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Carbon-13 NMR spectroscopy of heterocyclic compounds
Structure, 31 (1976) 161-167 OKlsevier Scientific Publishing Company, Amsterdam -
Journal of Molecular
Printed in The Netherlands
CARBON-13 NMR SPECTROSCOPY OF HETEROCYCLIC COMJ?OUNDS I. AN ITERATIVE ANALYSIS OF THE PROTON-COUPLED 20 MHz FOURIER TRANSFORM SPECTRA FOR FURAN AND THIOPHENE: PROTON-COUPLED SPECTRA FOR THE B-SUBSTITUTED ALDEHYDE AND METHYL KETONE DERIVATIVES OF FURAN AND THIOPHENE
T. N. HUCKERBY Department
of Chemistry,
Uniuersity oflancaster,
Bailrigg, LancasterLAl4YA
(Gt. Bn-fain)
(Received 14 March 1975; in revised form 30 May 1975)
ABSTRACT Improved carbon-hydrogen coupling constants together with standard errors are given for furan and thiophene via an iterative spectral fitting procedure from undecoupled 20 MHz carbon NMR spectra. Carbon chemical shifts and carbon--hydrogen coupling constants for the carbonyl compounds are presented and discussed_ It is concluded that long range carbon-hydrogen couplings in these systems are not as valuable for conformational assignments as the analogous proton-proton interactions_ INTRODUCTION
Many NMR investigations of furan and thiophene have been reported in the literature. Their proton NMR spectra are well known, but it is only recently in an elegant study by Lunazzi [l] that furan, a deceptively simple {AR)1 system, has been properly analysed in terms of an individual assignment of proton-proton coupling constants, from careful measurements of weak transitions. Carbon-13 coupling parameters have been approached by a study of the 13Csatellites on the proton spectra [2,3], and the carbon chemical shifts together with one-bond C-H coupling constants were reported early in the study of the 13Cnucleus 143 . High resolution protoncoupled 13CNMR spectra for furan and thiophene at 15 MHz have been described by Weigert and Roberts [ 53 _ These various studies provided information which was used [5] to present data for long range 13C-lH coupling interactions in the two heterocycles. The low sensitivity and CW presentation meant, however, that insufficient transitions were observed to perform iterative spectral analyses, and also, the (lrand p signals from thiophene overlapped quite badly. As part of a study on the spectroscopic properties of a range of heterocyclic systems it was decided to redetermine these spectra at 20 MHz in the Fourier-Transform mode in order that a measure of the accuracy limits for these long range couplings could be ascertained.
162
Furan-2-aldehyde (furfuraldehyde) ana thiophene-Zaldehyde, together with 2-furyl- and 2-thienyl-methyl ketone have been widely studied by proton NMR spectroscopic techniques. There has been much interest in the conformational preferences of the substituent groups and these have been investigated by a variety of specialised methods including nuclear Overhauser enhancements [6], low temperature observations [7,8], nematic phase studies [S] and the application of solvent effects [S, 10-121. Little has been recorded concerning 13C parameters for furan or thiophene derivatives. The one bond aldehyde CH coupling has been given (from proton satellite measurements) in a paper [13] where it was shown that a linear relationship existed between *J(CH) and v(CH-stretch), and it was suggested that the effective nuclear charge on carbon might play a major role in variations of ‘J(CH). Data for CH couplings other than those involving C2 have been described for a-methyl furan by Weigert and Roberts [5] . Similar data for a series of 2- and 3-substituted thiophenes have been presented by Takahashi et al. [14]. Thiophene-2-aldehyde was included in this study, but again no parameters were given for the substituted carbons in any of these compounds, and no couplings to the aldehyde proton or carbon were described. In this paper we discuss carbon spectra for furan and thiophene derivatives (I) bearing aldehyde and methyl ketone substituents at C,. ‘-‘4
H3 R
x
= 0,s
R
=
H,CH3
0 (1)
RESULTS
Fur-an and thiophene
In neither case were the low- and high-field halves of the basic doublets completely identical. For thiophene the signals were just separated, allowing an easier assignment of transitions than previously. The derived protoncarbon coupling constants are summarised in Table 1. Although it is now acknowledged that standard errors in NMR parameters as produced by iterative fitting of theoretical to observed spectra do not directly represent the absolute accuracy of measurement, they do serve to indicate the relative magnitudes of errors involved for each parameter. The carbonyl
