Effect of substituent on the ¹³C - NMR chemical shifts of substituted 5-benzylidenebarbituric acids

 

P. Mohandass¹, K. Radhakrishnan*¹, S.Manimekalai²,V. Suriyanarayanan³, S. Radhakrishnan⁴

¹Research Department of Chemistry, Saraswathi Narayanan College, Perungudi, Madurai –625022, Tamil Nadu, India

²Department of Chemistry, E.M.G. Yadava Women’s College,Madurai, Tamil Nadu, India

³Electro-organic Chemistry Division, CSIR-CECRI, Karaikudi-630003, Tamil Nadu, India

⁴Central Instrumentation Facilities, CSIR-CECRI, Karaikudi-630003, Tamil Nadu, India

*Corresponding Author E-mail: prskradha61@gmail.com

Substituted 5-benzylidenebarbituric acids have been prepared and characterized by ¹H and ¹³C NMR spectral analysis. Investigation of substituents effect and mode of transmission of electronic effects have been carried out by ¹³C NMR chemical shifts measurement and correlated it with LFER parameters in substituted 5-benzylidenebarbituric acids.

 

 

 

INTRODUCTION:

Correlation of the chemical shift with substituent constant has been widely used to investigate the nature of the effect of the substituent on the physical properties of compounds. Electron density around the nucleus of interest (H,C) is mostly affected by the electron-donating and electron-withdrawing ability of the substituent. Therefore, a correlation between the observed chemical shift and any parameter representing such ability seems to be well-founded, and there are numerous reports on the subject¹.

 

The single substituent parameter(SSP) approach which is represented by equation (1) has been used to correlate the chemical shift of m- and p- substituted compound to unsubstituted compound using the Hammettσ_mand σ_p values.  Values of ¹³C were reported for the system of m-andp-substituted cinnamic acids².

 

The dual substituent parameter (DSP) approach, on the other hand, divides the effect of substituent effect into inductive (σ_I) and resonance (σ_R) parameters as represented in Eq. (2).

 

 

Yukawa and Tsuno proposed Eq. (3) for dealing with enhanced resonance effects

 

 

The DSP analysis of sterically congested systems has been improved by TSP by using Charton’s steric parameter (ν) using Eq. (4).

 

 

 

The magnitude, as well as the sign of ρ, should have a profound meaning on the mechanism of the transmission of the substituent effect.  For example, correlations of the chemical shift of carbonyl carbon with the Hammett σfor the system of benzanilides, X-C₆H₄-CO-NH-C₆H₄-Y show normal correlation when Y is varied, but thereverse correlation is observed when X is varied. πpolarizations have been attributed for such observation.^3,4 The magnitude of ρ_x (-2.838 ppm, r=0.940) is larger than that of ρ_y(0.853 ppm, r= 0.975) in DMSO -d₆. The carbonyl carbon is directly bonded to the phenyl ring of X-C₆H₄, but it is separated by the nitrogen atom from C₆H₄-Y and therefore, the ρ_xshould be larger than ρ_y.  The estimation of sign and magnitude of the substituent chemical shift, however, seems to be complicated by many factors which affect the chemical shift.

 

Recently we reported the effect of substituents on the antimicrobial activities ofsubstituted 2-benzylidene 1,3-indandiones⁵ andsubstituted5-benzylidenebarbituric acids⁶.These above reports prompted us to undertake a detailed ¹³C NMR spectral and correlation study of substituted5-benzylidenebarbituric acids.

 

MATERIALS AND METHODS:

All chemicals used were purchased from Sigma Aldrich.  The purity of the compounds was checked by TLC on silica gel G plate.Substituted 5-benzylidenebarbituric acids (Figure-1) were prepared by reaction of the corresponding substituted benzaldehyde with barbituric acid as reported earlier⁷.

 

Figure - 1

 

Spectra:

¹H and ¹³C spectra of substituted 5-benzylidenebarbituric acids were obtained on a Bruker AMX 400 MHz spectrometer.  Chemical shifts of ¹H were measured with the peak ofDMSO at δ 2.51 as the internal reference, while those of ¹³C were recorded with the central peak of DMSO at δ 39.90 as the internal reference.The ¹H and ¹³C chemical shifts of the substituted 5-benzylidenebarbituric acids are presented in Tables 1 and 2 respectively.

 

Assignment of  ¹H and ¹³C chemical shifts:

Assignments of ¹H chemical shifts were made using considerations of splitting patterns, spin decoupling and additivity of chemical shifts.  The ¹³C chemical shifts were assigned by intensity and SCS considerations.

