Table of Contents
International Journal of Spectroscopy
Volume 2010, Article ID 246821, 6 pages
http://dx.doi.org/10.1155/2010/246821
Research Article

Styrylpyrylium Salts: and NMR High-Resolution Spectroscopy (1D and 2D)

Laboratoire de Chimie Organique : Structure et Réactivité, UFR/SEA, Université de Ouagadougou, 03 B.P. 7021, Ouagadougou 03, Burkina Faso

Received 28 December 2009; Revised 12 April 2010; Accepted 20 April 2010

Academic Editor: Karol Jackowski

Copyright © 2010 Jean Claude W. Ouédraogo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

and NMR high-resolution spectroscopy (1D and 2D) ( , -COSY, HSQC, HMBC) for four styrylpyrylium perchlorates were carried out and signal attributions are reported. Chemical shifts observed on NMR spectra for the styrylpyrylium salts were compared with net atomic charge for carbon obtained by AM1 semiempirical calculations. The position of the styryl group present low effect on chemical shifts for carbon atoms, while the presence of methyl group led to the unshielding of the substituted carbon.

1. Introduction

NMR spectroscopy reports for pyrylium salts from few authors are available [1, 2]. This investigation presents some complexity in the attribution of the chemical shifts to atoms. The data on pyrylium salts NMR spectroscopy with substitution effects analysis are useful to understand various properties well known for pyrylium cations [36]. Particularly emission properties (fluorescence and phosphorescence) of styrylpyrylium salts are reported in relation with proton, chemical shifts in 1H NMR of the pyrylium ring [7, 8]. However carbon chemical shift data relative to pyrylium ring, proton and carbon chemical shift for styryl group remain unknown, as for us.

In this study, we report high-resolution 1D and 2D 1H and 13C NMR analysis results for four styrylpyrylium salts (Figure 1).

246821.fig.001
Figure 1: Structures of studied styrylpyrylium salts.

13C chemical shifts of the pyrylium ring, phenyl, and styryl groups are presented with correlation with Austin Model 1(AM1) theoretical calculations. The substitution effects of methyl, phenyl, and styryl groups and their positions on the pyrylium ring were discussed as regards atomic chemical shifts.

2. Results and Discussion

Spectra are recorded with compounds dissolved in d6-DMSO. Proton NMR chemical shifts recording for studied styrylpyrylium salts were reported in Table 1 and those for carbon 13 in Table 2.

tab1
Table 1: 1H NMR (600 MHz) data: chemical shifts in ppm as unit of measurement; multiplicity and constant coupling J given in Hz for four styrylpyrylium salts 1, 2, 3 and 4.
tab2
Table 2: 13C-NMR data, 9.40 T (100.6 MHz) or 14.09 T (150.9 MHz) of compounds 1, 2, 3 and 4: Chemical shifts (in ppm as unit of measurement) and net atomic charge (q) of styrylpyrylium salts carbons.

On all spectra, we observed general pyrylium salts characteristics and also specific data due to styryl group with its extracyclic double bond. Data are comparable to those of previous work at low resolution obtained by A. R. Katritzky and coll [1]. Here the study at high resolution gives high precision on chemical shifts for proton and carbon and reveals correlation between atoms.

2.1. 1H NMR Spectra Data Analysis

or : x is the position number of carbon or hydrogen beard by carbon x.

All the protons of the phenyl groups resonate between 7.50 and 8.45. Pyrylium ring proton signals appear as singlets in the range 8.50–9.20. The signals were assigned and compared with data encountered in literature [810].

The two protons H3 and H5 of compound 1 are isochrones because of the symmetry of this molecule and appear at 8.92, whereas in compound 3, because of the asymmetry of the molecule, they are 9.02 for H3 and H5 shifted towards low frequency with 8.80 (Figure 2).

246821.fig.002
Figure 2: 1H NMR spectra of compound 3.

The high chemical shift value corresponding to H3 may be explained by the fact that it is in a paramagnetic anisotropy field of both phenyl rings, those in positions 2 and 4, while H5 is impacted by only one phenyl group anisotropy, the one in position 4 [11].

For NMR data analysis, comparison with triphenyl pyrylium: 2,4,6-triphenylpyrylium tetrafluoroborate encountered in the literature [12] shows the signal of the pyrylium ring protons at 8.50. These protons are more shielded than the styrylpyrylium perchlorate due certainly to the mesomeric effects.

The vinylic protons H7 and H8 constitute an AX system. Proton H8 for all the compounds appears like a doublet between 8.37 and 8.72.

The scalar coupling value between the two vinylic protons of each molecule varied between 15.90 Hz and 16.20 Hz. This value is an indication for the (E) stereochemistry of the double bond C=C of styryl group [13].

In the case of compound 3, H7 signal appears on the spectrum 2D (HMBC) like doublet at 7.81 (Figure 3).

