Conformational equilibrium of dicyclopropyl ketone

Spectrochimica Acta, Vol. 43A, No. 7, pp. 939-942, 1987.

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Conformational equilibrium of dicyciopropyl ketone GERD SCHRUMPF* and THOMAS ALSHUTHt * Institut fiir Organische Chemic der Universit/it, Tammannstr. 2, D-3400 G6ttingen, F.R.G.; and ~"MaxPlanck-Institut fiir Biophysikalische Chemie, Am Faflberg 11, D-M00 G6ttingen, F.R.G. (Received 21 July 1986; in final form 12 December 1986; accepted 19 December 1986) Abstract--The i.r. and Raman spectra of liquid dicyclopropyl ketone have been reinvestigated from 4000 to 200 cm- 1 with higher resolution than previously obtained. In addition, the i.r. spectrum of the polycrystaUine solid was recorded from 4000 to 400 cm- t. Contrary to published results, evidence has been obtained for a conformational equilibrium between the predominant cis-cis form and the oauche-gauche conformer.

INTRODUCTION In view of the hybridization of the cyclopropyl carbon atoms with the exocyclic orbitals being largely sp 2 hybrids, it is interesting to study the conformational structure of appropriate carbonyl-substituted cyclopropanes. In cyclopropyl aldehyde [1], cyclopropyi methyl ketone [2-4], cyclopropanecarboxylic acid [5-] and its derivatives [6-9], an equilibrium between cis and trans conformers was observed. This is in contrast to the trans-gauche equilibrium detected for vinylcyclopropane [10, 11-], which has also a s p 2 hybridized carbon atom directly attached to the ring and is able to participate in a conjugative interaction with the cyclopropane moiety. The vibrational spectra of dicyclopropyl ketone were interpreted as being consistent with the occurrence of the cis~is conformer only with C2v symmetry [-12]. The trans-trans con-

O

O

cc C2v

gg C2

former also having C2v symmetry was rejected on account of massive steric hindrance. The presence of a second rotamer was also excluded. However, in contrast to the statement of NEASE and WURREY [12] that no Raman bands disappear on cooling (which would have been proof of another conformer in the fluid phase), we note by inspection of their data three cases where obviously Raman bands do disappear (877, 728, 534 c m - 1). I.r. data of the solid phase should reveal the disappearance of further bands which might possibly be too weak in the Raman to be observed. Thus, we have reinvestigated the Raman spectra of the title compound with improved resolution and the i.r. spectra of the liquid and the solid with the aim of searching for evidence of a second conformer. EXPERIMENTAL

Commercial dicyclopropyl ketone was purified by distillation over a 100cm spinning hand column. The constant boiling middle fraction was redistilled using the same column but with a greatly reduced take-off ratio (1:100). No impurities could be detected by gas chromatography on different columns (5 % SE-30, 3 % dodecyl phthalate, 3 % FFAP, all on chromosorb W, 60-80 mesh, 2 m, 100°C isotherm) and on a capillary column (SE-30, 40 m, 70-200°C programmed at 2°C/min). Spectra were obtained as described previously [13] (Table 1). RESULTS AND DISCUSSION

0

0

cg Ci

gg' Cs

O

O

tt C2v

tg Ct

For the most part, the Raman and i.r. spectra of the liquid observed previously and in the present work do not differ greatly. However, we measured with improved resolution several bands that were not resolved formerly showing additional bands and a number of weaker signals not yet reported. Furthermore, the i.r. spectrum of the solid has been recorded here. These improvements led to results differing from those published. In contrast to the previous study, we observed six CH stretches above 3000 c m - 1 for the main conformer in the liquid. As no coupling of the CH2 modes in different rings is expected, the pair at 3096 and 3087cm -1 represents the locally antisymmetric stretches, the pair at 3024 and 3011 cm -1 the symmetric combinations. However, the CH stretching 939

GERD SCHRUMPF and THOMAS ALSHUTH

940

Table 1. Vibrational spectra of dicyclopropyl ketone* Raman Liquid 3096 m,p 3087 m,dp 3054 m,b,p 3024 sh,m 3011 vvs,p 1683 ms 1675 sh,m

