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Article

17O NMR Spectra of α,β-Unsaturated Carbonyl Compounds RCH=CHCOX: the Influence of Group X on the δ(17O) Value of the Carbonyl Oxygen and on the Shielding Effect of Group R

College of Pharmacy, The Ohio State University, 500 West 12th Avenue, Columbus, OH 43210, USA
Molecules 1999, 4(11), 320-328; https://doi.org/10.3390/41100320
Submission received: 25 August 1999 / Accepted: 12 September 1999 / Published: 26 October 1999

Abstract

:
17O NMR spectra of RCOX (R = Me, Ph), RCH=CHCOX (R = H, Ph, Me, CO2Et) and Me2C=CHCOX (X = H, Me, Et, i-Pr, t-Bu, Ph, PhCH=CH, OMe, OEt, Cl, CN, CF3, CO2Et) are reported. The 17O shift values of these carbonyl compounds depend mainly on the electron donating power of X and correlate well with the resonance constants σR+ of X. This suggests that the δ(17O) values can be used to reflect qualitatively the electrophilicity of the carbonyl group. The shielding effect of the substituent R in RCH=CHCOX is dimin-ished with increasing the electronic donating power of X and correlates with σ+ values of X.

Introduction

17O NMR spectroscopy is a particularly useful tool for investigating the electronic state of carbonyl groups, including their electrophilicity [1, 2]. In RCOX derivatives, the shielding of O atom is increased with increasing the donating power of X group [2]. It has been shown by Dahn that the activity of the electrophilic carbonyl group, diminished by electron donating groups X, can be qualitatively reflected by the 17O chemical shift values of the carbonyl oxygen [2].
Several types of α,β-unsaturated carbonyl compounds, as aldehydes, ketones and carboxylic acid derivatives, are important organic derivatives [3, 4]. They have been extensively studied and have found a broad application in synthesis [3, 4]. 1H and 13C NMR spectroscopic studies have been applied to these compounds [3a, 5]. The 17O spectra of a few α,β-unsaturated carbonyl compounds have been reported previously [1, 6], but they have so far not been systematically investigated. This paper de-scribes a study of the influences of substituents X in RCH=CHCOX (Scheme 1) on the δ(17O) value of the carbonyl O atom and on the shielding effect of group R.