derivatives
Chemical shifts
Carbon chemical shifts referenced to internal TMS are given in Table 2. Signals were assigned to individual carbons via the 13C-‘H coupling constants
163 TABLE
1
Derived proton-carbon
coupling
constants
J
Furan
Thiophene
C&
201.71 f 0.02a 6.94 f 0.05 11.07 + 0.04 6.86 t 0.02
184.70 F 0.02 7.612 0.03 10.04 ? 0.04 5.04 f 0.03
13.78 175.08 4.05 5.97
4.66 166.95 5.83 9.79
Cd% WI, w-b
CA C, Hz C, H, C,H,
-F 0.04 2 0.04 + 0.04 * 0.05
f 0.04 +_0.02 it 0.03 + 0.04
RMS deviations 0.032 0.052
0.054 0.077
CLY C, aAll values are given in Hz. TABLE
2
Chemical shifts for the carbonyl
C, CZ Cl C, C, CH,
derivatives (I)
x=0
x=0
x=s
x=s
R=H
R=CH,
R=H
R=CH,
178.4a 153.7 122.3 113.3 149.0 -
186.3 153.2 117.5 112.5 146.9 25.9
183.4 144.2 137.1 128.9 135.3 -
190.2 144.9 133.0 128.6 134.0 26.5
aAIl values are expressed
in p-p-m_ from TMS as internal reference.
(which are discussed below) using correlations with the observed magnitudes of ‘3C-1H couplings in furan and thiophene. Certain trends can be seen in the data. When changing from R = H to R = CH3, in both heterocycles, the carbonyl chemical shift moves to low field, while the shifts of C3 and C5 move to high field, a trend which is just observable (< 1 p-p-m_) for C4 also. The substituted ring carbon is little affected (< 0.7 p.p.m.) and there is no consistent trend. Measurement
of carbon-proton
couplings
In all cases the positions of individual lines in coupled multiplets were obtained by direct estimation of the band centres from appropriately expanded portions of spectra. Normally the signals were assumed first-order
164
in character and the couplings were obtained by averagingindividual line separations. The signal for CS in furfuraldehyde demonstrated a slight deviation Born first-order nature, but it did not prove possible to simulate a theoretical spectrum which reproduced any non-first-order character and the couplings given are thus again averaged values from line positions_ The signal from C, in thiophene-2-aldehyde showed strong deviation from the expected simple pattern. In this case, using proton parameters obtained via a separate 100 MHz study on the same sample, it was possible to reproduce the observed pattern quite precisely. This signal structure arises because the appropriate signals in the (unobserved) proton satellite spectrum are overlapped in a strongly ~~~ct~g manner due to close proximity of two of the proton chemical shifts. The carbonyl signal from this compound also exhibited a second-order spectrum, and parameters were again obtained by an iterative analysis. The assignments for J(C,H,) and J(C,H,) may be interchanged without affecting the spectral appearance, but other permutations are precluded. Except for the carbonyl and C, carbons in thiophene-2aldehyde, estimates of errors are not quoted explicitly. Table 3 summarises the carbon-proton couplings and gives the digital resolution for each individual signal. This separation between data points in the final transformed spectrum provides some measure of the observational precision.
Carbon-proton
couplings within the heterocyclic
ring
These parameters are summarised in Table 3. On changing R from H to CH3, for both ring systems, the ‘J couplings C3H3, C4H4 and CsHs all show a decrease in magnitude, the difference always being more marked for the furans. Some other trends in coupling constants are also observed. In all four compounds ‘J(&H,) is greater than ‘J(C3H3). This is not apparently true for 2-bromo-, 2-iodo-, and Zcarbomethoxythiophene [14], although a-methyl- and 2-methoxythiophene do comply. *J(GH,) is always greater than *J(C5H4) and 3J(C5H3) is similarly of greater magnitude than its partner
3J(CzH,) the difference in this second case being more prominent in the thiophen~s. Of the remaining two bond couplings, that between C3 and He is consistently larger than its partner between Ca and Hs. Here again the difference is heightened with the thiophenes, and the same order has also been observed for five other thiophenes 1141, but does not, apparently, hold for 2-methyl furan 153.
