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Natural abundance 17O NMR study of α-substituted methyl acetates
oss4-6539/89 13.00+0.00 0 1989Pergamon Pressplc
SpectrochimicaAcra, Vol. 4SA. No. 3, pp. 335-338, 1989. Printedin GreatBritain.
Natural abundance 170 NMR study of a-substituted methyl acetates D. W. BOYKIN, T. S. SUBRAMANIAN
and A. L.
BAUMSTARK
Department of Chemistry and LMBS, Georgia State University, Atlanta, GA 30303-3083, U.S.A. (Received 25 July 1988) Abstract-Natural abundance “ONMR chemical shift data for 10 substituted methyl acetates and 7 analogs, recorded in acetonitrile at 75”C, are reported. Variation in the carbonyl and single-bond oxygen signal are observed for formal c+substitution and do not correlate with inductive effects. The data appear to be consistent with y effects for analogous systems.
water at 75°C. The 2-butanone resonance (558 + 1 ppm) was used as an internal check on the chemical shift measurements for these compounds. The instrumental settings for the GX270 at 36.5 MHz were: spectral width 25 kHz, 2k data points, 90” pulse angle (28 ms pulse width), 200 ms acquisition delay, 40ms acquisition time, and 4000&100000 scans were required. The instrumental settings for the VXR-400 at 54.22 MHz were: spectral width 35 kHz, 2k data points, 90 pulse angle (4Oms. pulse width), 200ms acquisition delay, 29 ms acquisition time. and 20000-40000 scans. The spectra were recdrded with sample spinning and without lock The signal-to-noise ratio was improved by applying a 25Hz exponential broadening factor to the FID prior to Fourier transformation. The data point resolution was improved to k 0.1 ppm on the VXR-400 and &-0.2 ppm on the GX-270 by zero filling to 8k data points. The reproducibility of the chemical shift data is estimated to be better than k l.Oppm.
INTRODUCTION
“0 NMR studies of esters have shown that signals for both the carbonyl oxygen and the single-bonded oxygen are sensitive to electronic effects and steric effects independent of the point of attachment of the substituent. The “0 chemical shifts of aromatic esters have been shown to be sensitive to the electronic effects of substituents [l-3]; the double-bonded oxygen is approximately twice as sensitive to electronic effects as the single-bonded oxygen [2]. Quantitative relationships have been found between 1‘0 chemical shifts and torsion angles for aromatic esters [4]. Change in structure of the group directly attached to the carbonyl group produces significant changes in the position of the carbonyl “0 resonance; however, the magnitude of the shift for the signal from the singlebonded oxygen is somewhat smaller. Variation of the group attached to the single-bonded oxygen causes a large change in the chemical shift of the resonance for the single-bonded oxygen, and although less pronounced, a detectable effect is observed on the carbony1 oxygen resonance [S-7]. Recently, the influence of electronic and steric effects on the “0 NMR chemical shifts of alkyl isocyanates was reported and an interesting shielding effect was observed on the acetyl carbonyl signal for chloro- and trichloroacetyl isocyanate [S], which was in the opposite direction of expectation based upon a simple a inductive effect. As a consequence of this observation we chose to investigate the influence of substituents on the “ONMR chemical shifts of methyl acetates and wish to report the generality of the effects.
RESULTS
The “ONMR chemical shift data for 10 substituted methyl acetates and 7 analogs were recorded at 75°C in acetonitrile (0.5 M) at natural abundance and are listed in Table 1. As expected, two signals were observed for esters l-17; the signals for the carbonyl oxygen ranged from 345 to 363 ppm, while the singlebonded oxygen signals appeared between 128 and 142 ppm. Successive substitution of a chlorine atom tl to the carbonyl group of methyl acetate (Z-4) resulted in an approximately Sppm upfield shift per chlorine atom for the carbonyl signal and a somewhat smaller upfield shift for the single-bonded oxygen resonance (see Fig. 1). Traces of acid could potentially cause upfield shifts of the carbonyl resonance [4b]. In a control experiment employing methyl butyrate, 2% added trifluoroacetic acid resulted in only a modest 0.9ppm upfield shift for the carbonyl signal; the “0 NMR signal for the added acid (256 ppm) was also readily observed. No signals for traces of acids were observed in the ester spectra. Consequently, the upfield shifts noted here cannot result from traces of acid. It appears from the data for methyl trifluoroacetate (6) that the upfield influence of fluorine is less than that for chlorine. Small shielding effects have been noted for the “0 chemical values from the SO, group of methyl methanesulfonate and for methanesulfonyl chloride upon trifluorosubstitution [9]. Substitution
EXPERIMENTAL
The esters (1-17) used in this study were commercially available from Aldrich or were prepared by standard reactions from their corresponding acids. The “ONMR spectra were recorded on a JEOL GX-270 or on a Varian VXR-400 spectrometer equipped with a 10 mm broad-band probe. All spectra were acquired at natural abundance, at 75°C in acetonitrile (Aldrich, anhydrous gold label under nitrogen) containing 1% 2-butanone as an internal standard. The concentration of the esters employed in these experiments was 0.5 M. The signals were referenced to external deionized 335
336
D. W. Table
1. r’0 chemical and analogs
Compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
BOYKIN et al.
