Spectroscopic study of the conformational isomerism of 2-formylfuran derivatives

Journal o[ Molecular Structure, 197 (1989) 193-202

193

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

S P E C T R O S C O P I C S T U D Y OF THE C O N F O R M A T I O N A L I S O M E R I S M OF 2 - F O R M Y L F U R A N D E R I V A T I V E S

ARPAD I. KISS

Department of Physical Chemistry, Technical University Budapest, Budapest (Hungary) DAISY MACHYTKA

Central Research Institute of Chemistry, Hungarian Academy of Sciences, Budapest (Hungary) JULIUS BANKI*

Department of Organic Chemistry, Slovak Technical University Bratislava (Czechoslovakia) MIKLOS G/~L

Department of General and Inorganic Chemistry, EiStvSs University, Budapest (Hungary) (Received 3 October 1988)

ABSTRACT The conformational equilibria of 5-X-2-formylfurans (X = CH3, Br, CN, NO2, 4-02NC6H4) were studied by infrared (IR), Raman and 1H-NMR spectroscopy. The ratios of the conformers were determined from the solvent dependence of the IR spectra, and the energy differences of the conformers from the temperature dependence of liquid-state Raman spectra and IH-NMR spectra. Conclusions are drawn concerning the relative magnitudes of the rotational barriers. CNDO/ 2 calculations are included for comparison. All compounds investigated exist as equilibrium mixtures of the 0 " " 0 trans and 0 " - 0 cis forms, the former being predominant in non-polar media, the latter in polar solvents and the liquid state. The effect of substituents on the conformational properties are discussed.

INTRODUCTION

Many papers have been published on spectroscopic studies of the conformational equilibrium of 2-formylfuran (furfural). Reviews on the earlier publications are given in refs. 1 and 2. There is general agreement that furfural exists in an equilibrium mixture of two conformers: the O'-" O trans and O . . . O cis forms. There are, however, different opinions on the relative abundances of the two rotamers, their equilibrium is strongly dependent on the phase or solvent, respectively. Concerning the conformational equilibria of 5-substi*Present address: Department of Informatics and Cybernetics, Slovak Technical University, Bratislava, Czechoslovakia.

0022-2860/89/$03.50

© 1989 Elsevier Science Publishers B.V.

194 TABLE 1 Compounds investigated 10

X~

9

5~72

~Cx

x

0 " " 0 cis

0 . . "0 trans

Notation

X

Y

1

H

H

2 3 4 5

H CH.~ Br CN

CH:~ H H H

6

NO2

H

7

4-02NC6H4

H

tuted furfurals, only qualitative and partly contradictory results have been published [3-8]. The aim of the present work was to study the effect of different substituents in position 5 of the furan ring on the conformational equilibrium of furfural. The list of the compounds investigated is given in Table 1. The ratios of the conformers were determined from the solvent dependence of the infrared (IR) spectra. The liquid-state Raman spectra and the 1H-NMR spectra in different solvents were recorded at different temperatures in order to evaluate the energy differences of the conformers (the isomerization energies ). The isomerization energies, the heights of the rotational barriers and the atomic charges were calculated using the semiempirical CNDO/2 method. The atomic charges were used to determine the orienting effect of the bromo substituent. The conformational doublets in the IR spectra of furfural and the evaluation of its conformational properties have been reported previously [9,10]. For comparison with the 5-substituted derivatives, the corresponding quantities of furfural were reinvestigated in this study. Some properties of 2 are also included for comparison with 3* METHOD

All the compounds used were commercial products or prepared by standard methods. They were purified by vacuum distillation or recrystallization and their spectra were taken immediately after purification. *Part of this work was presented on the WATOC'87 World Congress, 12-19 August 1987, Budapest, Hungary. Summary in the Book of Abstracts, PA 76 (p. 129).