Couplings between
the ring and the substituent
In proton NMR studies, measurements
of the five-bond coupling between
an aldehyde proton and ]E14 or H5 have been used to determine the conformational preference of the aldehyde group. In particular, a very small value for the former and a large (ea. 1 Hz) coupling for the latter (across a W pathway) is taken to indicate a predominantly s-trans geometry [lZ] . Variations in these couplings with changes in solvent have been used to estimate population ratios 110, 111.
165 TABLE
3
Coupling
constants
for the carbonyl
derivatives (I)
x=0
x=0
x=s
x=s
R=H
R=CH,
R=H
R=CH,
32.0= 6.95 10.1 6.6
C,HR
C,H, W-b GH,
Digital resolution XP C:H: C,H, Digital resolution C,HR WA GH., GH, Digital
0.061 0.6 178.25 3.7 5.7 0.28 0.0 3.5 179.1 13.4
resolution C,H, CA GH, GH, Digital
0.28 0.4= 10.Sc 7.85c 205.7=
resolution C,HR
Other C, couplings
0.061 179.7 1.30 0.79 0.44
Digital resolution CRHR
Other Cn couplings Digital resolution
0.061
32.42 6.70 8.69 5.38
1.6 6.8 9.8 6.8 0.11 0.5 174.8 3.8 5.7
0.13
0.35 169.2 5.4 9.4
0.45 168.0 5-4 9.2
0.15
0.13
0.25 4.55 171.7 4.1
0.086 0.0 10.6 7.8 204.3
0.0 4.5 170.7 4.0 0.13
0.15 0.0 10.8 7.4 187.2
0.11 6.2 1.2e 0.7e 0.3e
1.4 6.4 8.8 5.9
0.11
0.086 0.0 3.5 177.7 13.45
r 0.03b + 0_05b _+0.03b 2 0.04b
< 0.2d 10.85 7.05 186.4
0.15 179.55 + O-03 C,H, or C,H, = 1.17 2 0.05 C&H, = 0.62 + 0.03 C,H, or C,H, =.3.66 f 0.04 0.061
0.061
0.13 -f
0.061
-
122.3
-
-
0.0
-
0.45
-
0.061
-
0.061
128.0
=All values in the Table are given in Hz. bFrom an iterative analysis, standard errors from the computation are given. CApproximate values obtained on a first order basis, signal shows perturbation. %ignals broadened, but splitting not resolved. eApproximate values from strongly overlapped signals. fSignal too weak to allow adequate resolution of the complex splittings.
166
It was hoped that the analogous carbon-hydrogen coupling constants &He, CSHR, C1H4 and CIH5 might be of sufficient magnitude to provide similar and parallel information. Observations of CzHR couplings might also show these to be sensitive to conformational preferences. Couplings between either C4 or Cs and aldehyde or methyl protons are observable in some cases, but exhibit variable and unpredictable magnitudes. If it is assumed that the s-tram conformation in the thiophenes will be more highly populated than in the furans, then it would appear that the two bond couplings between C2 and the aldehyde protons and the three bond couplings between C, and the methyl protons are relatively insensitive to conformational preferences. The carbonyl carbons in all the molecules exhibit coupling with the ring protons. These splittings are difficult to measure precisely because of the low signal intensities for these nuclei in the ketones, and because the lines in the multiplets are strongly overlapped. The splittings have not been assigned to individual ring proton-carbon interactions, although it is tempting to suggest that the largest in each case is the C1H5 coupling by analogy with proton studies. The difficulty of measurement and the fact that the magnitudes are not greatly larger than those observed in proton NMR studies of conformational preferences suggest that the clarification of assignments by a study of specifically deuterated molecules, the syntheses for which have recently been described [15], is not a worthwhile exercise. Since the completion of this work, two notes have appeared describing variable temperature studies on heterocyclic aldehydes and ketones, and have discussed rotational isomerism 1161 and rotational barriers [17] _ EXPERIMENTAL
The materials were reagent-grade commercial samples, used without further purification. Acetone-d, (10 % by volume) was added as lock material_ For the measurement of chemical shifts a quantity of tetramethylsilane sufficient to give an observable signal was added as an internal reference_ Carbon spectra were determined at 20 MHz using a Varian CFT-20 Fourier-Transform Spectrometer. Proton--carbon coupling was retained using a gated decoupling facility which permitted retention of the nuclear Overhauser enhancement. By ahowing spectra to “fold” it was possible to work with spectral acquisition widths of 400 Hz (furan) and 230 Hz (thiophene), i.e. digital resolutions (with 4K data points for acquisition) of 0.19 and 0.11 Hz, respectively. Line centres were estimated visually from expanded portions of spectra since the instrumental peak listing procedure proved inadequate. Theoretical spectra were calculated using the iterative computer program LAME [lS] using proton-proton couplings for furan and thiophene which were adopted from the literature [l,31.