shifts (f 1 ppm) for substituted methyl (R-CO&H,) in acetonitrile at 75”C*
R
6(c=o)*
CH,
CH,Br CH,OCH,t CH(OCH,),: CH,CN CH;CO,CH, CH,CH(OCH,M CH,CH,N(CH,), CH,CH, CH,CH,CH, CH(CH,), C(CH,),
*Shifts referenced to external water + 1 ppm) used as an internal check t - 27.7 ppm. $15.3 ppm. $21.8 ppm.
with a methoxy group, which is electronegatively comparable inductively to a chlorine atom, also produced upfield shifts of the carbonyl signal as indicated by 8 and 9. On the other hand, the monobromo compound 7 showed no change in the carbonyl chemical shift; however, a change was noted for its single bonded oxygen signal. Small upfield shifts of the carbonyl signal have been reported for several c(bromo t-butyl esters [7]. Other electron withdrawing groups which might be expected to exert a significant inductive effect only produce slight downfield shifts of the carbonyl resonance; compare the data for methyl cyanoacetate (10) and dimethylmalonate (11) with that of methyl acetate (1). Calculations (molecular mechanics) on the mono a-substituted methyl acetates showed no unusual conformational differences irrespective of the “0 chemical shift data. The effects of the various substituents were also noted for the resonance of the single-bonded oxygen; these effects were similar to those described above for the carbonyl “0 NMR chemical shift data. The effect of variation in alkyl groups (14-17) on the single bonded oxygen signal is consistent with y effects (shielding) deducible from data on analogous ethers [lo]. In several cases the single bonded oxygen signal is affected more than the carbonyl oxygen signal. For several of the monosubstituted acetates with electron withdrawing groups (2,7, S), a y-effect is noted on the single bonded oxygen signal which is also shielding relative to 1. In contrast, for compounds lo-12 little (slight deshielding) or no effect is noted. No data for model compounds have been reported. These results, at present, may be regarded as normal y-effects.
6(-O-CH,)*
361.0 357.1 351.0 345.5 346.5 352.8 361.0 352.5 354.1 362.0 363.1 359.1 356.3 353.5 355.5 348.3 347.6
CH,CI CHCI, CCI, CCI,F CF,
(Oppm);
acetates
141.2 135.8 133.8 128.6 130.5 133.0 138.1 131.7 134.8 140.7 142.3 141.3 139.1 136.7 138.3 134.7 131.1 1%
2-butanone
(558
DISCUSSION
The range in carbonyl “0 NMR chemical shift for alkyl substituted esters 1417 is generally less than noted for comparable aldehydes [ 111,roughly similar to that of analogous ketones [ll] and somewhat greater than that noted for alkyl group change in a series of cyclic amides [ 121. The chemical shift differences for the carbonyl resonances between 1 and 15 or 16 are about 20% less than noted for the corresponding t-butyl acetates [7]. Changes in substitution by varying alkyl groups follow chemical shift patterns which have been recognized for the carbonyl signal of substituted t-butyl acetates [7], for aldehydes [I 11, for ketones [I l] and cyclic amides [12] and which thus can be described in terms of y and 6 effects. It was suggested earlier [S] that 0 inductive effects significantly influence the I70 chemical shifts of alkyl isocyanates since the resonance of chloromethylisocyanate is downfield of that for ethyl isocyanate by 33ppm. This downfield shift was in the opposite direction to that expected for a 6 effect [S]. Although not as dramatic, a similar effect is apparent for the esters under study here since the carbonyl oxygen shift value for 2 is deshielded from that of 14 by 3Sppm. The influence of chlorine substitution on the carbony1 “0 chemical shift of methyl acetates is, as noted above, shielding by approximately 5 ppm per chlorine atom (14). A shielding effect for chlorine atom substituion has also been noted on the r3C chemical shifts for related compounds [13]. The substitution of methyl groups c( to the carbonyl of methyl acetate results in similar upfield shifts (ca 4ppm per methyl
337
“0 NMR of a-substituted methyl acetates
CLCHl
b(,.CH,
!