246821.fig.003
Figure 3: HMBC spectrum for compound 3.

The allocation of the proton H7 was confirmed by using proton-proton correlation spectra (COSY). The results show that this proton resonates at 7.80, in a broad peak for 2 and 4. On the 1D 1H NMR spectrum of 1, proton H7 resonates in a good resolution as doublet at 7.70 (Figure 4).

246821.fig.004
Figure 4: Partial view of 1H NMR spectra of compound 1 (600 MHz).

When the styryl group is attached in position 4 of the pyrylium ring, the proton H8 resonates at 8.70 as for 1 and at 8.72 as for 2; when it moves to position 6, H8 undergoes a slight shielding and resonates at 8.48 as for 3 and 8.37 as for 4.

Protons H14 and H18 of 1 are isochrones because of the symmetry of the molecule and resonate like doublet at 8.44.

Proton-proton correlation allowed allocation of signal in the form of triplet towards 7.79 to isochrones protons H15 and H19.

In compound 3, proton H14 is more unshielded than proton H8 from styryl group (Figure 1 and Table 1). However, on the 1H NMR spectra of the other compounds, H8 is more unshielded than all the other aromatic protons.

2.2. 13C NMR Spectra Data Analysis

Carbon chemical shifts attribution for pyrylium cation-carbons were made by correlation spectra (HSQC, HMBC) and by DEPT. Data are given in Table 2.

Carbons C2, C4, and C6 of pyrylium ring are unshielded than phenyl carbon and those of styryl group while carbons C5 and C3 are the most shielded as it is shown on Figure 5.

246821.fig.005
Figure 5: 13C NMR spectra of compound 1.

Indeed, the pyrylium cation is a hybrid of resonance between an oxonium form and three carboniums forms (Figure 6).

246821.fig.006
Figure 6: Pyrylium cation resonance structures.

So, C2, C4, and C6 exhibit a positive charge in the main resonance structures. The NMR chemical shifts (calculated by the GIAO method) [14] obtained for carbon atoms of the pyrylium cation are in good agreement with these results.

HMBC correlation spectrum based on the 1H NMR data observed with compound 3 shows that proton H8 is coupled with carbon C6, C4, and C9 but not with C7. Also, H7 is coupled with C6 and C5 but not with C8 (Figure 7).

246821.fig.007
Figure 7: Pertinent HMBC correlations observed with 3.

Carbon C7 resonates between 118 and 124 while C8 resonates between 145 and 149 for all the compounds. This difference is due to the mesomeric forms of the molecule given positive load on carbon 8 (Figure 8).

246821.fig.008
Figure 8: Mesomeric forms of 1.

This mesomeric form is the same for all the compounds. The net atomic charge for carbon atoms obtained by AM1 semiempirical calculation corroborates with allocations made (Table 2). Indeed, the highest is the net atomic charge, and the more unshielded carbon is observed [15].

3. Substitution Effect Analysis

Comparing the pyrylium cation structure and the obtained 13C NMR spectra, we were interested by methyl group position in pyrylium ring, effect on the chemical shifts of styryl group and pyrylium ring carbons.

Disturbance made by the electron donating methyl group led to the unshielding of substituted carbon with a gap of about 14, while affecting slightly chemical shifts of the other carbons (Table 3). The effect was noticed with isothiazol, where the presence of methyl group causes the unshielding of substituted carbon by 10 to 15 [16].

tab3
Table 3: Gap of chemical shifts of styryl group and pyrylium ring carbons for compounds 1, 2, 3 and 4.

Pyrylium ring 13C NMR of the 2,4,6-triphenylpyrylium is given in the literature [17]. When one phenyl group is substituted by a styryl group (formation of 1 or 3), we observe a shielding of about 4 of carbons C2 and C6, and about 6 of C4. This shielding could be explained by the paramagnetic anisotropy, which is more intense around 2,4,6-triphenylpyrylium which has 3 phenyls groups directly attached to the pyrylium ring.

When styryl group is in position 4, the methyl carbon resonates at 15.10; when the styryl group is bearing in position 6, the carbon resonates at 17.63. This slight unshielding is certainly due to a magnetic anisotropy effect [11].

4. Conclusion

Protons and carbons chemical shifts for four styrylpyrylium perchlorates were allocated notably thanks to correlation spectra and to DEPT. Carbon 13 chemical shift assignments have been confirmed by net atomic charge for carbon obtained by AM1 semiempirical calculation. The scalar coupling value indicates an (E) stereochemistry around the double bond C=C of the styryl group.

The effects of the styryl group position in the pyrylium ring and the substitution effects (methyl and phenyl groups) will be compared to their reduction potential obtained by cyclic voltammetry.

