, n2- n, --id azobenzene t1id substance V2C 2

where, id azobenzene is the original diffusion current for azobenzene, V I. C, and t1id are its volume, concen­tration and change in the diffusion current after elec­trolysis, id(substance) , V 2, C2 and t1id (substance) are the corresponding quantities for the electroactive spe­cies under investigation. Moreover, the value of ' n' was also determined by I ::: 1.67 n and in both cases the value was n ::: 2.

In order to perform cyclic voltammetric studies, the GC was polished with fine-grade emery paper fo l­lowed by polishing alumina (0.5 11m), washed and activated by triangular voltage sweeps from + 1.0 to -

CHO

j~ EnoL

CH3

Cone. H2S04

-C2HsOH

- H20

17 - 24

Where, Compd No R

1,9, 17 ,H

2, 10, 18 4-N02

3,11,19 4-0CHJ 4, 12, 20 3-CHJ 5, 13, 21 4-Br

6, 14, 22 4-CI

7,15, 23 3-Cl

8,16, 24 3-N02

Scheme I

2.0 V at the rate of 10 and 200 m V sec" for ferri­cynide/ferrocynide system in 0.1 M KCI'4. The elec­trode area of GC was evaluated (0.012 cm2

) from

KUMAR et al. : SYNTHESIS OF COUMARINS 365

Table I -Characterization data of compounds 9-24

Compd m.p. Yield Found (Caled) (%)

°c (%) C H N

9 86 78 61.10 (61.53 5.605.98 11.20 11.96)

10 122 75 51.59 (51.61 4.354.65 14.95 15.05)

11 89 76 59.00 (59.09 6.00 6.06 10.10 10.60)

12 88 80 69.32 (69.90 6.206.45 11.11 11.29)

13 98 78 45 .90 (46.00 4.104.15 8.89 8.94)

14 88 80 53.69 (53.73 4.504.85 10.20 10.44)

15 84 75 53.69 (53 .73 4.504.85 10.2010.44)

16 120 75 51.59 (51.61 4.354.65 14.95 15.05)

17 115 70 69.65 (69.86 4.024.10 9.72 9.58)

18 112 65 59.90 (60.50 3.113.26 12.21 12.46)

19 120 70 67.00 (70.50 4.304.34 8.608.69)

20 92 80 70.50 (70.58 4.504.57 9.009.15)

21 108 82 54.90 (54.98 2.902.96 7.507 .54)

22 78 75 62.52 (62.57 3.30 3.37 8.50 8.58)

23 86 80 62.52 (62.57 3.303.37 8.508.58)

24 11 6 80 60.20 (60.50 3.213.26 12.40 12.46)

Table II- Spectral assignments for some 3-arylazo-8-aldehydo-4-meth y Icoumari ns

Compd

17

18

22

R

-H

4'-CI

IR (vcm,l)

3059 (C-H, Sp2), 2950 (C-H, sp3 vCH3), 271 2 (C=O, aldehydic char­acteri stic), 1700 (C=O, six membered lac tone ring), 1590, 1500 (aro­matic stretch), 1560 (N=N), 790 (Phenyl )

3020 (C-H, Sp2), 2969 (C-H, Sp3), 2720 (C=O, aldehydic), 1720 (C=O, six membered lactone ring), 1690 (C=O alde­hyde), 1600, 1500 (aromatic stretch), 1570 (N=N), 1523, 1347 (N02), 852 (C-N, ArN02)

3050 (C-H, Sp2), 2989 (C-H, Sp3 vCH3), 2725 (C=O, aldehydic char­acteristic), 1725 (C=O, six membered lactone ring), 1710 (C=O, alde­hyde), 1587, 1527 (aromatic stretch), 1580 (N=N), 109 1 (C=C), 783 (C-Cl).

IH-NMR (0 ppm)

1.30 (s, 3H, CH3) 7.26-7.36 (m, 8H, Ar-H) 9.6 (bs, 1H, CHO)

1.32 (3H, s, CH3) 7. 12-7.33 (m, Ar­H) 9.2 (bs, I H, CHO)

1.28 (3H, s, CH3) 7.36-7.45 (m, Ar­H) 9.5 (bs, 1 H, aldehyde)

0.6 mM of ~Fe(CN)6 in aqueous solution using dif­fusion coefficientl5 of Fe(CNk 4 as 0.65xlO,5 cm2s· l. Preparative electrolysis was carried out (in 2 mL, 10.3

M solution) at d.m.e. keeping a blanket of nitrogen over the surface of the solution. The product of elec­trolysis was identified by comparison of properties with the expected product.