Fig. 1. Natural abundance “0 NMR spectra for methyl chloroacetate (2), methyl dichloroacetate (3) and methyl trichloroacetate (4).
group) of the carbonyl “0 resonance; compare 1, 14, 16 and 17. Consequently, despite the large difference in electronegativity there appears little difference in the y effect of a methyl group and a chlorine atom. The role that a chlorine atom plays in determining “0 chemical shifts varies as a function of the functional group that is being detected and the location of the chlorine atom relative to the functional group. For example, the effect of a directly-bonded chlorine (and other halogens) on the carbonyl “ONMR signal of acid halides (RCOX) is shielding [14]; however, the “0 resonance of sulphonyl chlorides (RSO,CI) are significantly deshielded [9]. The former was explained in terms of resonance interaction leading to an increase in electron density on the carbonyl oxygen whereas the latter was attributed to interaction between oxygen lone-pairs and the anti-bonding orbital of the SAtA)45:3-B
S-Cl bond such that a decrease in electronic charge on oxygen occurred. For the case in which chlorine is not directly bonded to the carbonyl group e inductive effects were expected to be important and to be deshielding, yet shielding is observed for the esters reported here and for a ketone recently published [14]. Consequently, it appears that any u inductive effect is overridden by other factors. The results for the esters are thus consistent with those obtained from the isocyanates [8]. SUMMARY
The y-effect of the alkyl substituents on the esters is consistent with previous results on other systems. Several classes of substituents have not been previously examined in detail. The a-inductive effects do
D. W. BOYKINet al.
338
not correlate with the “0 chemical shift data. A possible explanation of these effects may be found in the model of LI and CHESNUT[15] in which y-effects are postulated to result from competition between attractive and repulsive van der Waals interactions [ 153. Consequently, it appears that “0 chemical shift data for all the a-substituents can be regarded as resulting from y-effects.
[3] D. MONTI,F. ORSINIand G. S. RICCA,Spectrosc. Lett. 19, 91 (1986). r41 (a) A. L. BAUMSTARK, P. BALAKRISHNAN, M. DOTRONG, __ C. J. MCCLOSKEY,M. G. OAKLEYand D. W. BOYKIN, J. Am. them. Sot. 109.1059 (1987): (b) For a review see D. W. BOYKINand A. L. B,&Ms;A&, Tetrahedron, in press. f51_ T. SUGAWARA.Y. KAWADA and H. IWAMURA,Chem. _ Lett. 1371 (1978). f61 C. DELSETH.T. T.-T. NGUYEN and J. P. KINTZINGER. Helv. Chim. Acta 63, 498 (1980). [7] F. ORSINIand G. S. RICCA, Org. magn. Reson. 22, 653 (1984). B-l b. W: BOYKIN,J. Chem. Res. 338 (1987). C. CHATGILIALOGLU, S. ROSSINIand 593 G. BARBARELLA, V. TUGNOLI,J. magn. Reson. 70, 204 (1986). Cl01 C. DELSETHand J.-P. KINTZINGER,He/u. Chim. Acta 61, 1327 (1978). He/u. Chim. Acta 59, [Ill C. DELSETHand J.-P. KINTZINGER, 466 (1976). WI D. W. BOYKIN,Heterocycles 26, 773 (1987). 1131G. C. LEVY, R. L. LICHTERand G. L. NELSON,Carbon13 Nuclear Magnetic Resonance Spectroscopy, 2nd edn, p. 140. Wiley, New York (1980). Cl41 C. P. CHENG, S. C. LIN and G.-S. SHAW,.I. magn. Reson. 69, 58 (1986). [IS] S. LI and D. B. CHESNUT,Magn. Reson. Chem. 23,625 (1985). L
Acknowledyements~Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, to the NSF Instr;mentaiion Pro&m (CHEM8409599), and to the Georgia State University Research Fund.
REFERENCES
[l] E. E. LIEPINSH,1. A. ZITSMANE,L. M. IGNATOVICH,E. LUKEVITS,L. I. GUVANOVAand M. G. VORONKOV,Zh. ohshch Khim. 53, 1789 (1983). [Z] P. BALAKRISHNAN,A. L. BAUMSTARKand D. W. BOYKIN,Org. magn. Reson. 22, 753 (1984).