167 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
L. Lunazzi, Mol. Phys., 18 (1970) 413. G. S. Reddy and J. H. Goldstein, J. Amer. Chem. Sot., 84 (1962) 583. J. M. Read, C. T. Mathis and J. H. Goldstein, Spectrochim. Acta, 21 (1965) 85. T. E. Page, T. Aiger and D. M. Grant, J. Amer. Chem. Sot., 87 (1965) 5333. F. J. Weigert and J. D. Roberts, J. Amer. Chem. Sot., 90 (1968) 3543. L. Kaper and T. J. de Boer, Rec. Trav. Chim., 89 (1970) 825. K. I. Daiqvist and S. Forsen, J. Phys. Chem., 69 (1965) 1760 and 4062. B. Roques, S. Combrisson, C. Biche and C. Pascard-Billy, Tetrahedron, 26 (1970) 3555. L. Lunazzi and C. A. Veracini, J. Chem. Sot. Perkin II, (1973) 1739. R. J. Abraham and T. M. Siverns, Tetrahedron, 28 (1972) 3015. B. R. Larsen, F. Nicolaisen and J. T. Nielsen, Acta Chem. Stand., 26 (1972) 1736. M. L. Martin, J.-C. Roze, G. J. Martin and P. Fournari, Tetrahedron Lett., (1970) 3407. S. L. Rock and R. M. Hammaker, Spectrocbim. Acta Part A, 27 (1971) 1899. K. Takabashi, T. Sone and K. Fujieda, J. Phys. Chem., 74 (1970) 2765. D. J. Chadwick, J. Chambers, P. K. C. Hodgson, G. D. Meakins and R. L. Snowden, J. Chem. Sot. Perkin I, (1974) 1141. D. J. Chadwick, G. D. Meakins and E. E. Richards, Tetrahedron Lett., (1974) 3183. B. P. Roques, S. Combrisson and F. Wehrli, Tetrahedron Lett., (1975) 1047. A revised and improved version of LAOCNS with magnetic equivalence factorisation; C. W. Haigh, University of Swansea, personal communication.
