Intramolecular interaction in gibberic acid and its derivatives

Spectrochimiea Acta, Vol.42A,No. 4, pp. 405-408, 1986.

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lntramolecular interaction in gibberic acid and its derivatives SALMAN R. SALMAN,G. A. W. DERWISH* and SAHAR S. AL-SALIH Chemistry Department, College of Science, University of Baghdad, Baghdad, Iraq

(Received 31 January 1985; accepted 2 September 1985) Abstract--Spectroscopicinvestigation (u.v., i.r. and NMR) of gibberic acid, epigibberic acid, aliogibberic acid and epiallogibberic acid suggests a weak intramolecular interaction of the aromatic ring with the terminal methylene group in the case of epiallogibberic acid, and with the terminal earbonyl group in the case of epigibberic acid.

INTRODUCTION

NAGAKURA and TANAKA [1] and NAGAKURA [2] observed a new band, inferred as being due to intramolecular charge transfer, in the u.v. spectra of substituted nitrobenzene, acetophenone and benzoic acid. HOFFMAN et al. [3] and HOFFMAN [4] suggested that there is a through-space and through-bond interaction between n- or n-electron systems which are responsible for the appearance of this new charge-transfer band. Optimal through-bond interaction between two nelectron systems A and B, which are separated by two saturated carbon atoms (C1 and C2), can be expected [5] when the lone pair orbitals on A and B are parallel to each other and to the C1-C 2 sigma bond. It has been noted [6, 7] that systems fulfilling the conformational requirements outlined by HOFFMAN concerning the ~t-electron system for A and/or B often show an absorption band in the u.v. region. These absorption hands have been designated as sigmacoupled transitions [7]. Other studies [8] indicated that it is possible to detect a charge-transfer band in compounds such as trans-6-thioperhydronaphthylidene-2-malonitrile where the donor and the aeceptor groups are separated by five sigma bonds. In the present work gibberic acid (1) and its derivatives epigibberic acid (2), allogibberic acid (3) and epiallogibbericacid (4) were studied by u.v., i.r. and NMR spectroscopy to explore the possibility of intramolecular interaction between the donor and aeceptor groups which are contained within these compounds.

17 CH3

CH3

2

~

.

.

.

O

H

jHa

c\ ~HH"" ~ C ( ) O ' ~ / ~

Hb

3

~

...OH/Ha

4

The four compounds contain a n-donor group, the aromatic ring, and an acceptor ketonic group, C--O, in the molecules (1) and (2), or C=CHe, in the molecules (3) and (4). In these systems four sigma bonds separate the donor and the acceptor groups, and thus a weak intramolecular interaction is a possibility. EXPERIMENTAL

1

*Present address: Ministry of Industry and Minerals, Baghdad, Iraq. 405

Gibberellinic acid was prepared according to GRovE and MULHOLLAND[9]. From gibberellinic acid it was possible to prepare allogibberic acid and epiallogibberic acid [9]. Gibberic acid was prepared from allogibberic acid, and epigibberic was prepared from epiailogibberic acid [9]. The purity of these compounds were checked by TLC, GLC, u.v., mass spectral and NMR methods. Due to the fact that (1)-(4) are soluble with difficulty in suitable solvents for measurements of the concentration dependence of i.r. absorption

406

SALMAN R. SALMAN et al.

spectra, i.r. measurements for (1)-(4) were made using KBr discs and in Nujoll muls using Beckman IR-4250 and Perkin-Elmer 137 A spectrometers. Ultraviolet spectra were recorded in methanol on Beckman Acta MV 11 and PyeUnicam SP-800 spectrometers, using 1 cm silica cells. Proton NMR spectra were recorded in acetone-d e using Varian FT 80 A and Varian XL 200 machines. Ultraviolet, i.r. and NMR measurements were made at ambient temperature.

RESULTS AND DISCUSSION

Ultraviolet, i.r. and N M R data were obtained for compounds (1)-(4). jl

a. Ultraviolet spectra Table 1 gives the u.v. absorption values for compounds (1)-(4). Figure I indicates that the O--O transitions in (1) and (3) are more distinct than in compounds (2) and (4). Because the absorption assigned to the K - b a n d is expected to fall in the region where the solvent absorbs considerably, reference is made to the position of the absorption minima ('~min) in the region around 245 nm. The position of these 2~in indicate that the K-bands of compounds (2) and (4) undergo bathochromic shifts relative to (1) and (3). Also, the C = O absorption in (2) undergoes a blue shift relative to (1). Comparing these results with those of perturbed and unperturbed benzene ring [10], one could conclude that the aromatic rings in (2) and (4) are more perturbed than in (1) and (3).