A
SpectrochimicaAcra, Vol. 4SA. No. 3, pp. 335-338, 1989. Printedin GreatBritain.
Natural abundance 170 NMR study of a-substituted methyl acetates D. W. BOYKIN, T. S. SUBRAMANIAN
and A. L.
BAUMSTARK
Department of Chemistry and LMBS, Georgia State University, Atlanta, GA 30303-3083, U.S.A. (Received 25 July 1988) Abstract-Natural abundance “ONMR chemical shift data for 10 substituted methyl acetates and 7 analogs, recorded in acetonitrile at 75”C, are reported. Variation in the carbonyl and single-bond oxygen signal are observed for formal c+substitution and do not correlate with inductive effects. The data appear to be consistent with y effects for analogous systems.
water at 75°C. The 2-butanone resonance (558 + 1 ppm) was used as an internal check on the chemical shift measurements for these compounds. The instrumental settings for the GX270 at 36.5 MHz were: spectral width 25 kHz, 2k data points, 90” pulse angle (28 ms pulse width), 200 ms acquisition delay, 40ms acquisition time, and 4000&100000 scans were required. The instrumental settings for the VXR-400 at 54.22 MHz were: spectral width 35 kHz, 2k data points, 90 pulse angle (4Oms. pulse width), 200ms acquisition delay, 29 ms acquisition time. and 20000-40000 scans. The spectra were recdrded with sample spinning and without lock The signal-to-noise ratio was improved by applying a 25Hz exponential broadening factor to the FID prior to Fourier transformation. The data point resolution was improved to k 0.1 ppm on the VXR-400 and &-0.2 ppm on the GX-270 by zero filling to 8k data points. The reproducibility of the chemical shift data is estimated to be better than k l.Oppm.
INTRODUCTION
“0 NMR studies of esters have shown that signals for both the carbonyl oxygen and the single-bonded oxygen are sensitive to electronic effects and steric effects independent of the point of attachment of the substituent. The “0 chemical shifts of aromatic esters have been shown to be sensitive to the electronic effects of substituents [l-3]; the double-bonded oxygen is approximately twice as sensitive to electronic effects as the single-bonded oxygen [2]. Quantitative relationships have been found between 1‘0 chemical shifts and torsion angles for aromatic esters [4]. Change in structure of the group directly attached to the carbonyl group produces significant changes in the position of the carbonyl “0 resonance; however, the magnitude of the shift for the signal from the singlebonded oxygen is somewhat smaller. Variation of the group attached to the single-bonded oxygen causes a large change in the chemical shift of the resonance for the single-bonded oxygen, and although less pronounced, a detectable effect is observed on the carbony1 oxygen resonance [S-7]. Recently, the influence of electronic and steric effects on the “0 NMR chemical shifts of alkyl isocyanates was reported and an interesting shielding effect was observed on the acetyl carbonyl signal for chloro- and trichloroacetyl isocyanate [S], which was in the opposite direction of expectation based upon a simple a inductive effect. As a consequence of this observation we chose to investigate the influence of substituents on the “ONMR chemical shifts of methyl acetates and wish to report the generality of the effects.
RESULTS
The “ONMR chemical shift data for 10 substituted methyl acetates and 7 analogs were recorded at 75°C in acetonitrile (0.5 M) at natural abundance and are listed in Table 1. As expected, two signals were observed for esters l-17; the signals for the carbonyl oxygen ranged from 345 to 363 ppm, while the singlebonded oxygen signals appeared between 128 and 142 ppm. Successive substitution of a chlorine atom tl to the carbonyl group of methyl acetate (Z-4) resulted in an approximately Sppm upfield shift per chlorine atom for the carbonyl signal and a somewhat smaller upfield shift for the single-bonded oxygen resonance (see Fig. 1). Traces of acid could potentially cause upfield shifts of the carbonyl resonance [4b]. In a control experiment employing methyl butyrate, 2% added trifluoroacetic acid resulted in only a modest 0.9ppm upfield shift for the carbonyl signal; the “0 NMR signal for the added acid (256 ppm) was also readily observed. No signals for traces of acids were observed in the ester spectra. Consequently, the upfield shifts noted here cannot result from traces of acid. It appears from the data for methyl trifluoroacetate (6) that the upfield influence of fluorine is less than that for chlorine. Small shielding effects have been noted for the “0 chemical values from the SO, group of methyl methanesulfonate and for methanesulfonyl chloride upon trifluorosubstitution [9]. Substitution
EXPERIMENTAL
The esters (1-17) used in this study were commercially available from Aldrich or were prepared by standard reactions from their corresponding acids. The “ONMR spectra were recorded on a JEOL GX-270 or on a Varian VXR-400 spectrometer equipped with a 10 mm broad-band probe. All spectra were acquired at natural abundance, at 75°C in acetonitrile (Aldrich, anhydrous gold label under nitrogen) containing 1% 2-butanone as an internal standard. The concentration of the esters employed in these experiments was 0.5 M. The signals were referenced to external deionized 335
336
D. W. Table
1. r’0 chemical and analogs
Compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
BOYKIN et al.