Spectra and pKa determination At pH > 6, a pH independent spectrum with a dif­

fuse peak at 390 to 395 nm is always obtained. Beer's law holds over the concentration range from 0.5x 1 0'3 to LOx 1 0-4 M. The molarabsorbtivity (1 mole·1cm·I), and Sandell's sensitivity (Jlg cm·2) are 0.45x104 and 0.0089, respectively. The pK value of the 3-arylazo-8-aldehydo-4-methyl coumarins, calculated according to Eq. (1) and reported in Table III.

(A - A) pK = pH + log (min) .. . (1 )

A - Amax

where, Amax at pH < 6 and Amin at pH > 10.

Results and Discussion (a) Behaviour at dropping-mercury electrode

A single two-electron polarographic wav~ was ob­served in the B.R. buffer of pH range 2.5 - 11.5. The diffusion controlled nature of the limiting current was establi shed on the basis of the plots of id Vs ..Jh and id vs concentration. A comparison of the limiting current at various pH values indicate that the depolarizer is reduced consuming the same number of electrons in the entire pH range studied. The value of the tempera­ture coefficient (below 1.60% deg,l) further supported the diffusion controlled nature of the electrode proc­ess. The half-wave potentials were dependent on pH and shifted towards more negative potential value with an increase in pH. The plot of -E1/2 Vs pH was linear upto certain pH value (Ca - 8.5) . The linear representation of EI /2 - pH dependence can be de­scribed by the following linear equation :

EI I2 = 0.17 - 0.08 pH

The shift of -E1/2 towards more negative potential with increasing concentration of depolarizer, con­firmed the irreversible nature of the electrode process. However, confirmatory evidence comes from the logarithmic analysis '2. The slope of the log plot con­siderably exceeded 0.059/n.

The value of charge-transfer coefficient and rate constant (K\ h) have been calculated by the method of

366 INDIAN J. CHEM., SEC B, FEBRUARY 2003

Table III-Electrochemical characteristics of substituted 3-arylazo-8-aldehydo-4-methylcoumarins (m2J.lt 1/6 = 1.96 mg2J3sec-1/2, n = 2, t = 3.0 sec., C = 0.5x I 0.3 M, p H = 7 .5±0. 1)

SI. R -E1/2 (V) id (IlA) Slope an Doll2x 10.3 K Or.h pK (*) Temp. No. (mV) (cm2/sec) (cm/sec) Coeff. (%)

1. -H 0.43 3.28 90 0.553 2.36 I.7x lO·s 8.6 (8.5) 3.34 1.02

2. 4-N02 0.30 3.30 94 0.576 2.37 2.7 I X 10.4 8.9 (8.4) 3.36 1.40

3. 4-0CH3 0.47 3.25 90 0.602 2.34 4.7x I0·6 9.2 (9.0) 3.3 1 1.28

4. 4-CH.l 0.45 3.50 96 0.564 2.52 I.1 0x 10·s 8.6 (8.8) 3.57 1.08

5. 4-Br 0.39 3.62 87 0.622 2.60 3.03x I0·s 9.8 (9.5) 3.69 1.18

6. 4-CI 0.38 3.20 95 0.570 2.30 4.53x lO·s 8.6 (8.5) 3.26 1.27

7. 3-CI 0.36 3.86 94 0.576 2.78 8.30x I0·s 8.5 (8. 1) 3.93 1.3 1

8. 3-N02 0.32 3. 18 97 0.558 2.29 l.77x lO.s 9.2 (8.9) 3.24 1.26

* Spect rophotometric data.

Table IV - In fluence of depolarizer concentration on electrochemica l charac tcristics of 22

SI. No. Conc.x 1O·.lM - E1I2 id Siopc (V) (IlA) (mV)

1. 0.3 0.452 1.96 98

2. 0.5 0.470 3.25 90

3. 0.7 0.484 4.56 93

4. 0.9 0.490 5.77 95

5. 1.0 0.505 7.10 98

Meites and Israel16 and reported in Tables III, IV. The number of protons (P) involved in the rate deter­mining step of reaction was determined by the s lope of log plot expression II .

dE II2 =[0.059 16Jl

x p dpH a n

(b) Behaviour at glassy-carbon electrode Typical cyclic voltammogram of 3-arylazo-8-

aldehydo-4-methy l coumarin in 20% DMF containing 0.1 M BU4N+Br" at 100 mys-I scan rate is I vs yll2

graph as depicted in Figure 1. Cathod ic peak increases linearly with the concentration of the compound in the range 0.5 x 10.3 M to 1.0 x 1O-3M and the square root of sweep rate (yIl2) in the range of 30 mVs·1 to 200 mys·1 was noticed as depicted in Figure 2. The peak potential was found to shi ft cathodically with the scan rate and concentration. This behaviour clearly indicates the diffusion­controlled nature of the electrode process.