 

Table 1: ¹H NMR chemical shifts ofsubstituted 5-benzylidenebarbituricAcids

S. No.	Substituent (X)	H2,2’	H3,3’	H5	H8	H10
1	0	7.065	8.369	8.252	11.175	11.302
2	-OH	6.878	8.32	8.213	11.117	11.249
3	0	7.304	8.094	8.255	11.218	11.365
4	-H	7.485	8.073	8.285	11.238	11.397
5	-Cl	7.518	8.069	8.243	11.275	11.425
6	-Br	7.67	7.979	8.223	11.272	11.421
7	0	8.245	8.017	8.324	11.329	11.504

 

 

 

Table 2: ¹³C NMR chemical shifts of substituted 5-benzylidenebarbituricacid

 

RESULTS AND DISCUSSIONS:

Correlations with Lynch-Equation:

The SCS of monosubstituted benzenes has been very useful inthesignal assignment of polysubstitutedcompounds^8-11.  However, in many published studies summarized by Craik¹² it has been apparent that the SCS values of X and Y at positions 4’ and 1’ in disubstituted benzenes Fig. (2) are non-additive.  Lynch¹³ has proposed that the non-additivity of the chemical shifts of C-1’ and C-4’ is reflected in the relationship Eq. (5) where SCS_X(Y) is the substituent chemical shift of thecarbon para-to X in the series of 1’,4’-disubstituted benzenes (Fig. 2, X, Y ≠ H),

 

Figure -2

 

 

SCS_X(H) is the corresponding substituent chemical shift of the carbon parato X in monosubstituted benzenes (Fig.2, Y=H), ‘b’ is the slope parameter and ‘a’ is the shift calculated for the parent spices with X=Y.  A wide range of successful correlations (using Lynch equation) have been established for the SCS of carbons bearing Y in Fig. (2) with the value of ‘b’ ranging from 0.6 to 1.5¹⁴.  When the slope is close to unity, experimental results could be reproduced by additivity relationship and when b ≠ 1, the Lynch equation can be used to predict the SCS values through proportionality relationships.  The slope b of the Lynch equation Eq. (5) is less than one, then the fixed substituent at C-1’ Fig. (2) diminishes the substituent effect. When slope b is larger than one, showing that the fixed substituent undergoes an amplification of the substituent effect.

 

In case of C_1,there is agoodcorrelation with (r=0.988) appropriate SCS (S_i) values, slope b is 0.97, which reveals that the fixed substituent Y=-CH-C-C₃H₂N₂O₃, has little effect on the additivity of this shifts¹⁴.A satisfactory correlation exists between SCS of C_22’ and S_o withacorrelation coefficient (r=0.828) and the slope value (b=0.84), indicating that the fixed substituent Y, significantly diminishes the substituent effect¹⁴.A poor correlation exists between SCS of C_33’ and S_m withacorrelation coefficient (r=0.113) and the slope value (b=0.71) which reveals that the fixed substituent Y diminishes the substituent effect¹⁴.The C₄ carbon affords a good correlation with S_pwith correlation coefficient r=0.993 and the slope value b=0.94 demonstrates that the fixed substituent Y has little effect on the additivity of these shifts¹⁴.  The results of Lynch correlations are given in table (3) and the plots of Lynch correlations shown in fig. (3).

 

Table (3): Results of Lynch correlations^a of¹³C chemical shifts of substituted 5-benzylidenebarbituric acids Fig. (1) with SCS values for monosubstituted benzenes.

Where, ‘a’ is SCSX (Y) = a + b SCSX (H); ‘b’ is Correlation coefficient; ‘c’ is Number of data points; ‘d’ is Standard deviations

 

Table ( 4): Results of statistical treatment of ¹³C – Chemical shift with σ_p, σ_p^o_,σ_p⁺_, σ_p⁺_/ σ_p,     σ_p⁺_/ σ_p^-,σ_p⁺_/ σ_p/ σ_p^-substituent constants using single parameter equation (2)