I.r. Liquid 3095 m 3085 sh,mw 3046 m,b 3023 sh,m 3010s 1685 s 1672 sh,m

Solid 3088 mw 3061 w 3046 m 3018 m 3002m 1683 ms

Interpretation CH2 as str CH2 as str C H str C H str CH2 sym str CH2 sym str CO str

1662 sh,w 1654 w 1460 sh,w,p 1443 ms,p 1438 sh,w,dp? 1420 mw,dp 1404 w,dp

1350 ms,p 1341 w,sh,p? 1312vw,p

1460 w 1448 m 1442m

gg 1451 m CH 2 def 1433 ms

1418sh 1404 s 1391 sh 1378 sh 1350 w 1341 sh,vw

1405 s 1391 s 1375 sh, m 1349 ms 1338 vw

gg? C H 2 def CH2 def C H bend C H bend gg?

1302 vw 1220sh,w,p? 1203 vs,p 1187 m,p?

1220m 1204 w 1186m

1111 mw,b,dp

1111 sh,m

1092 w,dp

1095 s

1061 vw,dp

1065 s

1033s,p 1013 sh,w,dp

1031 ms 1013m

877 w,dp 848 mw,b,dp 824 sh? 815 s,b,dp

985 m 919m 911m 877 w 848 m 820 m, b 815sh

794 sh,w,p?

793 w

753 ms,b,p

756 vw

735 sh,vw 728 w,p 628 vw,b,dp

734 w 728 sh,w 629 mw

534 w,p 467 w,p 418 vw 387 sh,w 364 m,b,p 360 sh,w,dp?

534 w 466 w 420 m,b

985 vw,dp 921 s,b,p?

307 vw,dp

360w 355w 350w 308 sh

1272 w 1224vw 1204 ms 1185m 1181 mw 1178 sh,w lll7sh,m lll5ms 1099 sh,s 1096 s 1065 s 1060 sh,mw 1044w 1031 ms 1011sh,m 1006 ms

2 x 636 B1 ring puls A1 ring puls CH2 twist

C H 2 twist C H bend C H 2 wag

CH2 wag B~ CC str gg CC str

924 vw 918 w

A 1 ring def B~ ring def

846 s,vvb 821 m 816m 810 m 794 m 787 ms 784 ms 765 vw 761 m 751 w 740w

B 2 ring def CH2 rock C H 2 rock

636 s 631 sh,w 618w 600w 579 m 516w

CO wag

420m

CO wag?

gg ring def

CH2 rock

At CC str

gg CC str

C C C bend? gg C C C bend?

Conformational equilibrium of dicyclopropyl ketone

941

Table 1. (Contd.) Raman

l.r.

Liquid

Liquid

288 w,p 277 sh,vw,p 242 w,b,dp 225 sh,vw,dp 212 sh,vw,p? 203 w,b,p

Solid

Interpretation

302 m 289 sh,w 278 w 266 vw 240 w,b 223 w 205 sh,w

*Abbreviationsused:w, weak;m, medium;s, strong; b, broad; v, very;str, stretch;def, deformation; puls, pulsation; as, asymmetric; sym, symmetric. For details of the interpretation see text.

vibrations of the two rings do couple: the polarized Raman line at 3054 c m - 1 represents the in-phase, the i.r. band at 3046 c m - 1 the out-of-phase CH stretch. Since we observe two bands near 1200 cm- 1, some coupling of the breathing modes of the two rings occurs. On account of their intensities, we assign the signal at 1220 c m - t to the B1, the band at 1203 c m - 1 to the A1 ring pulsation fundamental. Evidence for a second conformer is scattered over the whole spectrum. In the CH2 scissoring region, four signals are expected in the Raman spectrum of the liquid, but at least five Raman and even more i.r. lines are observed. The polarized band at 1443 cm-1 (A1) has a polarized high frequency shoulder, that we consider to be due to the A1 scissors of a second conformer. It also occurs as a weak i.r. absorption, that is absent in the solid. The signal at 877 c m - ~ in the liquid disappears on crystallization in both types of spectra. We attribute this signal to one of the ring deformations of a second conformer. Since the line is depolarized, the minor rotamer must have an element of symmetry, which excludes the cis-gauche rotamer (C1). That form has no symmetry at all, and its normal modes in the Raman spectrum should all be polarized. There is a weak depolarized Raman signal at 985 cm - I in the liquid, which was not observed previously. The corresponding i.r. absorption was attributed to the B~ CCC stretch of the main conformer by NEASEand WURREY[12]. However, the i.r. band definitely and reversibly disappears on cooling. We assign the line to the antisymmetric CCC stretch of a second rotamer, which has been observed in this frequency region in dicyclopropyl methane, too [13]. Since the Raman band is depolarized, the minor form must have a symmetry element, i.e. it is the gauche-gauche (C2)or the gauche-gauche' form (C,) of dicyclopropyl ketone. These two cases cannot be distinguished by vibrational spectroscopy in molecules of this size. The symmetric CCC stretch at 753 c m - ~ is accompanied by a weak i.r. band at 728 c m - 1 in the liquid, which reversibly disappears on cooling. The cor-