Results and Discussion

Effects of the X groups

The 17O NMR data for a series of α,β-unsaturated carbonyl compounds (3-11) together with the data of the corresponding methyl and phenyl carbonyl compounds (1 and 2, respectively), obtained in acetonitrile solution at natural abundance, are summarized in Table 1 and Table 2. The 17O NMR spectra of 3a, 3b, 3c, 4f, 4h, 5a, 5b, 11b and most of 1 and 2 have been already reported in various of solvents [1, 2]. In order to estimate difference substituent chemical shifts, these compounds were also measured under the same experimental conditions used here for α,β-unsaturated carbonyl compounds. The dif-ferences between literature data and the present values may arise from the solvent effects, temperature effects and external reference systems (δref = 0 ppm).
The 17O signals of the O atoms of α,β-unsaturated carbonyl compounds generally appear at higher field (6-50 ppm) than those of the corresponding methyl carbonyl compounds (1). The shielding effects for α,β-unsaturated carbonyl compounds have been previously reported in comparison with their saturated analogs and can be attributed to the conjugation of the carbonyl group with C=C double bond [1, 8].
The shielding of the carbonyl O atoms in the vinyl and styryl alkyl ketones (3a-3e and 4a-4e, re-spectively) are enhanced with increasing volume of the substituent attached to the carbonyl group. The shielding trends are in the same direction as noted for those of the corresponding methyl ketones (1a-1e) and tertiary enaminones (8a-8e) [9]. The 17O NMR data for the styryl ketones give a comparable correlation with those for the corresponding substituted enaminones [Eqs. (1) and (2)].
δo(3) = 1.48 δo(8) - 103.0 (n = 5, r = 0.966, SD = 7.5)
δo(4) = 1.43 δo(8) - 92.2 (n = 5, r = 0.974, SD = 6.2)
In the more extented conjugation systems (4f and 4g), the 17O signals of the carbonyls are considerably shielded to the upfield. A shielding of -40.6 ppm for 4f and of -49.3 ppm for 4g was observed in relative to 4b. The shielding effects are consistent with the reduction of the double bond character of the carbonyl groups as a result of increased cross-conjugation with additional benzene ring or C=C double bond. The slight small shielding for phenyl substituent in 4f can be ascribed to the twisting of the carbonyl group out of the plane of the C=C bond, a torsion angle of 16.9° for 4f was noted previously by X-ray [10].
Introduction of an electron withdrawing group attached to the carbonyl group causes deshielding of the O atom. For example, the 17O signals of the carbonyl oxygens of 1k (573.8 ppm) and 4k (561.5 ppm) appear at lower field than those of 1b and 4b by 5.0 and 7.2 ppm, respectively. This is in agree-ment with the increased the double bond character of the carbonyl group produced by an electron withdrawing group. Surprisingly, a large shielding of -34.7 ppm was observed for 4-MeOC6H4CH=CHCOCF3 relative to 4-MeOC6H4CH=CHCOMe (Table 3). The same effect of greater shielding (-67 ppm) has been previously observed for 8k as compared with 8b [9].
For comparison, the 17O spectra of several compounds 4-YC6H4COX and 4-YC6H4CH=CHCOX (Y=H and MeO) are given in Table 3. The shielding effect of the 4-MeO group is defined as Δδ = δ(4-MeO) - δ(H). The Δδ value for 4-MeOC6H4COCF3 (-13.8 ppm) [2c] is close to that for 4-MeOC6H4COMe (-15.2 ppm). It has been descibed that a C=C double bond inserted between the 4-substituted phenyl and carbonyl group results in a reduction of the substituent effects by approximately 50% [11]. This is true for 4-MeOC6H4CH=CHCOX (X = Me and MeO). In contrast, the Δδ value for 4-MeOC6H4CH=CHCOCF3 (-49.4 ppm) is considerably increased instead of being reduced. The greater shielding observed for 4-MeOC6H4CH=CHCOCF3 is consistent with the reduction of the dou-ble bond character of the carbonyl group, suggesting the important contribution of the resonance structure B (Scheme 2). Accordingly, a deshielding of 6.7 ppm for the 4-MeO group in 4-MeOC6H4CH=CHCOCF3, as compared with 8b, is in agreement with the corresponding increased dou-ble bond character between the 4-MeO oxygen and phenyl group, as shown in B.
The CN group attached to the carbonyl group causes considerable deshielding of the carbonyl O atom, as compared with the corresponding Me derivative. The deshielding effect depends upon the structures and the number of β-substitutents. The deshielding is 34.6 ppm for acetyl cyanide (1m), as compared with 1b. In the enhanced conjugation systems 2m and 4m, the deshielding effect of the CN group is reduced: 9.5 ppm for 2m and 10.6 ppm for 4m as compared with 2b and 4b, respectively. The deshielding was diminished by increasing the number of methyl groups at β-position of acroloyl cya-nide (3m), ca. 7 ppm per methyl group: 21.0 ppm for 3m, 13.3 ppm for 5m and 7.4 ppm for 11m in relation to 3b, 5b and 11b, respectively. The diminished deshielding effects of the CN group in α,β-unsaturated system may attributed to the competition between the deshielding contribution arising from the inductive effects of the CN group and the shielding contribution arising from the cross-conjugation of the carbonyl group with CN group. The consequence of the latter effect would be ex-pected to reduce the double bond character of the carbonyl group. The other reason for the observed deshielding of the CN derivatives compared with Me analogs is probably an effect due to the change of the energy of n-π* transition.
Similar results were observed for the -COCO2R derivatives 1n, 2n, 3n, 4n, 5n, 10n and 11o (Table 1 and Table 2) as shown by their 17O data. A deshielding of 14.1 ppm is observed for ethyl pyruvate (1n) relative to acetone (1b); whereas the shielding (-5.8 to -22.7 ppm) is observed for 2n-10n and 11o relative to the corresponding -COMe derivatives 2b-10b and 11b. The large shielding effect indicates an enhanced interaction of the substituent with the electron withdrawing CO2R. The greater shielding of -51 ppm previously observed for 8n (404.7 ppm) as compared with 8b (455.7 ppm) has been explained in terms of the extended bond polarization and the increased n,π-conjugation in the N-C=C-C=O system [9].
In esters (3h, 3i, 4h, 4i, 5i and 6i) and chlorides (3j, 4j, 5j and 6j), as previously noted for saturated analogs [1], the increase in shielding of the O atom is attributed to the decrease in the π-bond order of C=O, explained by the resonance of the lone pair of electrons on O or Cl atom with the π-bond of the C=O group. The smaller shielding for the chlorides results from the interaction of the Cl atom with the C=O group is weaker, as compared with the ester derivatives.
The 17O chemical shifts of MeCOX (1), PhCOX (2) and α,β-unsaturated compounds RCH=CHCOX (3-6) show that the shielding of the O atoms has the same trend and is enhanced with increasing the electron donating power of the X group. Good correlations are found between the δ(17O) values of 2-6 and those of 1 [Eqs. (3) and (7)], indicating that the factors which affect the shielding of the O atom in 2-6 are parallel to those in 1. The near-unity values of the slope of the correlation lines suggest that the steric and electronic effects of the substituents X on δ(17O) values are essentially similar in MeCOX (1), PhCOX (2) and RCH=CHCOX (3-6).
δ(17O)(2) = 0.96 δ(17O)(1) - 0.9 (n = 13, r = 0.981, SD = 16.0)
δ(17O)(3) = 1.00 δ(17O)(1) - 20.4 (n = 11, r = 0.995, SD = 8.9)
δ(17O)(4) = 0.97 δ(17O)(1) - 10.8 (n = 13, r = 0.990, SD = 11.7)
δ(17O)(5) = 0.95 δ(17O)(1) - 5.0 (n = 6, r = 0.995, SD = 9.9)
δ(17O)(6) = 1.03 δ(17O)(1) - 16.1 (n = 4, r = 0.999, SD = 7.1)
(point for 7o was included)
Resonance constants σRo, σR, σR- and σR+ are considered to represent the resonance donor capacity of X [12]. The best correlations of the 17O shift values of the carbonyl O atoms of 1-6 are with σR+ constants of X [Eqs. (8) and (13)]. There are no acceptable correlations between the 17O shift values of 1-6 and σ+ constants [12] of X. These results demonstrate that the contribution of resonance structure D [R-CO-X (C) ↔ R-C(O-)=X+ (D)] in various carbonyl compounds RCOX is very important in the ground state of the molecules; the donor power of X plays a decisive role in determining the δ(17O) values of the carbonyl -COX group and in their electrophilicities.
δ(17O)(1) = 200.4 σR+ + 599.7 (n = 12, r = 0.979, SD = 17.7)
δ(17O)(2) = 188.4 σR+ + 572.8 (n = 12, r = 0.941, SD = 29.1)
δ(17O)(3) = 193.9 σR+ + 569.7 (n = 12, r = 0.968, SD = 21.5)
δ(17O)(4) = 204.9 σR+ + 584.6 (n = 12, r = 0.972, SD = 21.8)
δ(17O)(5) = 185.6 σR+ + 561.6 (n = 6, r = 0.956, SD = 29.2)
δ(17O)(6) = 201.8 σR+ + 592.0 (n = 4, r = 0.984, SD = 24.8)
(point for 7o was included)

