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Synthesis and Substituent Effects in Substituted Styryl 4-Methoxy-1-Naphthyl Ketones
Synthesis and Substituent Effects in Substituted Styryl 4-Methoxy-1-Naphthyl Ketones
Journal of the Korean Chemical Society. 2006. Jun, 50(3): 183-189
Copyright © 2006, The Korean Chemical Society
  • Received : December 23, 2005
  • Published : June 20, 2006
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G. Thirunarayanan
P. Ananthakrishna Nadar

Abstract
A series of substituted styryl 4-methoxy-1-naphthyl ketones [(2E)-1-(4-methoxy-1-naphthyl)-3-phenyl-2-propen-1-ones] were synthesized using facile method of microwave assisted condensation reaction. The yield of chalcones is more than 90%. They are characterized by their physical constants, micro analysis, infrared (KBr, 4000-400 cm –1 ) and NMR both 1 H and 13 C spectral data. From infrared spectra, the s-cis and s-trans stretching vibrations of carbonyl group, from NMR spectra the ethylenic proton and carbon chemical shifts (ppm) are assigned. These spectral data are correlated with various Hammett substituent constants. From the results of statistical analysis the effect of substituents on CO, α and β proton and carbons are explained.
Keywords
INTRODUCTION
With microwave irradiation of substituted benzaldehydes and aromatic ketones in presence of anhydrous zinc chloride gave exclusively high yield of substituted styryl chalcones. 1 Their basic skeletons of chalcones are widely figured in natural products and are known to have multi pronged activity. 2 Many of the chalcones are used as agrochemical and drugs. 3 Recently much attention has paid on the synthesis of chalcones mainly from acetophenone analogs 4 with various aromatic benzaldehydes. Several catalysts 5 such as basic alumina, Al 2 O 3 -AlPO 4 , P 2 O 5 -piperidone ultrasonic rays using C-200 and Lewis acids have been used for knovenogal condensation and bases or quaternary ammonium salts have also been employed. Further studies on the efficient synthesis of chalcones are of current interest because of their wide range of application. Thus the author to report here the first time a simple facile approach to synthesis high yield of substituted styryl 4-methoxy-1-naphthyl chalcones and investigate the substituent effects from infrared and nuclear magnetic resonance spectra were recorded.
EXPERIMENTAL SECTION
A mixture of substituted benzaldehydes (0.01mol) and 4-methoxy-1-naphthyl ketones (0.01 mol) and anhydrous zinc chloride (0.001mol) was taken in ACE tube and flushed with Argon and tightly capped. The mixture is subjected to microwave oven heating for 5-8 minutes in a domestic microwave oven (LG Microwave Oven MG-395WA) and then it is allowed to reach to room temperature. The reaction mixture was treated with ethanol and the separated solid was filtered, washed with n-Hexane and dried. The solid was recrystallised by benzene-hexane mixture 6 . The reaction is shown in 1 . Compounds a-f and h are unknown and the remaining compounds are known.
Melting points were determined in open glass capillaries on Mettler FP51 melting point apparatus and are uncorrected. Infrared spectra (KBr, 4000-400 cm -1 ) were recorded on JASCO IR-700 Japan model spectrophotometer. The nuclear magnetic resonance spectra both 1 H and 13 C of chalcones were recorded using UNITYPLUS-300 “KIBSIPS” 300MHz spectrometer, operating at 200MHz frequency for recording 1 H NMR spectra and 75.45 MHz frequency for recording 13 C NMR Spectra. The micro analysis of the chalcones were performed in Perkin Elmer 240C Analyzer.
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Based on Hays and Timmons 7 infrared carbonyl stretching frequencies of s- cis and s-trans conformers are assigned. The NMR chemical shifts (ppm) of ethylene α, β protons and carbons are assigned based on reported in earlier literature 8 . The Physical constants, micro analysis and spectral data of all chalcones are summarized in 1 .
- Substituent effects
Correlation study involves the prediction of ground state molecular equilibrations 11 of organic substrates such as s-cis and s-trans isomers of alkenes, alkynes, benzoylchlorides, styrenes and α, β-unsaturated ketones from spectral data. Their use in structure parameter correlations has now becomes popular for studying transition state study of reaction mechanisms, 12 biological activities and normal coordinate analysis. 13 Dhami and Stothers 14 have extensively studied the 1 H NMR spectra of a large number of acetophenones and styrenes with a view to establish the validity of the additivity of substituent effects in aromatic shieldings, first observed by Lauterber. 15 Savin and coworkers 16 obtained the NMR spectra of unsaturated ketones of the type RC 6 H 4 -CH=CH-COCMe 3 and sought Hammett correlations for the ethylenic protons. Solcaniova and coworkers 17 have measured 1 H and 13 C NMR spectra of substituted phenyl styrenes and substituted styryl phenyls and obtained good Hammett correlations for the olefinic protons and carbons. Now a day’s scientists 18 have paid more interest to correlate the group frequencies of spectral data with Hammett substituent constants to explain the substituent effect of organic compounds. Recently Dae Dong Sung and Ananthakrishna Nadar 19 investigate elaborately the single and multi substituent effects by spectral data of biphenyl and 9H-Fluorenyl chalcones. With in the above view there is no information available in the literature in the past with substituted styryl 4-methoxy-1-naphthyl ketones. Hence the authors have synthesized thirteen chalcones of the above type using microwave irradiation technique. And the substituent effects of above compounds are investigated from infrared and NMR spectra were made.
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- Substituent Effects from infrared spectra
The carbonyl stretching frequencies (cm -1 ) of s-cis and s-trans isomers of present study are shown in 1 and the corresponding conformers are shown in (I).