195

The IR spectra of the solid-state compounds were measured on K B r discs, that of furfural in the liquid state. In addition, solution spectra were taken of all compounds in three solvents of different permittivities (carbon tetrachloride, chloroform, acetonitrile), except 6 for which benzene was used as the non-polar solvent because of its poor solubility in carbon tetrachloride. The spectra were recorded on a 75IR Zeiss J e n a instrument at room temperature. The concentration of the solutions varied between 0.1 and 0.3 mol dm -3, the thickness of the NaCl cell was 0.42 mm. 5-(4-Nitrophenyl)-furfural (7) was recorded in 0.01-0.03 mol dm -a concentrations in a cell of 0.63-mm thickness owing to poor solubility. The accuracy of the sharp peaks can be considered as + 2 c m -1. The R a m a n spectra of 1, 2 and 3 were measured in the liquid state at several temperatures between 20 and 80 ° C on a Jobin-Yvon H G 2 S spectrometer using the 514.5-nm line of a 120-roW Ar + laser for excitation. The resolution of the spectra was + 2 c m - 1. The 1 H - N M R spectra were recorded on a Varian XL-100 spectrometer in deuterated acetone and diethyl ether solutions of 1-2.5 mol dm -3 concentration. The most important requirement for the solvents was their very low freezing point. Compounds 1, 3, 4 and 6 were measured in acetone, and 1 and 3 also in diethyl ether. The spectra of the ether solutions were recorded between 100 ° C and + 40 ° C, that of the acetone solutions between - 90 ° C and + 40 ° C. T M S was used as reference. For the C N D O / 2 calculations the bond distances and angles of the trans form of furfural were taken from ref. 9. The trans, gauche (perpendicular) and cis forms of 1, 3, 4 and 6 were calculated starting from these data for the furfuryl part of the molecules. For the substituents the following estimated data were used [11]: C ( 5 ) - B r 186 pro, C ( 5 ) - N 140 pm, N - O 124 pro, C ( 5 ) C ( M e ) 152 pro, C ( M e ) - H 109 pro, C ( 5 ) - N - O and O - N - O 120 °, C ( 5 ) C ( M e ) - H and H - C ( M e ) - H 109.5 °. The geometries of the trans and cis forms of I and 2 were optimized. The optimized geometries showed 1-2.5 pm deviations in the bond lengths and 2-4 ° deviations in the bond angles from the experimental geometries. The results of the C N D O calculations for 1, have been reported previously [9]. -

RESULTS AND DISCUSSION

Infrared spectra The complete band assignment of the vibrational spectra of 2-substituted furans has been given by Sdn~chal and Saumagne [12]. The skeletal vibrations, the aldehyde C - H vibrations and the C=O stretching vibration show conformational splitting. The components of all doublets observed in the spectra of 1 were assigned to the trans or cis forms [10]. According to these inves-

196

tigations the trans form is more abundant in non-polar media, and the cis form is more abundant in polar media and in liquid and solid states. An approximate normal-coordinate analysis for furfural has been performed by Adamek et al. [ 13 ], using the same force-constant set for both conformers. Recently, a complete CNDO/2 force field of the trans- and cis-furfural was calculated [ 14 ]. The differences between the experimental and calculated frequencies of the two rotamers are larger than the band splitting caused by the isomerism. Thus the CNDO calculations were unable to reproduce these splittings even qualitatively. For 2,5-disubstituted furans the first assignment of the 10 most characteristic vibrations was given by Grigg et al. [3]. These authors concluded, from the solvent dependence of the carbonyl stretching bands of 3, 4 and 6, the presence of rotational isomerism [4]. In contrast, Chadwick et al. [5] found no doublets in a study of the carbonyl bands of 3 and 4 in carbon tetrachloride and acetonitrile solutions. On the other hand, studying the first overtones of the carbonyl stretching vibrations, Ballester et al. [6 ] observed doublets in the spectra of 3, 4 and 6 which they interpreted as arising from conformational isomerism. For the interpretation of the spectra of compounds 1-7 the band assignment of furfural [ 12 ] and that of the 2,5-disubstituted furans [3 ] were used. For 5 and 7 no spectra were found in the literature, their vibrations could be easily assigned by comparison with analogous molecules. Comparing the spectra of furfural and its substituted derivatives it was found that the substituents cause only minor (5-20 cm -1) frequency shifts. The complete spectra and their assignment are not given here, but are available from the authors on request. The band pairs showing intensity changes as a function of the solvent polarity are given in Table 2 together with their assignments to vibrations and conformers. The results suggest that the bands at 1216 and 1198 cm-1 in the spectrum of 4 and those at 1210 and 1200 cm-1 in the spectrum of 3 are conformational doublets, in contrast to Grigg et al. [3 ] who assigned these bands to different normal modes. The doublets mentioned in Table 3 were used for the calculation of the band areas. The ratios of the conformers in the different solvents were calculated from the areas of the components of the band pairs. In the spectra of solutions of 7 in the same solvents used for 1-4 an insufficient number of band pairs was found for the calculation to be possible. Therefore its spectra were recorded in acetone and in DMSO. The carbonyl stretching bands consisting of 3 or 4 components have been omitted from Table 2 for all molecules. The ratios of the areas of the components show no similarity to those of the other band pairs. Ballester et al. [ 15 ] stated that the carbonyl stretching absorption is caused both by Fermi resonance and by rotational isomerism. Fermi resonance seems to be more impor-