5. Experimental Part

Styrylpyrylium perchlorates are prepared as described in literature by Simalty et al. [18] or with a procedure via -diketone for compounds 1 and 2 [19].

The NMR spectra were recorded at 298 K in DMSO-d6 on a Bruker Avance spectrometer operating at 7.05 T (300 MHz for 1H and 75.4 MHz for 13C) or Varian VNMRS spectrometers operating at 9.40 T (400 MHz for 1H and 100.6 MHz for 13C) and 14.09 T (600 MHz for 1H and 150.9 MHz for 13C).

The chemical shift scales were calibrated using the signal of the solvent (2.50 ppm for 1H and 39.5 ppm for 13C) [20] or the signal of internal TMS (0.00 ppm).

Mass spectra are recorded using ES ionization with a Waters QTOF2 spectrometer.

Calculations were carried out using the CS MOPAC programs version 5.0. All the structures were completely optimized by the AM1 method [21].

1
2,6-diphenyl-4-styrylpyrylium perchlorate [18, 19].
Red compound; yield: 31%; Mp: C.
IR (cm-1): 1638.35; 1603.67; 1593.16; 1576.99; 1517.97; 1495.32; 1469.78; 1191.17; 1083.42; 984.77; 776.86; 714.24; 683.39; 646.12; 621.12.
MS ES: m/z (%): 337(5); 336(35); 335 [M+] (100); 254(11); 253(55); 231(11); 178(21); 157(22); 129(28); 100(5).
1H-NMR; (ppm): 8.94 (s, 2H, pyr H3 and H5); 8.70 (d, 1H, =C8H–); 7.50–8.50 (15H, Ar and 1H, d, –C7H= at 7,70).
NMR 13C: 168.88 (C2, C6); 114.90 (C3, C5); 163.23 (C4); 123.77 (C7); 148.67 (C8).

2
3-methyl-2,6-diphenyl-4-styrylpyrylium perchlorate [18, 19]
Red compound; yield: 20%; Mp: C.
IR (cm-1): 1626.16; 1601.89; 1590.82; 1574.97; 1508.48; 1080.41; 978.00; 738.37; 728.39; 698.87; 623.78.
MS ES: m/z (%): 350(25); 349 [M+] (100); 335(10); 267(11); 254(15); 253(100); 244(5); 159(5); 157(58); 120(8).
1H-NMR; (ppm): 9.17 (s, 1H, pyr H5); 8.70 (d, 1H, =C8H–); 7.7–8(m, 16H, Ar and –CH7=); 2.67 (s, 3H, –CH3).
NMR 13C: 168.97 (C6); 167.19 (C2); 163.61 (C4); 149.22 (C8); 127.52; 121.18; 112.75.

3
2,4-diphenyl-6-styrylpyrylium perchlorate [18].
Red compound; yield: 22%; Mp: C.
IR (cm-1): 1629.89; 1592.25; 1504.82; 1492.34; 1467.63; 1215.65; 1092.70; 975.90; 766.67; 679.75; 622.87.
MS ES: m/z (%): 337 (5); 336 (30); 335 [M+](100)
1H-NMR; (ppm): 9.02 (s, 1H, pyr H5); 8.8(s, 1H, pyr H3); 8.5(d, 1H, =C8H–); 7–8(m, 16H, Ar and –C7H=)
NMR 13C: NMR 13C: 170.15 (C6); 169.12 (C2); 163.69 (C4); 145.80 (C8); 134.96; 134.87; 134.38; 132.53; 132.17; 129.96; 129.76; 129.53; 129.47; 129.25; 129.15; 128.71; 118.85 (C7); 116.93 (C5); 114.28 (C3).

4
3-methyl-2,4-biphenyl-6-styrylpyrylium perchlorate [18]
Red compound; yield: 23%; Mp: C.
IR (cm-1): 1626.30; 1606.20; 1594.25; 1504.85; 1091.11; 972.04; 764.06; 698.70; 624.03.
MS ES: m/z (%): 350 (12); 349 [M+] (45); 244 (15); 243 (100); 179 (12); 101 (6).
1H-NMR; (ppm): 8.52 (s, 1H, pyr H5) 8.4 (d, 1H, =C8H–); 7.5–8 (m, 16H, Ar and –C7H=); 1.9 (s, 3H, –CH3)
NMR 13C: 169.60 (C6); 169.46 (C2); 167.94 (C4); 146.40 (C8); 135.14; 134.26; 133.04; 132.23; 131.88; 130.65; 130.24; 129.44; 129.33; 129.26; 129.24; 129.13; 128.39; 121.01 (C7); 118.34 (C5); 17.63 (CH3).

Acknowledgments

The authors gratefully acknowledge Professor Michel Luhmer and Mrs. Rita D’Orazio (CIREM, Université Libre de Bruxelles, Belgium) for the NMR measurements.

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