S.No.	Carbons	Scale	ρ	R	s	F	log δo	n
1	C4	σp	0.043±0.006	0.949	0.006	43.33	2.115±0.002	7
		σpO	0.038±0.010	0.882	0.008	13.99	2.114±0.004	6
		σp+	0.029±0.002	0.986	0.003	179.37	2.121±0.001	7
		σp+/σp	0.033±0.006	0.924	0.007	29.11	2.117±0.003	7
		σp+/σp-	0.024±0.002	0.989	0.003	226.34	2.118±0.001	7
		σp+/σp/σp-	0.026±0.004	0.938	0.006	37.12	2.115±0.002	7
2	C5	σp	-0.011±0.001	0.984	0.001	152.18	2.189±0.001	7
		σpO	-0.015±0.001	0.988	0.001	163.39	2.190±0.001	6
		σp+	-0.007±0.001	0.916	0.002	25.94	2.187±0.001	7
		σp+/σp	-0.008±0.002	0.887	0.002	18.40	2.188±0.001	7
		σp+/σp-	-0.006±0.001	0.938	0.002	36.47	2.188±0.001	7
		σp+/σp/σp-	-0.006±0.001	0.917	0.002	26.68	2.189±0.001	7
3	C6	σp	0.025±0.004	0.950	0.004	45.95	2.073±0.001	7
		σpO	0.021±0.005	0.900	0.004	17.16	2.073±0.002	6
		σp+	0.018±0.001	0.995	0.001	508.52	2.077±0.001	7
		σp+/σp	0.019±0.003	0.917	0.005	26.46	2.075±0.002	7
		σp+/σp-	0.014±0.002	0.970	0.003	80.18	2.075±0.001	7
		σp+/σp/σp-	0.015±0.003	0.902	0.005	21.75	2.075±0.002	7
4	C7	σp	-0.004±0.002	0.722	0.001	5.43	2.211±0.001	7
		σpO	-0.004±0.002	0.620	0.002	2.51	2.211±0.001	6
		σp+	-0.003±0.001	0.820	0.001	10.24	2.210±0.001	7
		σp+/σp	-0.004±0.001	0.917	0.001	26.41	2.210±0.001	7
		σp+/σp-	-0.002±0.001	0.775	0.001	7.51	2.210±0.001	7
		σp+/σp/σp-	-0.003±0.001	0.836	0.001	11.63	2.210±0.001	7
5	C11	σp	-0.003±0.001	0.979	0.0003	116.72	2.24±0.001	7
		σpO	-0.003±0.001	0.945	0.0004	33.30	2.215±0.001	6
		σp+	-0.002±0.001	0.996	0.0001	640.41	2.214±0.001	7
		σp+/σp	-0.002±0.001	0.935	0.0005	34.80	2.214±0.001	7
		σp+/σp-	-0.002±0.001	0.983	0.0003	145.26	2.214±.0001	7
		σp+/σp/σp-	-0.002±0.001	0.930	0.0005	32.23	2.215±0.001	7
6	C22’	σo	0.044±0.024	0.632	0.020	3.33	2.094±0.008	7
		σo*	0.038±0.020	0.691	0.020	3.66	2.095±0.009	6
		Es	-0.042±0.013	0.825	0.016	10.69	2.103±0.006	7

“n=6 means calculated without -OH group”

 

 

Fig. (3). Lynch plot of SCS of C₄ vs S_p

 

Hammett and dual substituent parameter correlations:

The ¹³C chemical shifts of the carbon atoms of the 5-benzylidenebarbituric acid series afford several reasonable correlations using Hammett substituent parameters Eq.(1) and the data are given in Table (4).The results of the correlations of ¹³C SCS values of 5-benzylidenebarbituricacidserieswith σ_Iand σ_R constants according to equation (2) are presented in Table (5).The results of multiple regression analysis of¹³C chemical shifts with σ_p,( σ_p⁺- σ_p)  and σ_p^o,(σ_p⁺- σ_p^o) constants using Yukava – Tsuno equation (3) are presented in Table (6).

Table 5: DSP analysis of chemical shift data with dual parameter equation (3 )