197 TABLE 2 Conformational doublets in the IR spectra Notation of doublets

Band position (cm- 1)

Assignment

CC14

a a' b b' c d e f g g' h

c t c t c t t c c t t C c t t c t C t c c t

Benzene

1

2

3

4

5

1480 1468

1480 sh 1471

1517 1507 1450 1438

1460 1452

1506 sh 1500

1393 1383

6

7 ~N ~a~cm

1351 1344

1354 1344 sh

Uj

1381 1376

1246 sh 1240 1158 1150 1015 sh 1011 945 929

1289 1277 1233 sh 1228 1170 1161 1020 1008

~ cH3 VN+S

1210 1200

1216 1198

1220 1205 1194 1182

1018 1005 954 944

6c H+UN ~C-H+ VN ~cH-n+ un

962 954

972 960

970 960

967 953 915 903

VN+S rI, CH 3

VN+S

c, Cis; t, trans, sh, Shoulder; u, stretching; 8, deformation; N, ring vibration; S, vibration mainly of the substituent. tant in some cases because the bands show minor temperature

dependence

[10]. I n o r d e r t o f i n d t h e trans a n d cis c o m p o n e n t s o f t h e d o u b l e t s , t h e f o l l o w i n g c o n s i d e r a t i o n w a s a p p l i e d : a s t h e trans f o r m h a s s m a l l e r d i p o l e m o m e n t t h a n t h e cis f o r m , t h e i n t e n s i t y o f t h e c o m p o n e n t o f e a c h b a n d p a i r w h i c h b e l o n g s t o t h e trans f o r m d e c r e a s e s i n f a v o u r o f t h e o t h e r w i t h i n c r e a s i n g s o l v e n t polarity. T h e c a l c u l a t e d r a t i o s o f t h e c o n f o r m e r s a r e g i v e n i n T a b l e 3. A s i t c a n b e s e e n , i n e a c h c a s e t h e trans f o r m is m o r e a b u n d a n t i n n o n - p o l a r s o l v e n t s a n d t h e cis f o r m is m o r e a b u n d a n t i n p o l a r m e d i a o r c o n d e n s e d p h a s e s . F r o m 1HN M R i n v e s t i g a t i o n s , R o d r i g u e z a n d B e r t r a n [ 7 ] f o u n d t h a t t h e cis f o r m is m o r e s t a b i l i z e d b y p o l a r s o l v e n t s t h a n is t h e trans o n e . O u r r e s u l t s s h o w t h a t

198 TABLE 3 Ratios (cc,~ c ...... ) of the conformers No.

Doublet

Solvent CC|4

CHCla

CHaCN

1.9 0.9 1.1 1.2 0.8

3.2 1.9 1.8 1.3 1.1 1.1 1.1

1

e,g

2 3 4 5

c,e,g' a',d b,g g

0.9 0.4 0.8 0.9 0.7

6

g

0.9 ~

1.0

7

d

0.9 ~

1.0 b

aIn benzene, bin acetone. TABLE 4 Energy differences of the conformers No.

1 2 4 6

AGo (kJ mo1-1)

AHo (kJ mo1-1) 'H-NMR (ether)

Raman (liquid)

'CNDO/2

1H-NMR (acetone)

1.4 2.4

6.9 2.3

3.26 3.28 3.62 4.05

2.8 2.6 3.2

the change caused by the solvent polarity is largest with 2 and 3, smallest with 6 and 7. It follows that the cis form is more stabilized in polar solvents by electron-donor substituents than by electron-acceptor groups. This trend seems to run in parallel with the increase in the calculated cis-trans energy differences (Table 4). Preliminary gas electron-diffraction results on 3 by B~inki et al. [ 16 ] showed a ratio of conformers very similar to that observed in carbon tetrachloride solution. R a m a n spectra For the study of temperature dependence the region of 900-1450 c m - i seemed to be the most appropriate. In the liquid state the cis form is predominant at room temperature, and its amount apparently decreases with increasing temperature.