shifts (f 1 ppm) for substituted methyl (R-CO&H,) in acetonitrile at 75”C*
R
6(c=o)*
CH,
CH,Br CH,OCH,t CH(OCH,),: CH,CN CH;CO,CH, CH,CH(OCH,M CH,CH,N(CH,), CH,CH, CH,CH,CH, CH(CH,), C(CH,),
*Shifts referenced to external water + 1 ppm) used as an internal check t - 27.7 ppm. $15.3 ppm. $21.8 ppm.
with a methoxy group, which is electronegatively comparable inductively to a chlorine atom, also produced upfield shifts of the carbonyl signal as indicated by 8 and 9. On the other hand, the monobromo compound 7 showed no change in the carbonyl chemical shift; however, a change was noted for its single bonded oxygen signal. Small upfield shifts of the carbonyl signal have been reported for several c(bromo t-butyl esters [7]. Other electron withdrawing groups which might be expected to exert a significant inductive effect only produce slight downfield shifts of the carbonyl resonance; compare the data for methyl cyanoacetate (10) and dimethylmalonate (11) with that of methyl acetate (1). Calculations (molecular mechanics) on the mono a-substituted methyl acetates showed no unusual conformational differences irrespective of the “0 chemical shift data. The effects of the various substituents were also noted for the resonance of the single-bonded oxygen; these effects were similar to those described above for the carbonyl “0 NMR chemical shift data. The effect of variation in alkyl groups (14-17) on the single bonded oxygen signal is consistent with y effects (shielding) deducible from data on analogous ethers [lo]. In several cases the single bonded oxygen signal is affected more than the carbonyl oxygen signal. For several of the monosubstituted acetates with electron withdrawing groups (2,7, S), a y-effect is noted on the single bonded oxygen signal which is also shielding relative to 1. In contrast, for compounds lo-12 little (slight deshielding) or no effect is noted. No data for model compounds have been reported. These results, at present, may be regarded as normal y-effects.
6(-O-CH,)*
361.0 357.1 351.0 345.5 346.5 352.8 361.0 352.5 354.1 362.0 363.1 359.1 356.3 353.5 355.5 348.3 347.6
CH,CI CHCI, CCI, CCI,F CF,
(Oppm);
acetates
141.2 135.8 133.8 128.6 130.5 133.0 138.1 131.7 134.8 140.7 142.3 141.3 139.1 136.7 138.3 134.7 131.1 1%
2-butanone
(558
DISCUSSION
The range in carbonyl “0 NMR chemical shift for alkyl substituted esters 1417 is generally less than noted for comparable aldehydes [ 111,roughly similar to that of analogous ketones [ll] and somewhat greater than that noted for alkyl group change in a series of cyclic amides [ 121. The chemical shift differences for the carbonyl resonances between 1 and 15 or 16 are about 20% less than noted for the corresponding t-butyl acetates [7]. Changes in substitution by varying alkyl groups follow chemical shift patterns which have been recognized for the carbonyl signal of substituted t-butyl acetates [7], for aldehydes [I 11, for ketones [I l] and cyclic amides [12] and which thus can be described in terms of y and 6 effects. It was suggested earlier [S] that 0 inductive effects significantly influence the I70 chemical shifts of alkyl isocyanates since the resonance of chloromethylisocyanate is downfield of that for ethyl isocyanate by 33ppm. This downfield shift was in the opposite direction to that expected for a 6 effect [S]. Although not as dramatic, a similar effect is apparent for the esters under study here since the carbonyl oxygen shift value for 2 is deshielded from that of 14 by 3Sppm. The influence of chlorine substitution on the carbony1 “0 chemical shift of methyl acetates is, as noted above, shielding by approximately 5 ppm per chlorine atom (14). A shielding effect for chlorine atom substituion has also been noted on the r3C chemical shifts for related compounds [13]. The substitution of methyl groups c( to the carbonyl of methyl acetate results in similar upfield shifts (ca 4ppm per methyl
337
“0 NMR of a-substituted methyl acetates
CLCHl
b(,.CH,
!
