Laser-Raman spectroscopy of disubstituted spiro-cyclopropane-1,9′-fluorene stereoisomers

SpMmchimica Acta, Vol. 35A, pp. 1303 to 1306 @ Pergamon Press Ltd., 1979. Printed in Great Britain

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Laser-Raman spectroscopy of disubstituted Spiro-cyclopropane-1,9’-fluorenestereoisomers CHRISTIAN

Laboratoire de Photochimie

DECKER

Get&ale, Equipe associee au CNRS, Ecole Nationale Superieure de Chimie, 68093 Mulhouse, Cedex, France (Received 14 February 1979)

Abstract-Raman spectra are reported for 2,3-dicarbomethoxy and 2,3-dimethyl-spiro-cyclopropane1,9’-fluorene stereoisomers in the crystalline state. A tentative assignment is proposed based on comparison with the spectra of fluorene and of substituted cyclopropanes. The significant changes observed in the region 20-100 cm-’ allow a clear distinction between the cis and trans isomers. INTRODUCITON

investigating the cis-trans photoisomerization of some disubstituted Spiro-cyclopropane-1,9’fluorenes [l, 21, we have considered Raman spectroscopy as a possible technique to distinguish stereoisomers in the crystalline state. Gas-liquid chromatography has been previously used by VON DOERING and JONES [3] to separate the cis and tram isomers of 2,3-dimethyl-spirocyclopropane1,9’-fluorene. However, we have shown [l] that, under those analytical conditions (22O”C), thermal isomerization takes place extensively in the GLC inlet and column thus providing misleading results. Since both U.V. and i.r. absorption spectra of the cis and trans derivatives do not differ significantly enough to allow a clear distinction between them, the most reliable method appeared to be NMR spectroscopy. While

R: CHj, COOCH,. Cis and trans isomers were easily differentiated by monitoring the methyl protons which appear at 3.77 and 3.70ppm for the cis and trans diesters respectively, and at 1.32 and 1.47 ppm for the cis and tmns dimethyl derivatives, respectively. This technique was used successfully to study the photoisomerization of these disubstituted cyclopropanes [l, 21, but it applies only to experiments carried out in solution. We are therefore interested in techniques of high sensitivity which will enable us to detect and distinguish the stereoisomers also in the solid state. Laser Raman spectroscopy is possibly such a technique allowing direct analysis of the crystalline sample at ambient temperature. Although the vibrational spectra of both cyclopropane [4] and fluorene [5,6] were studied by many workers, no previous spectroscopic work has

been done on disubstituted spiro-cyclopropanefluorene compounds. In this paper, I will present the Raman spectra of these derivatives which have proved useful to differentiate and quantitatively analyze the crystalline cis and trans dicarbomethoxy and dimethyl substituted stereoisomers. JSXPERIMFBTAL

Samples used in this work were prepared according to the procedure described by BRAUNand coworkers [7]. Cis and tram 2,3-dicarbomethoxy-Spiro-cyclopropane-1,9’fluorene were synthetized by thermal addition of fluorene carbene on maleic or fumaric acid derivatives. Cis and tmans 2,3-dimethyl-Spiro-cyclopropane-1,9’-fluorene were synthetized by photolysis of 9-diazofluorene in the presence of cis or fmns butene at low temperature. The purity of each of the stereoisomers was checked by NMR spectroscopy [8]. The Spiro-cyclopropane sample, in the form of a crystalline powder, was examined directly on the 180” viewing platform of a Jobin-Yvon Ramanor HG 2s spectrometer equipped with a Spectra-Physics model 171 argon ion laser. The power at the sample was adjusted to 400 mW at 5145 A; under those conditions no thermal isomerization was observed, even after extended analysis time. A scan rate of 0.33 cm-‘/s and time constant of 0.5 s or less were used to record all the spectra. Plasma lines from the laser were routinely used to verify the calibration of the spectra to *2 cm-‘.