S.No	Carbons	Scale	ρI	ρR	R	SE	F	logδo	n	λ=ρR/ρI
1	C4	σI ,σR	0.024±0.003	0.051±0.003	0.995	0.002	154.44	2.122±0.001	6	0.25
		σI ,σRo	0.032±0.017	0.036±0.017	0.849	0.01	3.89	2.118±0.007	6	1.33
		σI ,σR+	0.010±0.030	0.018±0.016	0.640	0.016	1.39	2.122±0.015	7	1.80
		σI ,σR-	0.019±0.011	0.037±0.009	0.943	0.007	12.10	2.120±0.005	6	2.00
		F,R	0.025±0.003	0.054±0.002	0.996	0.002	279.61	2.123±0.001	7	2.00
2	C5	σI ,σR	-0.013±0.001	-0.007±0.001	0.998	0.0003	349.25	2.191±0.001	6	0.70
		σI ,σRo	-0.014±0.003	-0.005±0.002	0.956	0.002	16.09	2.191±0.001	6	0.36
		σI ,σR+	-0.011±0.006	-0.002±0.001	0.838	0.003	4.74	2.191±0.003	7	0.20
		σI ,σR-	-0.012±0.002	-0.005±0.001	0.980	0.001	36.20	2.191±0.001	6	0.42
		F,R	-0.014±0.001	-0.009±0.001	0.993	0.001	146.55	2.191±0.001	7	0.71
3	C6	σI ,σR	0.016±0.002	0.026±0.002	0.996	0.001	176.46	2.077±0.001	6	1.63
		σI ,σRo	0.019±0.010	0.016±0.010	0.810	0.006	2.86	2.074±0.004	6	0.80
		σI ,σR+	0.008±0.019	0.010±0.009	0.636	0.01	1.36	2.076±0.009	7	1.25
		σI ,σR-	0.014±0.008	0.017±0.007	0.885	0.005	5.43	2.075±0.003	6	1.21
		F,R	0.015±0.003	0.032±0.002	0.990	0.002	105.40	2.078±0.001	7	2.13
4	C7	σI ,σR	-0.001±0.001	-0.008±0.002	0.928	0.001	9.34	2.209±0.001	6	2.00
		σI ,σRo	-0.002±0.003	-0.004±0.003	0.550	0.002	0.649	2.210±0.002	6	2.00
		σI ,σR+	0.001±0.002	-0.003±0.002	0.669	0.002	1.618	2.208±0.002	7	0.21
		σI ,σR-	-0.001±0.001	-0.004±0.003	0.648	0.002	1.087	2.210±0.001	6	4.00
		F,R	-0.001±0.002	-0.005±0.002	0.806	0.001	3.719	2.209±0.001	7	5.00
5	C11	σI ,σR	-0.003±0.001	-0.003±0.001	0.998	0.0001	341.86	2.214±0.001	6	1.50
		σI ,σRo	-0.003±0.001	-0.002±0.001	0.859	0.0007	4.22	2.214±0.001	6	0.66
		F,R	-0.002±0.001	-0.004±0.001	0.993	0.002	152.18	2.214±0.001	7	2.00
6	C22’	σI ,σR	0.010±0.022	0.079±0.020	0.891	0.01	5.81	2.115±0.010	6	8.00
		σI ,σRo	0.020±0.001	0.031±0.042	0.438	0.03	0.35	2.104±0.018	6	1.50
		F,R	0.006±0.019	0.081±0.024	0.903	0.01	8.85	2.117±0.010	7	1.33

“n=6 means calculated without -OH group”

 

Table (6):Results of multiple regression analysis of ¹³C chemical shifts with σ_p,( σ_p⁺- σ_p) and σ_p^o,(σ_p⁺- σ_p^o) constants using Yukava – Tsuno equation (3).

“n=6 means calculated without -OH group”

 

 

Substituent effects on the ¹³C NMR chemical shifts of the substituted 5-benzylidenebarbituric acids:

The use of ¹³C NMR SCS is to monitor the transmission of electronic effects in the molecular frame works in general and to understand the mode of transmission of long-range substituent effects in extended π-electron systems, in particular, are topics of current interest. Thus long-range ¹³C SCS have been reported for N-benzylideneanilines¹⁵, N-benzylidenebensylamines¹⁶, chalcones¹⁷, benzophenones¹⁸, phenylacetylenes¹⁹, β-nitrostyrenes^20,21, cinnamic acids^22,23, cinnamates²⁴, benzonitrile²⁵, cyanopyridine²⁶, (E)-1-benzylidene-2-(diphenyl methylene) hydrazines²⁷, aryl hydrazides²⁸,(E)-1-benzylidene-2-(4-bromophenyl) hydrazines²⁹, N-(substituted phenyl)-2-cyano acetamides³⁰, 4-bromo-1-naphthyl chalcones³¹and symmetrical diimines³².The ¹³C chemical shift data of several carbon atoms of substituted 5-benzylidenebarbituric acids has been correlated with SSP equation (Eq.1.), DSP equations (Eq.2.) and Yukawa-Tsuno equation (Eq.3.).

 

C₄ - Carbon atom.

The Chemical shift of C₄carbon appeared over a relatively narrow range of 16.24 ppm. The result of SSP analysis shown in the table (4), an excellent correlation affords by σ_p⁺ / σ_p^- constant given in equation (6), and the plot of log δ C₄ vs σ_p⁺ / σ_p^- as shown in Fig. (4).\

 

 

The result of DSP analysis in thetable (5), afford an excellent correlation are shown in equations (7) and (8)

.