Journal of Molecular
Printed in The Netherlands
CARBON-13 NMR SPECTROSCOPY OF HETEROCYCLIC COMJ?OUNDS I. AN ITERATIVE ANALYSIS OF THE PROTON-COUPLED 20 MHz FOURIER TRANSFORM SPECTRA FOR FURAN AND THIOPHENE: PROTON-COUPLED SPECTRA FOR THE B-SUBSTITUTED ALDEHYDE AND METHYL KETONE DERIVATIVES OF FURAN AND THIOPHENE
T. N. HUCKERBY Department
of Chemistry,
Uniuersity oflancaster,
Bailrigg, LancasterLAl4YA
(Gt. Bn-fain)
(Received 14 March 1975; in revised form 30 May 1975)
ABSTRACT Improved carbon-hydrogen coupling constants together with standard errors are given for furan and thiophene via an iterative spectral fitting procedure from undecoupled 20 MHz carbon NMR spectra. Carbon chemical shifts and carbon--hydrogen coupling constants for the carbonyl compounds are presented and discussed_ It is concluded that long range carbon-hydrogen couplings in these systems are not as valuable for conformational assignments as the analogous proton-proton interactions_ INTRODUCTION
Many NMR investigations of furan and thiophene have been reported in the literature. Their proton NMR spectra are well known, but it is only recently in an elegant study by Lunazzi [l] that furan, a deceptively simple {AR)1 system, has been properly analysed in terms of an individual assignment of proton-proton coupling constants, from careful measurements of weak transitions. Carbon-13 coupling parameters have been approached by a study of the 13Csatellites on the proton spectra [2,3], and the carbon chemical shifts together with one-bond C-H coupling constants were reported early in the study of the 13Cnucleus 143 . High resolution protoncoupled 13CNMR spectra for furan and thiophene at 15 MHz have been described by Weigert and Roberts [ 53 _ These various studies provided information which was used [5] to present data for long range 13C-lH coupling interactions in the two heterocycles. The low sensitivity and CW presentation meant, however, that insufficient transitions were observed to perform iterative spectral analyses, and also, the (lrand p signals from thiophene overlapped quite badly. As part of a study on the spectroscopic properties of a range of heterocyclic systems it was decided to redetermine these spectra at 20 MHz in the Fourier-Transform mode in order that a measure of the accuracy limits for these long range couplings could be ascertained.
162
Furan-2-aldehyde (furfuraldehyde) ana thiophene-Zaldehyde, together with 2-furyl- and 2-thienyl-methyl ketone have been widely studied by proton NMR spectroscopic techniques. There has been much interest in the conformational preferences of the substituent groups and these have been investigated by a variety of specialised methods including nuclear Overhauser enhancements [6], low temperature observations [7,8], nematic phase studies [S] and the application of solvent effects [S, 10-121. Little has been recorded concerning 13C parameters for furan or thiophene derivatives. The one bond aldehyde CH coupling has been given (from proton satellite measurements) in a paper [13] where it was shown that a linear relationship existed between *J(CH) and v(CH-stretch), and it was suggested that the effective nuclear charge on carbon might play a major role in variations of ‘J(CH). Data for CH couplings other than those involving C2 have been described for a-methyl furan by Weigert and Roberts [5] . Similar data for a series of 2- and 3-substituted thiophenes have been presented by Takahashi et al. [14]. Thiophene-2-aldehyde was included in this study, but again no parameters were given for the substituted carbons in any of these compounds, and no couplings to the aldehyde proton or carbon were described. In this paper we discuss carbon spectra for furan and thiophene derivatives (I) bearing aldehyde and methyl ketone substituents at C,. ‘-‘4
H3 R
x
= 0,s
R
=
H,CH3
0 (1)
RESULTS
Fur-an and thiophene
In neither case were the low- and high-field halves of the basic doublets completely identical. For thiophene the signals were just separated, allowing an easier assignment of transitions than previously. The derived protoncarbon coupling constants are summarised in Table 1. Although it is now acknowledged that standard errors in NMR parameters as produced by iterative fitting of theoretical to observed spectra do not directly represent the absolute accuracy of measurement, they do serve to indicate the relative magnitudes of errors involved for each parameter. The carbonyl
derivatives
Chemical shifts
Carbon chemical shifts referenced to internal TMS are given in Table 2. Signals were assigned to individual carbons via the 13C-‘H coupling constants
163 TABLE
1
Derived proton-carbon
coupling
constants
J
Furan
Thiophene
C&
201.71 f 0.02a 6.94 f 0.05 11.07 + 0.04 6.86 t 0.02
184.70 F 0.02 7.612 0.03 10.04 ? 0.04 5.04 f 0.03
13.78 175.08 4.05 5.97
4.66 166.95 5.83 9.79
Cd% WI, w-b
CA C, Hz C, H, C,H,
-F 0.04 2 0.04 + 0.04 * 0.05
f 0.04 +_0.02 it 0.03 + 0.04
RMS deviations 0.032 0.052
0.054 0.077
CLY C, aAll values are given in Hz. TABLE
2
Chemical shifts for the carbonyl
C, CZ Cl C, C, CH,
derivatives (I)
x=0
x=0
x=s
x=s
R=H
R=CH,
R=H
R=CH,
178.4a 153.7 122.3 113.3 149.0 -
186.3 153.2 117.5 112.5 146.9 25.9
183.4 144.2 137.1 128.9 135.3 -
190.2 144.9 133.0 128.6 134.0 26.5
aAIl values are expressed
in p-p-m_ from TMS as internal reference.