Effects of the β-substituent R in RCH=CHCOX

The influence of the β-substituent R in RCH=CHCOX is clearly reflected by the 17O shift value: an electron donating group (R = Me, OR, NMe2) causes shielding; whereas an electron withdrawing causes deshielding (R = CO2Me, CO2Et). The shielding of the β-phenyl group (R = Ph, Table 1) in various α,β-unsaturated systems (4a - 4n), as compared with those having the corresponding β-unsubstituted analogs (3a - 3n), can be explained as resulting from the extended conjugation of the C=C-C=O system with the benzene ring. The shielding effect of the β-substituent R, i.e. the chemical shift difference between RCH=CHCOX and the corresponding CH2=CHCOX: Δδ = δ(17O)R - δ(17O)H, depends on a function of the substituent X attached to the carbonyl carbon. In β-phenyl series (4), the Δδ value for the electron-withdrawing CN group (4m) is -19 ppm, whereas for the electron donor EtO group (4i) is only -3 ppm. The negative value of the Δδ is increased in the order EtO < Me < H < Cl < CO2Et < CN. The best correlation of the Δδ values is with σ+ constants [12] of the corresponding X (Δδ = -11.5 - 10.3 σ+, N = 11, r = 0.982, SD = 0.9). The shielding direction of a β-methyl group (R = Me) in various compounds MeCH=CHCOX is similar to that of the β-phenyl group, and shows slightly more sensitive to X.
It has been shown by Dahn that the resonance effects of electron donating geminal groups X in p-YC6H4COX are particularly important [2a,c]. The shielding of the O atom is increased and the sensi-tivity of the Y substituent is diminished with an increase in the donating power of the X groups. The present results are consistent with this conclusion.

Experimental

Materials

Compounds 3d-3f [13], 3m [14], 4m [14], 5m [14], 6m [14], 3n [15], 4c-4e [16], 4g [16], 4n [17], 4k [18], 5n [19], 7o [20], 11o [21], 4-(4-methoxyphenyl)-but-3-en-2-one [22] and 4-(4-methoxyphenyl)-1,1,1-triflorobut-3-en-2-one [23] were prepared by literature procedures. The re-maining coumpounds were commercially available (Fluka AG).

