Spcchoehimico Acta. Vol. 49A. No. 8, pp. 1105-1108, 1993 0584-8539/93 $6.00+0.00 Printed in Great Britain @ 1993 Pergamon Press Ltd

Substituent effects in IR spectroscopy-XII. Field effects of 4-substituents on the cyclopropyl r(CH) vibrator of tricyclenes*

CHRISTIAN LAURENCE and MARYVONNE LUCON

Laboratoire de Spectrochimie, Universite de Nantes, 44072 Nantes Cedex 03, France

and

HAN JOONG KOH and DAVID G. MORRIS Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, U.K.

(Received 16 June 1992; in final form 24 October 1992; accepted 28 October 1992)

Abstract-The variation of the equivalent C2-H and C6-H force constants of 4X-substituted tricyclenes depends mainly on the electrostatic forces exerted by the C4-X dipole. The sensitivity of the cyclopropanic bonds to electronic substituent effects in these tricyclenes is compared to the substituent dependence of v(CH) in other systems.

PREVIOUSLY [l, 21 we have shown that 4-substituted camphors 1 and 4-substituted isobomeols 2 constitute good models for substituent field effects since the rigid bicyclic skeleton ensures that a fixed disposition is maintained between the functional group and the CA-X bond; also steric effects are minimized. In the present work we have employed tricyclenes 3a-k with variable substituents at the C4 bridgehead position in order to investigate the influence of the substituent field effect on the IR stretching frequency, Y(CH) of the cyclopropyl C-H bond. This is of particular interest since the cyclopropyl C-H stretch is one of the stiffest that involves carbon and hydrogen and also because of the unusual nature of the cyclopropane ring. In cyclopropane itself the C-H bond is recognized as having enhanced s character. Values of 30% s and 70% p character have been cited [3]; also hybridization of the carbon-hydrogen bonding orbitals in cyclopro- pane has been cited [4] as SP’.‘~.

To this end we have synthesized a series of 4-substituted tricyclenes 3a-k in which the substituent nature ranges from the electron-donating COO- to the electron-withdrawing NO2 . In these compounds the band observed between 3040 and 3065 cm-‘, on the high wavenumber side of the intense Y(CH~) and v(CH3) bands, as illustrated in Fig. 1, is attributed to the stretching vibration of the C2-H and C6-H bonds of the cyclopropanic ring.

In the absence of Raman spectral data it is uncertain whether the 3040-3065 cm-’ absorptions are to be assigned to the symmetric or asymmetric stretching modes, or both. We are not aware of IR results on this subject for 1,1,2,3 tetra-substituted cyclopro- panes. However, cyclopropene, 4, can assist in understanding the coupling of C2-H and C6-H in tricyclenes (part structure shown in 5) whose dimensions are extrapolated from 4-chloronortricyclene [5]. The vibrational study of cyclopropene [6] indicates that v, and vS are separated by only 34 cm-’ for the vinylic stretching on a xenon matrix, and that the IR intensity of vS is much lower than v,. We suggest that the lower HCC angle in tricyclene, as compared with cyclopropene, 138” instead of SO”, must lead to a still weaker coupling; due both to a smaller separation between v, and v, and also a low IR intensity of vS, these modes cannot be differentiated in an IR spectrum of a solution in carbon tetrachloride. Presently we study the substituent dependence of this v(CH) vibration.

l For Part XI of this series, see Spectruchim. Acta 47A, 1649 (1991).

1105

1106 CHRISTIAN LAURENCE et al.

3ooo 2900 2 IO

Fig. 1. Infrared spectrum of the CH stretching region of tricyclene (3b) as a solution in carbon tetrachloride. The band at 3053cm-’ (extinction coefficient s=50Lmol-’ cm-‘) is caused by cyclopropanic CH stretching vibrations. The remaining bands, below 3000 cm-‘, are due to the

two methylene and the three methyl groups.

EXPERIMENTAL

4Substituted tricyclenes 3a, 3c-f, 3h and 3j were synthesized as described in the literature [7], as was 3k [8]. Tricyclene (3b) was obtained by a literature method [9]. Compounds 3g and 31 were prepared from 1-iodocamphene [lo] and 1-bromocamphene [ 111, respectively, using the previously described methodology [7]; full details will be reported elsewhere.

Infrared measurements were recorded on dilute solutions of the tricyclenes in carbon tetrachlor- ide; a 1 mm path-length was employed. Spectra were recorded with a Bruker IFS 45 high resolution FUR spectrometer with 256 scans at a resolution of 0.5 cm-‘.

1 2 3

13k

D H

li X D C -C=N2

I D H H

6 7

X -a- CCC-H

8 Formulae.

Substituent effects in IR spectroscopy-XII

RESULTS

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In Table 1 we present the wavenumbers of the stretching vibration Y(CH) of the cyclopropane C”H and C&H bonds, together with the values of the corresponding substituent field constant OF. TheSe SUbStitUent COUStaUtS are principally derived from gas phase basicity and acidity measurements [12] and agree well with values calculated theoretically [13]. The oF value of the -COO- group was determined from the v(C0) wavenumber of the tetrabutylammonium camphor-4-carboxylate [ 141. The strong depen- dence of field effects on the distance betweeen the substituent and the function and on the dielectric environment restricts the use of this value to systems, such as the 4- substituted tricyclenes 3, related to this definition model and to solvents of dielectric constant similar to that of carbon tetrachloride.