(which are discussed below) using correlations with the observed magnitudes of ‘3C-1H couplings in furan and thiophene. Certain trends can be seen in the data. When changing from R = H to R = CH3, in both heterocycles, the carbonyl chemical shift moves to low field, while the shifts of C3 and C5 move to high field, a trend which is just observable (< 1 p-p-m_) for C4 also. The substituted ring carbon is little affected (< 0.7 p.p.m.) and there is no consistent trend. Measurement
of carbon-proton
couplings
In all cases the positions of individual lines in coupled multiplets were obtained by direct estimation of the band centres from appropriately expanded portions of spectra. Normally the signals were assumed first-order
164
in character and the couplings were obtained by averagingindividual line separations. The signal for CS in furfuraldehyde demonstrated a slight deviation Born first-order nature, but it did not prove possible to simulate a theoretical spectrum which reproduced any non-first-order character and the couplings given are thus again averaged values from line positions_ The signal from C, in thiophene-2-aldehyde showed strong deviation from the expected simple pattern. In this case, using proton parameters obtained via a separate 100 MHz study on the same sample, it was possible to reproduce the observed pattern quite precisely. This signal structure arises because the appropriate signals in the (unobserved) proton satellite spectrum are overlapped in a strongly ~~~ct~g manner due to close proximity of two of the proton chemical shifts. The carbonyl signal from this compound also exhibited a second-order spectrum, and parameters were again obtained by an iterative analysis. The assignments for J(C,H,) and J(C,H,) may be interchanged without affecting the spectral appearance, but other permutations are precluded. Except for the carbonyl and C, carbons in thiophene-2aldehyde, estimates of errors are not quoted explicitly. Table 3 summarises the carbon-proton couplings and gives the digital resolution for each individual signal. This separation between data points in the final transformed spectrum provides some measure of the observational precision.
Carbon-proton
couplings within the heterocyclic
ring
These parameters are summarised in Table 3. On changing R from H to CH3, for both ring systems, the ‘J couplings C3H3, C4H4 and CsHs all show a decrease in magnitude, the difference always being more marked for the furans. Some other trends in coupling constants are also observed. In all four compounds ‘J(&H,) is greater than ‘J(C3H3). This is not apparently true for 2-bromo-, 2-iodo-, and Zcarbomethoxythiophene [14], although a-methyl- and 2-methoxythiophene do comply. *J(GH,) is always greater than *J(C5H4) and 3J(C5H3) is similarly of greater magnitude than its partner
3J(CzH,) the difference in this second case being more prominent in the thiophen~s. Of the remaining two bond couplings, that between C3 and He is consistently larger than its partner between Ca and Hs. Here again the difference is heightened with the thiophenes, and the same order has also been observed for five other thiophenes 1141, but does not, apparently, hold for 2-methyl furan 153.
Couplings between
the ring and the substituent
In proton NMR studies, measurements
of the five-bond coupling between
an aldehyde proton and ]E14 or H5 have been used to determine the conformational preference of the aldehyde group. In particular, a very small value for the former and a large (ea. 1 Hz) coupling for the latter (across a W pathway) is taken to indicate a predominantly s-trans geometry [lZ] . Variations in these couplings with changes in solvent have been used to estimate population ratios 110, 111.