The infrared spectra were all recorded on the KBr disc in order to avoid the shoulder formation 20 on carbonyl doublets. The s-cis conformers exhibit higher frequencies than the s-trans conformers due to the bulkier naphthalene group causes greater strain and they enhance the higher absorption of carbonyl group of s-cis isomer than the s-trans isomer. These frequencies are separately analyzed through various Hammett sigma constants.
From the statistical analysis, there is no significant correlation obtained with Hammett sigma constants in s-cis conformers. This is the conjugation between the C=O and the -CH=CH- parts is less important due to non-co planarity arising out of non bonded repulsion between naphthalene and styryl parts in the systems. In s-cis conformers there issignificant correlation is obtained (r=0.995) with σ R constants. Further no significant correlations are obtained for both conformers with σ + , σ I and σ R parameters except s-trans with σ R constants. This is due to the cross conjugation of methoxy substituent in fourth position of naphthyl ring as shown in (II)
In view of the inability of some σ constants to produce individually satisfactory correlations, it was thought worthwhile to seek multiple correlations involving either σ I and σ R constants or Swain-Lupton’s F and R parameters. The correlation equations (1,2) generated are
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. 1
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Plot of νC=O (s-trans) of substituted styryl 4-methoxy-1-naphthyl ketones versus σR.
Some cases where both the group parameters were fail to predict collectively the substituent effects. This may treated exceptional and by large it is to be realized that the collective participation of either σ I and σ R parameters or F and R parameters is more dependent than that of any single parameters role to predict the substituent effects. A good single parameter correlation is shown in . 1 .
- Substituent effects from NMR Spectra
- 1H NMR Spectra
The 1 H NMR spectral signals of ethylenic protons in all chalcones investigated are assigned. The chemical shifts of H α protons are at higher field than those of H β protons in all chalcones. The ethylenic proton signals give an AB pattern and the β protons doublet in most cases is well separated from the signals of the aromatic protons. The chemical shifts of α, β protons are given in 1 . The observation that H α protons appear at higher field than that of H β protons makes the subject very interesting. This may possibly due to the polarization of C=C double bond in the system being predominantly caused by the carbonyl group so as to make electron density greater at the α position than that of β position.
Physical constants, micro analysis and spectral data of substituted styryl 4-methoxy-1-naphthyl ketones.
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Physical constants, micro analysis and spectral data of substituted styryl 4-methoxy-1-naphthyl ketones.
Results of statistical analysis of chemical shifts of H-αand H-βprotons of substituted styryl 4-methoxy-1-naphthyl ketones
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Results of statistical analysis of chemical shifts of H and H protons of substituted styryl 4-methoxy-1-naphthyl ketones
Results of statistical analysis of chemical shifts of H-αand H-βprotons of substituted styryl 4-methoxy-1-naphthyl ketones with σIand σRor F and R parameters
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Results of statistical analysis of chemical shifts of H and H protons of substituted styryl 4-methoxy-1-naphthyl ketones with σI and σR or F and R parameters
The results of statistical analysis are presented in 2 . All the attempted correlations involving substituent parameters gave only positive ρ values. This shows normal substituent effects is operates in all the chalcones. The Chemical shifts observed for H α and the H β protons in the present investigation are correlated satisfactorily with Hammett sigma constants. In some cases correlation of H β with σ values is slightly better. By and large the necessity of enhanced σ values for correlation is not demanded by substituents. That the correlation with σ I and σ R parameters is not satisfactory in H α implies that such σ values are incapable of predicting chemical shifts individually due to the domination of cross conjugation between carbonyl group and methoxy group in naphthyl ring. In 3 the multiple correlations involving either σ I and σ R or F and R values for these ketones are presented. It is indeed satisfactorily that in most cases the multiple correlations are successful. Some of the single parameter correlations are shown in . 2 , 3 .
. 2
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Plot of δH-α (ppm) of substituted styryl 4-methoxy-1-naphthyl ketones versus σ.
. 3
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Plot of δH-β (ppm) of substituted styryl 4-methoxy-1-naphthyl ketones versus σ+.
- 13C NMR Spectra
From 13 C NMR spectra the observed 13 C Chemical shifts of C α and the C β carbons are presented in 1 . These chemical shifts are correlated with various Hammett substituent constants. The results of statistical analysis of substituent effects on C α and C β carbons are shown in 4 . There is a fair degree of correlation obtained for C α and the C β carbon chemical shifts with Hammett sigma constants. The degree of transmission of electronic effects is found to be higher with C α carbon than C β carbon. Uniformly σ I and σ R parameters or F and R values are adequately explain the substituent effects in all chalcones are evidenced from the correlation equations which are given in 5 .
Results of statistical analysis of chemical shifts of C-αand C-βprotons of substituted styryl 4-methoxy-1-naphthyl ketones
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Results of statistical analysis of chemical shifts of C and C protons of substituted styryl 4-methoxy-1-naphthyl ketones
Results of statistical analysis of chemical shifts of C-αand C-βcarbons of substituted styryl 4-methoxy-1-naphthyl ketones with σI and σR or F and R parameters
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Results of statistical analysis of chemical shifts of C and C carbons of substituted styryl 4-methoxy-1-naphthyl ketones with σI and σR or F and R parameters
. 4
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Plot of δC-α (ppm) of substituted styryl 4-methoxy-1-naphthyl ketones versus σ.
In single parameter correlations, there are satisfactory correlations obtained with σ + values for C α and C β carbons. This implies that one need not attach any significance to the correlations involving σ values. A good single parameter correlation is shown in . 4 .
Acknowledgements
The authors are thanks to Dr.D.D.Sung, South Korea, financial support for recording NMR spectra of all chalcones.
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