199

The following band pairs were used for the area calculations: trans cis 1 1466 c m - 1 1476 c m - 1 2 905 c m - 1 915 c m - 1 3 1011 cm -~ 1026 cm -1 From the band areas the energy differences of the conformers, AHo (Table 4), were determined via the temperature dependence of the equilibrium constant. The AHo values of 2 and 3 were found to be smaller than that of furfural. Comparison with results obtained using other methods is given in the following section. 1H-NMR spectra Rodriguez and Bertran [7] concluded from their N M R spectroscopic study of 5-substituted derivatives that the cis form is predominant in case of an electron-donor substituent. N M R studies on 4 and 6 showed the presence of two conformers in carbon tetrachloride at room temperature, whereas 3 is almost totally in the cis form in deuterated acetone [8]. The acetone solution of 6 was studied at different temperatures. The coalescence point was not detected, but it was estimated to be below - 90 ° C. The decrease of the barrier is attributed to the electron-acceptor property of the nitro group [8]. In order to determine the ~Ho values a more detailed investigation was made of the temperature dependence of the spectra. The spectra recorded at 40°C did not show the presence of two conformers. Below - 50 ° C, depending on the substituent, the signal belonging to the aldehyde proton and that of the H (3) proton began to broaden. Below the coalescence point, two signals appeared with different intensities. The behaviour of 6 is quite different from the other two derivatives, its acetone solution showed only very weak broadening until - 9 0 ° C, which means that the coalescence point is even lower, in accordance with the finding of Sheinker et al. [8 ]. The chemical shift of the H (3) proton is decreased in the trans form by the screening effect of the carbonyl group. In the compounds investigated the cis form is dominant at low temperatures according to the slow exchange, supporting the idea the cis form is more stable in polar solvents than is the trans form. Owing to the low resolution of the instrument used in this study, it was not possible to observe the stereospecific coupling constants between the aldehyde and other protons. For this reason the ratios of the conformers were set equal to the ratios of the band areas, of which those of the aldehyde proton were used in the calculations. The energy differences of the conformers (AHo) evaluated from the temperature dependence of the equilibrium constants are given in Table 4. for 1 and 3 it was possible to determine accurately the band areas at three different temperatures. With 4 the calculation could be performed only at a single tem-

200

perature, therefore only AGo could be performed. For comparison the AGo values are given for all the three derivatives. The AHo and AGo values obtained by different methods can be seen in Table 4. The liquid phase is a strongly polar medium, therefore higher AHovalues are expected in the liquid state than in solvents. This expectation is verified by the IR result for furfural [ 10 ]. The AHo value obtained from the Raman measurement, however, seems to be too high. The reason may be the less-accurate determination of the band areas from Raman spectra than from NMR, caused by the smaller splitting of the Raman doublets because of band broadening in the liquid state. The CNDO/2 method was proved to give reliable results for furfural ( [9] and references cited therein) and, therefore, it was applied for comparative calculations (Table 4). More sophisticated methods (minimal basis set ab initio [17], MINDO/3 [18]) give the same results as CNDO/2, in agreement with the experimental data. Our calculated AHo values for 1 and 3 are nearly the same, and those of the 5-substituted derivatives were found to increase with the electron-acceptor property of the 5-substituent. The coalescence temperatures are given in Table 5. The higher the coalescence temperature, the higher is the rotational barrier. Sheinker et al. [8] stated TABLE 5 Height of rotational barriers No.

1 3 4 6

Coalescence temperature ( ° C )

E~.auch e -- Etran s

Ether

Acetone

CNDO/2 (kJ m o l - 1)

-75 -65

-70 -60 -70 -90

23.7 25.7 25.4 18.1

TABLE 6 C N D 0 / 2 net charges on atom 0 (1) F u r a n derivative

Qt .....