DISCUSSION

The least-squares method leads to Eqn (1) for the correlation between v(CH) and oF

v(CH) = 17.750, + 3050.2. (1)

For the 10 substituents in our series the correlation coefficient is 0.944 and the standard deviation is 1.7 cm-‘. From the last column in Table 1 it is seen that the substituents H, NO* and OH are the most deviant. The quality of the fit of Eqn (1) is not the highest and accordingly we have checked this equation by using a 4-substituted tricyclene with a very strongly electron-withdrawing substituent, I[OCO(3-ClC&I.,)]2. A value of or= 1.19 is obtained for this substituent when one inserts the value for the cyclopropanic v(CH), 3071.4cm-‘, into Eqn (1). This value compares rather well with that of oF= 1.18 reported recently for I(OCOCF,), [15].

Equation (1) demonstrates that variation of the carbon-hydrogen stretching fre- quency, and fundamentally the C-H force constant, derives mainly from electrostatic forces exerted longitudinally on the C2-H and C6-H dipoles by the field of the Cl-X dipole.

The sign and magnitude of the regression coefficient, p= +19.7 cm-‘, of the above equation are to be compared to other regression coefficients found for the dependence of Y(CH) vibrations of different systems on electronic substituent constants. Thus for 2,4,6- trideutero-l-substituted benzenes 6, p= +12.6 cm-‘, for 3- or Csubstituted diazoaceto- phenones 7, p=O and for 4-substituted phenylacetylenes 8, p= -6.9cm-’ [13]. The change of sign of p in passing from the series 4 to 6 has been attributed [15] to the increase in the polarity of the C-H bond, which manifests itself in the ability of the C-H bonds marked bold in 7 and 8 to function as hydrogen bond donors, whereas compounds

Table 1. Wavenumbers (cm-‘) of the cyclopropanic C-H stretching vibration of 4-substituted tricyclenes 3a-k and substituent constants a,

Compound Substituent (X) v(CH) uF Deviations*

3a COO- +NBu~ 3043.7 -0.29 -1.3 3b H 3053.1 0 2.9 3c CHzOH 3053.9 0.14 1.2 3d COOH 3056.0 0.28 0.8 3e OH 3053.6 0.30 -1.9

z NHCOOMe I 3055.1 3056.1 0.31 0.40 -0.6 -1.2 3h Cl 3057.3 0.45 -0.9 31 Br 3057.0 0.45 -1.2 Y ~~o(3-clC&14)]2 3063.8 0.65 2.1 3k 3071.4 t

* In cm-‘. Deviations are v&I-I) observed - v(CH) calculated. t Unknown value.

h) 49:9-E

1108 CHRISTIAN LAURENCE et al.

of type 6 do not form detectable hydrogen bonds, as monitored by IR spectroscopy [16]. Consistent with this, the positive sign of p obtained in the present work accords with the cyclopropyl C-H bonds of the tricyclenes 3a-j presently studied not being hydrogen bond donors. In effect we have been unable to find any evidence of hydrogen bonding acidity of the cyclopropyl C-H bonds by the method of ALLERHAND and SCHLEYER [17].

The magnitude of p 19.6cm-‘, currently observed for the tricyclenes 3a-j is only slightly greater than that, p = + 12.6 cm-‘, found for m-disubstituted benzenes. Since spatial separation is the principal determinant of the field effect of a substituent on a functional group, this similarity may in part arise from the same number of carbon- carbon bonds between the dipoles C-H and C-X in the two systems 3 and 6.

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[2] M. Berthelot, C. Laurence, J.-Y. Le Questel, G. W. Bradley and D. G. Morris, Spectrochim. Acta 47A, 1649 (1991).

[3] K. B. Wiberg, in The Chemistry of the Cyclopropyl Group (Edited by Z. Rappoport), Chap. 1. John Wiley and Sons, Chichester (1987).

[4] M. A. Bemett, J. Chem. Educ. 44, 17 (1967). [5] J. F. Chiang, C. F. Wilcox and S. H. Bauer, Tetrahedron, 369 (1969). [6] R. W. Mitchell, E. A. Dorko and J. A. Merritt, J. Molec. Spectrosc. 26, 197 (1968). [7] D. G. Morris and A. M. Murray, J. Chem. Sot., Perkin Trans. 2,734 (1975). [8] D. G. Morris and A. G. Shepherd, J. Chem. Sot., Chem. Commun., 1250 (1981). [9] W. Reusch, M. W. Di Carlo and L. Traynor, J. Org. C/rem. 26, 1711 (1961).

[lo] A. Press and S. Stemhell, Awtr. J. Chem. 23,989 (1970). [ll] A. Pross and S. Stemhell, Austr. J. Chem. 24, 1437 (1971). [12] R. W. Taft and R. D. Topsom, Prog. Phys. Org. Chem. 16, 1 (1987). [13] R. D. Topsom, Prog. Phys. Org. Chem. 16, 125 (1987). [14] C. Laurence, in Similarity Models in Organic Chemistry, Biochemistry and Related Fields (Edited by R. I.

Zalewski, T. M. Krygowski and J. Shorter), p. 231. Elsevier, Amsterdam (1991). [15] L. Yagupol’skii, A. Y. Il’chenko and N. V. Kondvatenko, Russ. Chem. Res. 43,32EE (1974); C. Hansch,

A. Leo and R. W. Taft, Chem. Reu. 91, 165 (1991). [16] C. Laurence, M. Berthelot, L. L. Leveson and C. W. Thomas, Spectrochim. Acta 38A, 487 (1982). [17] A. Allerhand and P. von R. Schleyer, .I. Am. Chem. Sot. 88,950 (1966).