RESULT AND DE3CLJSSION The spectral results from the cis and tram isomers of dicarbomethoxy and dimethyl-substituted Spiro-cyclopropane-1,9’-fluorene are given in Table 1. For illustration, the Raman spectrum of the trans diester derivative is shown in Fig. 1. In the absence of full normal coordinate calculations, we will only make a few comments on the assignments for these Spiro-cyclopropane-fluorene molecules. By comparison with the Raman spectra of fluorene and of l,l-diphenyl-2,3-dicarbomethoxy-cyclopropane and making full use of earlier work [9,10], we were able to attribute most of the Raman bands of these derivatives. The specific vibrations of onho-substituted benzenes are observed in the Raman spectra of

1303

C. DECKER

1304

Table 1. Raman frequencies of 2,3 disubstituted Spiro-cyclopropane-1,9-fluorene

:i;[Tm*I I Tj2,i/ 66

52 :

100 a? 6s

225

210

L15

!n

Y

1

P

6

205 CL4 105 m ”

2‘4

%” 67 90 sI8 41 Y

123

220

”

405 m

s

Y

k13 m

LlL s

418 n

51.3 Y

518 Y

5lla Y

Le.0 ”

b.50 v

518 T1

510 Y

655 m

652 Y

690 s

695 s

693 s

691 s

755 m

751 m

750 Y

752 ”

741 8

768 y

790 Y

795 v

790 Y

7.35 ”

515 8

8117 m

820 Y

a45 Y

890 Y

875 ”

865 Y

a60 Y

860 Y

;i:

w y

960 931 Y ”

970 w

960 Y

975 930 Y ii

998 Ic

997 m

980 Y

985 y

1026 s

1029 s

1028 s

1028 8

1020 *

1075 m

1060 Y

:::z : 1162 m

1090 1140 vs 1161 m

tcao lllr9 m s 1I’TI m

1088 m lllr9 9 1170 m

1090 Y 1147 m 1165 Y

,215 Y

1200 Y

123c a 12.30 m

1226 s 1275 m

1225 m 1290 m

1225 9 1290 m

1234 8 129o m

13Oh s

130,

s

1300 8

1300 I

1345 5

13L3 s

13LO 8

1340 s

1342 m

,L*0 w le.5 *

l&O II l&30 s

lb43 Y 14’18 0

1442 Y 1477 m

llIL5 Y 1476 m

1578 c 1609 9

z :z;:

‘573 m 1608 s

157h m 1606 s

,735 il

1735 ”

2945 Y

2945 Y

2955 Y

2955 ”

3035 Y 3050 Y

3040 Y 3055 Y

3OLO 74 3050 w

3045 Y 3055 ”

3&T Y

3080 Y

3076 Y

3018 w

:

3045 m 3055 m

and fluorene (cm-l)

Lasar-Raman spectroscopy of disubstituted Spiro-cyclopropane-1,9’-fluorene

/ 1500

stereoisomers

1305

I

1000 Wave

number,

cm-’

Fig. 1. Raman spectrum of &ans 2,3-dicarbomethoxy-spiro-cyclopropane-l,9’-fluorene.

fluorene and of all the four spiro-cyclopropanefluorene compounds investigated. The stretching of the benzene ring gives rise to characteristic prominent bands at 1480 and 1607 cm-‘, while the medium intensity bands at 415 and 520 cm-’ are attributed to out-of-plane ring bendings by quadrants. The band at 750 cm-’ which is particularly strong in fluorene is assigned to the out-of-plane C-H bending vibration with 4 adjacent hydrogens. In-plane CH bendings of orth+substituted benzene give rise to prominent bands at 1028, 1147, 1230, 1280 and 1345 cm-’ while the CH stretching vibration is observed in the range 3035-3055 cm-‘. The symmetric and asymmetric cyclopentane ring stretchingsappear at 860 and 790 cm-’ respectively as weak bands, probably because of complete substitution. The very strong band at 1300 cm-’ must be the symmetric cyclopropane ring breathing. This vibration which appears at 1180 cm-r in cyclopropane is shifted to the region 1300-1350 cm-’ in l,l-disubstituted alkyl cyclopropanes. In these compounds, there is also an intense band in the region 650-700 cm-l, characteristic of the quaternary carbon and which appears in our Spiro derivatives at 690 cm-‘. The asymmetric cyclopropane ring deformation, which seems to be sensitive to conformation, can be associated with the band at 815-845 cm-‘. The weak bands appearing at 480, 655, 1215 and 1725 cm-’ in the diesters isomers only are assigned to in-plane bending and stretching of the ester group [ll]. For distinction between the stereoisomers, the 800-850 cm-’ region may provide a possible ansS.*.(A) 32/12--8