,<

240

260

280

300

320

Wavelength(nm) Fig. 1. The u.v. spectra for compounds (1)-(4).

b. Infrared spectra Table 2 gives the i.r. results for the four compounds. Because these compounds are only soluble with difficulty in suitable solvents, the results obtained can only be tentatively discussed in terms of inter- or intramolecular interaction. In (1) beside the O H . . . O H absorption band, which is due to an intermolecular hydrogen bonding interaction, there is an absorption band at 3500 era- t which probably arises totally or in part from an intramolecular O H . . . . n interaction [11]; no such band was observed in (2), (3) and (4). This is explained as due to the fact that the C O O H group in (2), (3) and (4) is situated away from the benzene ring.

Figure 2 which gives the i.r. spectra of (1) and (2) clearly supports this contention. The C - H out-of-plane bending vibrations in (2) and (4) are shifted to higher frequencies relative to those in (1) and (3). In (2) the keto group absorption and the aromatic C----Cstretching are shifted to higher frequencies relative to (1). The same results were obtained for C=C stretching in (4) relative to (3). It is known that such changes of stretching frequency ofaceeptor groups occur in compounds that contain a donor that interacts with the acceptor group [12].

Table 1. Ultraviolet absorption spectra (nm) for compounds (1)-(4) R-band Compound 1

2 262 266 297 262 266 293 261 265 261 266.5 270

e 273 320 35 273 320 35 287 320 280 32O 280

K-band 2min e

O-O transition 2 emax

242

86

274.5

275

Sharp

249

176

275

279

Broad

239

72

274.5

232

Sharp

248

180

273.5

230

Broad

407

Intramolecular interaction in gibberic acid Table 2. Infrared frequencies (crn-t) of compounds (1)-(4)

Compound

VOH

Vco in ketonic

Vco in COOH

Vcffic aromatic

VC_H in-plane bending flCH

1725

1705

1590

890

1735

1715

1600

905

1694

1587

VC_H out-ofplane bend YCH 785 765

36(X)(sh) 3500(m) 3420(m) 3420(vw) 3400(sh) 3330(s) 3220(sh) 34OO 3330

1690

1607

C:=~H2 C-H bending in

C=:CH2

800 775 790 770

905

807

9OO

770

100

80

8

60

[..

40

\

20

0 4000

I 3500

i 25 O0

3000

I 2000

Wavenumber (cm-l) Fig. 2. Infrared spectra of compounds (1)

Table 3. Chemical shifts (ppm) of compounds (1)--(4)

c. N M R spectra

The proton chemical shifts for the four compounds are given in Table 3. They indicate that for the terminal protons of the C---CH2 group and proton 9 in (4), there is a shift to the low field relative to (3). This is explained as arising from the fact that the shielding effect of the aromatic ring [13] is more effective in (3), due to the geometry of this molecule, than in (4) [14]. This change in the geometry affects JH.,H, which is equal to 6.0 Hz in (3) and 3.4 Hz in (4). This change in Jn,,n, might be due to the interaction between the terminal C---H2 with the ~-electron of the aromatic ring [15]. The results obtained for the four compounds (1)-(4) using u.v., i.r. and NMR results indicate that there is an intramolecular interaction between the donor and the acceptor groups present in these four compounds, and this interaction is more pronounced in (2) and (4). The reason for the absence of an intramolecular charge$A(A)42:4-B

and (2). . . .

Compound

66

69

6axomatic

1 2 3

4.11 3.76 3.88

2.96 3.44 2.78

7.07 6.97 6.88

4

3.65

3.45

7.07

617

t~CH 3

2.15 2.20 2.21

4.9 4.6 5.17 2.26 5.00

transfer hand in the u.v. spectra of (2) and (4) is that the donor and the acceptor orbitals are not parallel to each other, but are twisted by about 60°. Thus the geometrical requirement mentioned by HOFFMAN for optimum interaction between the donor and the aceeptor is not present in these compounds. In addition the separation between the donor and the acceptor groups exceeds three o bonds.

408

SALMAN R. SALMAN et al.

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[8] P. PASMAN, J. W. VERHOEVENand T. J. DEBOER, Tetrahedron Lett. 7,07 (1977). [9] J. F. GROVEand T. P. C. MULHOLLAND,J. chem. Soc. 300s

(1960).

[10"] F.A. MATSEN,Chemical Application of Spectroscopy, p. 667. W. West, London (1956). [11"[ M. OKI, H. IWAMURA,T..ONODA and M. IWAMURA, Tetrahedrorg 24, 1905 (1968). [12] A. W. J. D. DF_gKERS,J. W. VERItOEVENand W. N. SPECIO,MP, Tetrahedron 29, 1691 (1973). [13] R.J. PRYCE,J. chem. Soc. Perkin Trans. I 1179 (1974). [14"] J. F. GROVEand T. P. C. MULHOLLAND,J. chem. Soc. 3007 (1960). [15] L. M. JACKMANand S. STERNHELL,Application of N M R spectroscopy in Oroanic Chemistry, 2nd ¢dn, pp. 273-274. Pergamon Press, Oxford (1969).