17O NMR Spectroscopy

The 17O NMR spectra were recorded on a Bruker-WH-360 spectrometer, equipped with a 10-mm probe, at 48.8 MHz, in the fourier transform (FT) mode without lock. System control, data acquisi-tions, and data managements were performed by an Aspect-2000 microcomputer. Instrumental set-tings: spectral width 50 000 Hz (1025 ppm), 2 K data points, pulse width 33 μs, acquisition time 20 ms, preacquisition delay 5 μs, 200 000 - 500 000 scans, sample spinning (28 Hz). An even number (12-28) left-shifts (LS) were applied to FID signal; the latter was zero-filled to 8 K words and expo-nentially multiplied with 100-Hz line-broadening factor (LB) before being subjected to the FT. The chemical shifts δo, measured in 0.5 M acetonitrile solution at 40°C at natural isotopic abundance, are reported relative to δo(H2O) (=0.0 ppm); dioxane (δo = 0 ppm) was used as an external standard; downfield shifts are positive. The general reproducibility of chemical shifts values is ca. ± 1 ppm (± 0.2 ppm within the same series).

Acknoledgement 

The author is indebted to Professor Hans Dahn for valuable comments, and to Professor Hugo Wyler for helpful discussions and financial support (from the Swiss National Science Foundation).