Qc,~

1 2 2-methyl 3 2-bromo 4 6

-0.156 -0,160 - 0,165 -0,170 -0,256 -0.164 -0.141

-0.145 -0.146 -0.159 -0.153 -0.131

201 t h a t electron-acceptor substituents decrease and electron-donor substituents increase the height of the energy barrier. Thus the order of the barriers is: 6 < 1 < 4 < 3. The C N D O / 2 barriers correlate fairly well with the experimental order (Table 5). Correlation of the experimental order with the factors which determine the height of the barrier exists only for the bond strength of the rotational axis but not for the symmetry or mass of the rotating group. The higher the bond order between C ( 2 ) and C (6) the higher the rotational barrier. W i t h an electron-acceptor group in position 5 of the furan ring, the bond order decreases between C (2) and C (6) and the barrier becomes lower. W i t h an electron-donor group the situation is the reverse. The data in Table 5 suggest t h a t the bromo group is a weak donor. In order to support this statement we have presented the calculated net charges for 1, 3, 4 and 6 together with the same values for the corresponding monosubstituted furans. Table 6 shows the charges on the O (1) atoms of the trans and cis forms. The charges on 0 (1) give the same order as do the heights of the barriers, showing the bromo group to be a weak donor with respect to O(1). CONCLUSIONS It can be concluded t h a t all the compounds investigated exist as equilibrium mixtures of the two conformers 0 " . O trans and O'-" O cis at room temperature. The results of the IR, R a m a n and 1H-NMR studies is t h a t for all compounds the trans form is more stable in non-polar solvents and the cis form is more stable in polar solvents and in liquid phase. The c/s form is more stabilized by electron-donor groups t h a n by electron-acceptor groups. The isomerization energy is lowered by a methyl group and is increased by an acceptor group in position 5 of the furan ring. The rotational barriers correlate well with the electron-orienting effect of the 5-substituent. The bromo group is a weak electron donor with respect to the furan oxygen atom.

REFERENCES 1 2 3 4 5

J.M. Angelelli,A.R. Katritzky, R.F. Pinzelliand R.D. Topsom, Tetrahedron, 28 (1972) 2037. V.N.Sheinker, A.D. Garnovskijand O.A. Osipov, Usp. Khim., 50 (1981) 632. R. Grigg,J.A. Knight and M.V. Sargent, J. Chem. Soc., (1965) 6057. R. Grigg, M.V. Sargent and J.A. Knigth, Tetrahedron Lett., (1965) 1381. D.J.Chadwick,J. Chambers, G.D. Meakinsand R.R. Snowden,J. Chem. Soc., Perkin Trans. 2, (1975) 13. 6 L. Ballester, B. Caballero,J.F. Bertran and R. Gra, Rev. CENIC, Ciens. Fis., 7 (1976) 113. 7 M. Rodriguezand J.F. Bertran, Rev. CENIC, Ciens, Fis., 6 (1975) 37. 8 V.N.Sheinker,E.G. Merinova,M.E. Pererson and O.A.Osipov,Zh. Obshch. Khim.,46 (1976) 1582.

202 9

Gy. Schultz, I. Fellegwlri, M. Kolonits,/i~.I. Kiss, B. Pete and J. B~nki, J. Mol. Struct., 50 (1978) 325. 10 B. Pete, ,~.I. Kiss, J. Mink, M. Gdl, and J. B~nki, Spectrochim. Acta, Part A, 36 (1980) 633. 11 L.E. Sutton, Tables of Interatomic Distances, Spec. Publ. No. 11, Chem. Soc. London (1958); Supplement No. 18 (1965). 12 M. S~ndchal and P. Saumagne, J. Chim. Phys., 69 (1972) 1246. 13 P. Adamek, K. Volka, Z. Ksandr and I. Stibor, J. Mol. Spectrosc., 47 (1973) 252. 14 J. B~inki, F. Billes, M. G~l, A. Grofcsik, Gy. Jalsovszky and L. Sztraka, Acta Chim. Hung., 123 (1986) 115. 15 L. Ballester and J.F. Bertran, Spectrochim. Acta, Part A, 34 (1978) 377. 16 J. Bdnki, Gy. Schultz and I. Hargittai, personal communication. 17 C. Petrongolo, Chem. Phys. Lett., 42 (1976) 512. 18 J. Capdevila and E. Canadell, J. Heterocycl. Chem.,18 (1981) 1055.