(c) Redox mechanism Keeping in view the feasibilitics of the different

sites of reduction, i.e. , -N=N- or -C=C or C=O and on the basis of DCP, CY and CPE, it was concluded that

an Dol12x I0·3

(cm2/sec)

0.553 2.35

0.602 2.34

0.582 2.35

0.570 2.31

0.553 2.56

0.3

K ° f.h

(cm/sec)

10.7x I0·6 3.33

4.7x I0·6 3.31

4.lx I0·6 3.32

3.9x I0·6 3.27

3.7x I0·6 3.62

Temp. Coeff. (% )

1.25

1.28

1.38

1.25

1.12

Ip = 4.65 /J.A Ep = -D.54 V

0.1 ~.1 ~.3 ...a 5 -0. 7 ....a9 -- (-EN ) to

Figure 1 - Cyclic voltammogram of 3-(4' -ch loro)phcnylazo-8-aldchydo-4-methylcoumarin

possible reduction site is _N=N_I7, since -C=C and C=O require much higher potential for reduction . The dependence of wave height and half-wave potential on p H was investigated in buffer solution of varying pH (2.5 - 11 .5). For all the compounds the wave height was practically independent of pH, whereas, half wave potentia! shifted towards negative side with increase in pH.

To get an indication of reversibility, the polaro­grams of these compounds were taken at different

KUMAR et al.: SYNTHESIS OF COUMARINS 367

N __ ~R

o H H ~ __ J~R I~ Slow

o H

[Rate determining step]

N __ Lg-A Where, R = As reported in Table III

Scheme II

a = -H b=4-CI c =4-OCH3

8

6

« fr' ::s. ,g:

2

o

(b)

(a) "':8

(c)

o f 4

2

tion of 3-arylazo-8-aldehydo-4-methyl coumarin (Scheme II). The solution after controlled potential electrolysis gave negative test for amino group thereby confirming the above reduction mechanism.

(d) Structural effects

It is apparent from the data that the values of dE1/2/dpH and a.n are practically in the same range for the entire reaction series. Thus, it was possible to dis­cuss the effect of substituents quantitatively in the terms of Hammett substituent constant (cr)1 8. It is ob­served that meta and para substituents have shown good correlation with the substituent constants and they appreciably effected the reduction of azo group by polar and mesomeric effects through the aromatic

~-"""------.--"""----.---r-----'- 0 ring. 0.1 0.2 0.3 0.4 0.5

yll2, V/Sec.

Figure 2 - Plots of Jp vs yY2 for some substituled 3-arylazo-8-aldehydo-4-methyl coummins

concentration of depolarizer. The concentration was varied in the range O.3x IO-3 M to l.Ox lO-3 M. The shift towards more negative potential with increase in con­centration indicates irreversible nature of the process . Since two electrons and one proton were required for the rate determjning step, a mechanism similar to that reported in literature l3 may be proposed for the reduc-

References I Raue R & Brack A, Belg 621380 (1963); Chem Abstr, 58,

1963, 11506a. 2 Moffett R B, J Med Chem, 7, 1964,446. 3 Raev L, Voinov E, Ivanov I & Popov D, Pharmazie, 45 ,

1990,696; Chem Abstr, 114, 1990, 74711 B. 4 Kun E & Aurelian L, US Pat 412783 (1991); Chem Abstr,

115,1991,970711. 5 Bohdem S & Elzbieta B, Pol PL 136525 (1986); Chem Abstr,

113,1990, 40455K. 6 Baizer M M & Chruma J L, Tetrahedron Lett, 5209, 1973. 7 Hallcher R C & Baizer M M, J Liebigs Ann Chem, 737, 1977. 8 Barnes J H, Triebe F M & Hawiey M D, J Electroanal Chem,

139, 1982, 385.

368 INDIAN J. CHEM., SEC B, FEBRUARY 2003

9 Florence T M, Johnson D A & Bailey G E, J Electroanal Chem, 54, 1974, 411.

10 Britton-Robinson H T S, Hydrogen Ions, Vol I. (D Van Nostrand, New York), 1956.

II Jain R & Pandey P, J Indian Chem Soc, 66, 1989,486. 12 Meiles L, Polarographic Techniques (interscience, New

York), 1967. 13 DeVries T & Kroon J L, J Am Chen! Soc, 75, 1953, 2484.

14 Noel M & Ananlhraman P N, Analyst, 11 0, 1985, 1095. IS Von Slackelberg M, Pilgram M & Tcome V, Z Electrochem,

57,1953,342. 16 Meiles L & Israel Y L, J Amer Chem Soc. 83, 1961 ,4403. 17 Malik W U, Mahesh V K & Goyal R N, J Electroanal Chem,

54, 1974,4 11. 18 Zuman P, Substituent effects in organic polarography, Ple­

num Press, New York, 1967.