165 TABLE
3
Coupling
constants
for the carbonyl
derivatives (I)
x=0
x=0
x=s
x=s
R=H
R=CH,
R=H
R=CH,
32.0= 6.95 10.1 6.6
C,HR
C,H, W-b GH,
Digital resolution XP C:H: C,H, Digital resolution C,HR WA GH., GH, Digital
0.061 0.6 178.25 3.7 5.7 0.28 0.0 3.5 179.1 13.4
resolution C,H, CA GH, GH, Digital
0.28 0.4= 10.Sc 7.85c 205.7=
resolution C,HR
Other C, couplings
0.061 179.7 1.30 0.79 0.44
Digital resolution CRHR
Other Cn couplings Digital resolution
0.061
32.42 6.70 8.69 5.38
1.6 6.8 9.8 6.8 0.11 0.5 174.8 3.8 5.7
0.13
0.35 169.2 5.4 9.4
0.45 168.0 5-4 9.2
0.15
0.13
0.25 4.55 171.7 4.1
0.086 0.0 10.6 7.8 204.3
0.0 4.5 170.7 4.0 0.13
0.15 0.0 10.8 7.4 187.2
0.11 6.2 1.2e 0.7e 0.3e
1.4 6.4 8.8 5.9
0.11
0.086 0.0 3.5 177.7 13.45
r 0.03b + 0_05b _+0.03b 2 0.04b
< 0.2d 10.85 7.05 186.4
0.15 179.55 + O-03 C,H, or C,H, = 1.17 2 0.05 C&H, = 0.62 + 0.03 C,H, or C,H, =.3.66 f 0.04 0.061
0.061
0.13 -f
0.061
-
122.3
-
-
0.0
-
0.45
-
0.061
-
0.061
128.0
=All values in the Table are given in Hz. bFrom an iterative analysis, standard errors from the computation are given. CApproximate values obtained on a first order basis, signal shows perturbation. %ignals broadened, but splitting not resolved. eApproximate values from strongly overlapped signals. fSignal too weak to allow adequate resolution of the complex splittings.
166
It was hoped that the analogous carbon-hydrogen coupling constants &He, CSHR, C1H4 and CIH5 might be of sufficient magnitude to provide similar and parallel information. Observations of CzHR couplings might also show these to be sensitive to conformational preferences. Couplings between either C4 or Cs and aldehyde or methyl protons are observable in some cases, but exhibit variable and unpredictable magnitudes. If it is assumed that the s-tram conformation in the thiophenes will be more highly populated than in the furans, then it would appear that the two bond couplings between C2 and the aldehyde protons and the three bond couplings between C, and the methyl protons are relatively insensitive to conformational preferences. The carbonyl carbons in all the molecules exhibit coupling with the ring protons. These splittings are difficult to measure precisely because of the low signal intensities for these nuclei in the ketones, and because the lines in the multiplets are strongly overlapped. The splittings have not been assigned to individual ring proton-carbon interactions, although it is tempting to suggest that the largest in each case is the C1H5 coupling by analogy with proton studies. The difficulty of measurement and the fact that the magnitudes are not greatly larger than those observed in proton NMR studies of conformational preferences suggest that the clarification of assignments by a study of specifically deuterated molecules, the syntheses for which have recently been described [15], is not a worthwhile exercise. Since the completion of this work, two notes have appeared describing variable temperature studies on heterocyclic aldehydes and ketones, and have discussed rotational isomerism 1161 and rotational barriers [17] _ EXPERIMENTAL
The materials were reagent-grade commercial samples, used without further purification. Acetone-d, (10 % by volume) was added as lock material_ For the measurement of chemical shifts a quantity of tetramethylsilane sufficient to give an observable signal was added as an internal reference_ Carbon spectra were determined at 20 MHz using a Varian CFT-20 Fourier-Transform Spectrometer. Proton--carbon coupling was retained using a gated decoupling facility which permitted retention of the nuclear Overhauser enhancement. By ahowing spectra to “fold” it was possible to work with spectral acquisition widths of 400 Hz (furan) and 230 Hz (thiophene), i.e. digital resolutions (with 4K data points for acquisition) of 0.19 and 0.11 Hz, respectively. Line centres were estimated visually from expanded portions of spectra since the instrumental peak listing procedure proved inadequate. Theoretical spectra were calculated using the iterative computer program LAME [lS] using proton-proton couplings for furan and thiophene which were adopted from the literature [l,31.
167 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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