wer on the cis-tram composition of an irradiated sample since the band assigned to the asymmetric ring deformation shifts from cyclopropane 815 cm-’ in the tram isomer to 847 cm-’ in the cis isomer, for both diester and dimethyl substituted derivatives. However, this peak cannot be used in a simple way for distinction purposes because of the large intensity difference between the two diesters stereoisomers and of its too weak intensity in the dimethyl derivatives. The other main difference between the Raman spectra of cis and trans isomers appears in the very low energy region corresponding to skeleton deformations (Fig. 2). The most interesting band to distinguish the diester stereoisomers consists in the sharp peak which appears at 42 cm-r in the cis compound while it is absent if the substituents are trans. The calibration curve obtained by plotting the intensity of the peak at 42 cm-‘, ratioed against the 1607 cm-’ peak, as a function of the fraction of cis isomer for various cis-trans mixtures permits to determine the cis/trans ratio of irradiated samples. This method is particularly appropriate to detect small amounts of the cis isomer in tram and cis mixtures and was successfully used to determine precisely the photostationary state composition which consists of 3% cis and 97% trans [l]. Similar determinations of the cisjtrans ratio can be carried out for the dimethyl derivatives by using the sharp peak at 28cm-’ and for l,l-diphenyl2,3-dicarbomethoxy-cyclopropane by using the peak at 37 cm-‘, both of which appear only in the trans isomers (Fig. 2).

C. DECKER

1306

100

50

100

100

50

Wave number,

50

100

50

cm-’

Fig. 2. Raman spectra near the exciting line of disubstituted cyclopropanes and of fluorene.

CONCLUSION

PI C. DECKER, G. RAVIER, A. M. BRAUN and J.

Comparisons of the Raman spectra of disubstituted Spiro-cyclopropane-1,9’-fluorenes to fluorene and to polysubstituted cyclopropanes spectra permit to assign most of the observed bands. The differences between the spectra of corresponding cis and rruns compounds arise partly from coupling of vibrations of the substituent group and the cyclopropane ring and mostly from different torsional modes which are apparent in the low energy region. It is possible to use these findings to distinguish the cis and trans isomers and to analyze quantitatively the cis and trans mixtures resulting from photoisomerization.

r-31W. VON DOEFUNGand M. JONES,JR., Tetrahedron L-en. 12, 791 (1963). r41 See, for example, G. HERZBERGin: Infrared and Raman Spectra, p. 352. Van Nostrand, New York (1969). 151 A. BREE and R. ZWARICH,J. Chem. Phys. 51,912

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FAURE,Noun. J. Chim. 2, 515 (1978).

(1969).

[61B. WYNCKEand A. HADNI, Spectrochim. Acta, 27A, 1929 (1971). 171 A. M. .BRAUN, H. G. CASSIDY,R. C. SCHULZand H. TANAKA.Makromol. Chem. 146. 195 (1971). @I H. FRITZ, T. WINCKLER,A. M. B&m ‘and C. DECKER,Helu. Chim. Acta 61, 661 (1978). [91 N. B. COLTHUP,L. H. DALY and S. E. WIBERLEY, in: Zntroduction zo Infrared and Raman Spectroscopy.

Academic Press, New York (1975). F. F. BENTLEY,in: [lOI F. R. DOLLISH,W. G. FATFZLEY, Characteristic Raman Frequencies of Organic Compounds,. Wiley, New York (1974). r111 D. L. POWELL,P. K. LABOE,and D. H. CHRISTENSEN, J. Mol. .%ucture 15, 77 (1973).