 

The sign of ρ_I and ρ_R are positive reveals that the normal substituent effect operates on C₄ carbon atom, i.e., an electron withdrawing substituent decrease the C₄ carbon atom shielding and an electron releasing substituent increase it. Examination of chemical shift of data in table (2), electron-withdrawing substituent causes downfield shift and electron releasing substituent causes up field. The magnitude of ρ_Ris greater than ρ_I indicate that the predominance of resonance effect over inductive effect in the chemical shift of C₄ carbon atom. This result is also shown from Yukawa-Tsuno equation (3) is given in table (6). The result of best fit eq. (3) is given in equation (9)

 

Fig. 4. The plot of log δ C₄ vs σ_p⁺ / σ_p^-

 

C₅ - Carbon atom:

The chemical shift of C₅ carbon appeared over a relatively narrow range of 4.42 ppm. The SSP analysis with various σ parameters, σ_p^o gave satisfactory correlation, results shown in the table (4). It gave negative correlation (Eq.10) and negative slope indicates that a reverse substituent effect operates on the carbonyl carbon. The Hammett plot of log δ C₅ vs σ_p^o gives,

 

Examination of chemical shift data (table 2) of C₅ carbon, electron releasing group causedownfield shift by increasing the shielding and electron withdrawing group cause upfield shift by decreasing the shielding.

 

The DSP analysis of C₅ carbon with various σ_R scale is given in table (8),the best fit is given in equations (11) and (12)

 

 

The sign of slopes ρ_I and ρ_R are negative, reveals that the reverse substituent effect operates on C₅ carbon atom.The results of Yukawa-Tsuno equation (13), also indicate that the magnitude of ‘r’ is very low (< 1).

 

 

C₆ – Carbon atom

The chemical shift of C₆ carbon atom appeared over a relatively narrow range of 7.15 ppm. The results of SSP analysis shown in table (4), excellent correlation afford by σ_p⁺ constant given in equation (14).

 

The results of DSP analysis in table (5), afford an excellent correlation are shown in equations (15) and (16).

 

The sign of ρ_Iandρ_Rarepositive reveals that the normal substituent effect operates on C₆ carbon atom, i.e., an electron withdrawing substituent decrease the C₇ carbon atom shielding and an electron releasing substituent increase it. Examination of chemical shift data in table (2), electron withdrawing substituent causes downfield shift and electron releasing substituent causes upfield. The magnitude of ρ_Rgreater thanρ_Iindicate that the predominance of resonance effect over inductive effect in the chemical shift of C₆ carbon atom.

 

The sign of slopes ρ_I and ρ_R are positive reveals that the normal substituent effects operate on C_22’ carbon atom. The magnitude of ρ_Ris higher than the magnitude of ρ_Iindicate that the resonance effect is predominant over resonance effect on C_22’ carbon atom. The TSP analysis of Charton steric parameter (ν) gave poor correlations that are given in table (7).

 

Table (7): TSP analysis of SCS data of C_22’ carbon atom ofsubstituted 5-benzylidenebarbituric acids employing equation (4).

“n=6 means calculated without -OH group”

 

The best fit of TSP analysis is taken from high correlation coefficient (R) and least standard error (SE) of the regression equation and the results are given in Eqs. (30) and (31).

 

A better description of the composition of the electrical effect³³ is given by the equation,

P_R =

 

P_R is the percentage of delocalized effect. The result of TSP analysis is given in table (10) shows that the delocalized effect is predominant over the localized effect. The magnitude (%) of the steric factor³³ is given by the equation,

 

P_S =

 

The results of table (7) shows that steric effect is also operating to some extent.

 

CONCLUSIONS:

A series of seven numbers of substituted 5-benzylidenebarbituric acids have been synthesized by condensation of barbituric acid with substituted benzaldehydes. These synthesized substituted 5-benzylidenebarbituric acids have been characterized bythe¹H and¹³C spectral data. The ¹³C NMR spectral data of these substituted 5-benzylidenebarbituric acids have been correlated with Hammett substituent constants and F and R parameters. From the results of statistical analysis, the effects of the substituent on the spectral data have been studied. The ¹³C NMR spectral correlations produced the most number of satisfactory correlations.

ACKNOWLEDGEMENTS:

One of the authors (P.Mohandass) is grateful to the University Grants Commission (UGC), Hyderabad for the award of Faculty Development Programme. Authors are also thankful to the Principal and Managing Board of Saraswathi Narayanan College, Madurai for providing research facilities.

 

CONFLICT OF INTEREST STATEMENT:

We declare that we have no conflict of interest.

 

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Received on 17.09.2017         Modified on 05.11.2017

Accepted on 20.11.2017         © AJRC All right reserved