Fig. 1. Natural abundance “0 NMR spectra for methyl chloroacetate (2), methyl dichloroacetate (3) and methyl trichloroacetate (4).
group) of the carbonyl “0 resonance; compare 1, 14, 16 and 17. Consequently, despite the large difference in electronegativity there appears little difference in the y effect of a methyl group and a chlorine atom. The role that a chlorine atom plays in determining “0 chemical shifts varies as a function of the functional group that is being detected and the location of the chlorine atom relative to the functional group. For example, the effect of a directly-bonded chlorine (and other halogens) on the carbonyl “ONMR signal of acid halides (RCOX) is shielding [14]; however, the “0 resonance of sulphonyl chlorides (RSO,CI) are significantly deshielded [9]. The former was explained in terms of resonance interaction leading to an increase in electron density on the carbonyl oxygen whereas the latter was attributed to interaction between oxygen lone-pairs and the anti-bonding orbital of the SAtA)45:3-B
S-Cl bond such that a decrease in electronic charge on oxygen occurred. For the case in which chlorine is not directly bonded to the carbonyl group e inductive effects were expected to be important and to be deshielding, yet shielding is observed for the esters reported here and for a ketone recently published [14]. Consequently, it appears that any u inductive effect is overridden by other factors. The results for the esters are thus consistent with those obtained from the isocyanates [8]. SUMMARY
The y-effect of the alkyl substituents on the esters is consistent with previous results on other systems. Several classes of substituents have not been previously examined in detail. The a-inductive effects do
D. W. BOYKINet al.
338
not correlate with the “0 chemical shift data. A possible explanation of these effects may be found in the model of LI and CHESNUT[15] in which y-effects are postulated to result from competition between attractive and repulsive van der Waals interactions [ 153. Consequently, it appears that “0 chemical shift data for all the a-substituents can be regarded as resulting from y-effects.
[3] D. MONTI,F. ORSINIand G. S. RICCA,Spectrosc. Lett. 19, 91 (1986). r41 (a) A. L. BAUMSTARK, P. BALAKRISHNAN, M. DOTRONG, __ C. J. MCCLOSKEY,M. G. OAKLEYand D. W. BOYKIN, J. Am. them. Sot. 109.1059 (1987): (b) For a review see D. W. BOYKINand A. L. B,&Ms;A&, Tetrahedron, in press. f51_ T. SUGAWARA.Y. KAWADA and H. IWAMURA,Chem. _ Lett. 1371 (1978). f61 C. DELSETH.T. T.-T. NGUYEN and J. P. KINTZINGER. Helv. Chim. Acta 63, 498 (1980). [7] F. ORSINIand G. S. RICCA, Org. magn. Reson. 22, 653 (1984). B-l b. W: BOYKIN,J. Chem. Res. 338 (1987). C. CHATGILIALOGLU, S. ROSSINIand 593 G. BARBARELLA, V. TUGNOLI,J. magn. Reson. 70, 204 (1986). Cl01 C. DELSETHand J.-P. KINTZINGER,He/u. Chim. Acta 61, 1327 (1978). He/u. Chim. Acta 59, [Ill C. DELSETHand J.-P. KINTZINGER, 466 (1976). WI D. W. BOYKIN,Heterocycles 26, 773 (1987). 1131G. C. LEVY, R. L. LICHTERand G. L. NELSON,Carbon13 Nuclear Magnetic Resonance Spectroscopy, 2nd edn, p. 140. Wiley, New York (1980). Cl41 C. P. CHENG, S. C. LIN and G.-S. SHAW,.I. magn. Reson. 69, 58 (1986). [IS] S. LI and D. B. CHESNUT,Magn. Reson. Chem. 23,625 (1985). L
Acknowledyements~Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, to the NSF Instr;mentaiion Pro&m (CHEM8409599), and to the Georgia State University Research Fund.
REFERENCES
[l] E. E. LIEPINSH,1. A. ZITSMANE,L. M. IGNATOVICH,E. LUKEVITS,L. I. GUVANOVAand M. G. VORONKOV,Zh. ohshch Khim. 53, 1789 (1983). [Z] P. BALAKRISHNAN,A. L. BAUMSTARKand D. W. BOYKIN,Org. magn. Reson. 22, 753 (1984).
A