References and Notes

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  2. Dahn, H.; Péchy, P.; Toan, V. V. Angew. Chem. Int. Ed. Engl. 1990, 29, 647. Dahn, H.; Carrupt, P.-A. Magn. Reson. Chem. 1997, 35, 577. Dahn, H.; Péchy, P.; Toan, V. V. Magn. Reson. Chem. 1997, 35, 589. Christ, H. A.; Diehl, P.; Schneider, H. P.; Dahn, H. Helv. Chim. Acta 1961, 44, 865.
  3. Patai, S.; Rappoport, Z. (Eds.) The chemistry of Enones; Wiley: Chichester, 1989. Habermas, K. L.; Denmark, S. E.; Jones, T. K. Org. React. 1994, 45, 1. Reich, H. J.; Wollowitz, S. Org. React. 1993, 44, 1. Crimmins, M. T.; Reinhold, T. L. Org. React. 1993, 44, 297. Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135. Chapdelaine, M. J.; Hulce, M. Org. React. 1990, 38, 225. Nagata, W.; Yoshioka, M. Org. React. 1977, 25, 225. Caine, D. Org. React. 1976, 23, 1. House, H. O. Acc. Chem. Res. 1976, 9, 59.
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  15. Jacobi, P. A.; Cai, G. Heterocycles 1993, 35, 1103.
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  17. Le Corre, M. C. R. Acad. Sci. Paris Serie C 1970, 270, 1312. Meijer, L. H. P.; Pandit, U. K. Tetrahedron 1985, 41, 467.
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  23. Zhuo, J.-C. unpublished results.
  • Samples Availability: Not available.
Scheme 1.
Scheme 1.
Molecules 04 00320 sch001
Scheme 2.
Scheme 2.
Molecules 04 00320 sch002
Table 1. 17O NMR chemical shifts of 1-4 (solvent MeCN at 40°C)a.
Table 1. 17O NMR chemical shifts of 1-4 (solvent MeCN at 40°C)a.
XMeCOXPhCOXCH2=CHCOXPhCH=CHCOXΔδb
H1a596.62a561.43a584.14a572.811.3
Me1b568.82b548.43b562.64b554.38.3
Et1c556.42c537.23c545.74c535.610.1
i-Pr1d553.62d532.53d533.34d523.89.5
t-Bu1e556.62e563.2c3e522.14e514.57.6
Ph1f548.42f549.63f523.44f513.79.7
PhCH=CH1g554.32g513.7--4g505.0-
OMed1h359.02h337.53h339.24h335.93.3
OEte1i358.52i337.03i338.14i335.52.6
Cl1j504.42j483.43j492.74j478.913.8
CF31k573.82k543.6f--4k561.5-
CN1m603.42m557.93m583.64m564.918.7
CO2Etg1n582.92n538.2f3n553.94n538.914.7
a) Line-width (in Hz) at half-height: 1a-n 40-240, 2a-n 80-270, 3a-n 40-190, 4a-n 120-540 except for 4g 730; b) Δδ = δ (4) - δ (3); c) Taken from Ref. 7; d) δ(OMe) for 1h: 140.1, 2h: 127.3, 3h: 133.4, 4h: 133.2 ppm; e) δ(OEt) for 1i: 170.9, 2i: 157.4, 3i: 163.2, 4i: 164.8 ppm; f) Taken from Ref. 2a. g) COOEt: δ(CO) for 1n: 345.4, 3n: 350.2, 4n: 349.5 ppm; δ(OEt) for 1n: 161.2, 3n: 164.3, 4n: 170.1 ppm.
Table 2. 17O NMR chemical shifts of 5-11 (solvent MeCN at 40°C)a.
Table 2. 17O NMR chemical shifts of 5-11 (solvent MeCN at 40°C)a.
XMeCH=CHCOXRO2CH=CHCOXbMe2NCH=CHCOXcROCH=CHCOXbMe2C=CHCOX
H5a564.6--8a459.7----
Me5b549.1--8b455.79bd521.711b547.3
OEt5ie333.36if351.68i295.2--11hg342.6
Cl5j476.66jh511.9------
CN5m562.46mi608.3----11m554.7
CO2R5nj536.67ok579.38n404.710nl499.011om541.5
a) Line-width (in Hz) at half-height: 50-310; b) R = Et unless indicated otherwise; c) Taken from Ref 9; d) R = Me, δ(OMe) = 75.2 ppm; e) δ(OEt) = 161.9 ppm; f) δ(OEt) = 168.3 ppm; g) Compound 11h: Me2C=CHCOX, δ(OMe) = 168.3 ppm; h) COOEt: δ(CO) = 357.4 ppm; δ(OEt) = 171.1 ppm; i) COOEt: δ(CO) = 359.5 ppm; δ(OEt) = 171.9 ppm; j) COOEt: δ(CO) = 350.2 ppm: δ(OEt) = 164.7 ppm; k) R = Me. 1- and 2-COOMe: δ(CO) = 354.2 and 351.3 ppm; δ(OMe) = 140.0 and 134.6 ppm; l) R = OEt, δ(3-OEt) = 121.7 ppm; COOEt: δ(CO) = 347.1 ppm, δ(OEt) = 164.1 ppm; m) R = Me: δ(CO) = 344.4 ppm; δ(OMe) = 132.0 ppm.
Table 3. 17O NMR data of 4-YC6H4COX and 4-YC6H4CH=CHCOX (solvent MeCN, unless indicated otherwise, at 40°C)a.
Table 3. 17O NMR data of 4-YC6H4COX and 4-YC6H4CH=CHCOX (solvent MeCN, unless indicated otherwise, at 40°C)a.
X4-YC6H4COX Δδb4-YC6H4CH=CHCOX Δδb
Y = HY = OMeY = HY = OMe
Me548.4533.2c-15.2554.3546.8d-7.5
OMe337.5e330.5f-7.0335.9g332.5h-3.4
CF3554.0i530.8i-13.2561.5512.1j-49.4
a) Line-width (in Hz) at half-height: 120-440; b) Δδ = δ(4-MeO) - δ(H); c) δ(4-OMe) = 59.9 ppm; d) δ(4-OMe) = 56.3 ppm; e) δ(OMe) = 127.3 ppm; f) δ(OMe) = 125.9 ppm; δ(4-OMe) = 57.5 ppm; g) δ(OMe) = 133.2 ppm; h) δ(OMe) = 132.5 ppm; δ(4-OMe) = 54.7 ppm; i) Measurement in CCl4, taken from Ref 2a; j) δ(4-OMe) = 63.0 ppm.

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MDPI and ACS Style

Zhuo, J.-C. 17O NMR Spectra of α,β-Unsaturated Carbonyl Compounds RCH=CHCOX: the Influence of Group X on the δ(17O) Value of the Carbonyl Oxygen and on the Shielding Effect of Group R. Molecules 1999, 4, 320-328. https://doi.org/10.3390/41100320

AMA Style

Zhuo J-C. 17O NMR Spectra of α,β-Unsaturated Carbonyl Compounds RCH=CHCOX: the Influence of Group X on the δ(17O) Value of the Carbonyl Oxygen and on the Shielding Effect of Group R. Molecules. 1999; 4(11):320-328. https://doi.org/10.3390/41100320

Chicago/Turabian Style

Zhuo, Jin-Cong. 1999. "17O NMR Spectra of α,β-Unsaturated Carbonyl Compounds RCH=CHCOX: the Influence of Group X on the δ(17O) Value of the Carbonyl Oxygen and on the Shielding Effect of Group R" Molecules 4, no. 11: 320-328. https://doi.org/10.3390/41100320

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