responding Raman line in the liquid is also absent in the solid [12]. We attribute that band to the symmetric CCC stretch of the minor conformer. All of this is conclusive evidence for the presence of a second conformer at room temperature along with the main cis-cis conformer. From the observation of depolarized bands, the minor rotamer is probably a gauche-gauche form. From inspection of molecular models, the Cs gauche-gauche' conformer with dihedral angles much larger than 90 ° relative to the cis-cis form would suffer from strong steric repulsion between the secondary hydrogen atoms on different ring, which ultimately leads to the sterically impossible trans-trans form. Gauche-gauche' rotamers having angles smaller than 90 ° are less strained but still higher in total energy than the gauche-gauche form. This was also observed in dicyclopropyl methane [13]. Thus, we conclude that the minor conformer is the gauche-gauche form. That gauche conformers are observed in dicyclopropyl ketone is not totally unexpected. Generally, in ct,fl-unsaturated carbonyi compounds, the coplanarity of the carbonyl and the double bond system for optimum n-orbital overlap is achieved only by a destabilizing ecliptic relation between the substituents attached to the carbonyl group and to the or-carbon. This also holds for the cis conformer of cyclopropylcarbonyl derivatives. If two successive ecliptic relations occur as in dicyclopropyl ketone, the increase in Pitzer strain will further destabilize the bisected cis relative to the gauche conformation. In the present study, we did not find evidence for the unsymmetrical cis-gauche conformer. This is surprising, because, if the cis form is generally favoured in carbonyl-substitued cyclopropanes, the next stable rotamer in dicyclopropyl ketone, where a trans conformation is energetically impossible, should be the cis-gauche form. This is found in many molecules with two successive CC bonds, about which rotational isomerism occurs as for instance the linear saturated hydrocarbons. In dicyclopropyl ketone, the minimum energy path for conformational interconvcrsion is the one, in which the two cyclopropyl groups leave the

942

GERD SCHRUMPFand THOMASALSHUTH

energy minimum at the cis-cis form by simultaneous torsion in opposite directions. Our result implies that the two rotations are strongly coupled. At the moment, it is not yet clear whether this cooperative conformational interconversion is sterically or electronically controlled. Acknowledgements--We are grateful to Dr M. STOCKBURGER, G6ttingen, for the use of the Raman equipment of his research group. This work was supported by the Fonds der Chemischen Industrie.

REFERENCES

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[3] P.L. LEEand R. H. SCHWENDEMAN,J. molec. Spectrosc. 41, 84 (1972). [4] J. R. DURIG, H. D. BIST and T. S. LITTLE, J. molec. Struct. 116, 345 (1984). [5] J. MAILLOLS,V. TABACIKand S. SPORTOUCH,J. molec. Struct. 32, 173 (1976). ['6] J. R. DURIG,H. D. BISTand T. S. LITTLE,J. chem. Phys. 77, 4884 (1982). ['7] J. E. KATON,W. R. FEAIRHELLER,Jr and J. T. MILLER, Jr, J. chem. Phys. 49, 823 (1968). [8] J. R. DURIG, H. D. BIST, J. A. SMOOTERSMITH, S. V. GRANTand T. S. LITTLE,J. molec. Struct. 99, 217 (1983). [9] L.S. BARTELL,J. P. GUILLORYandA. T. PARKS,J. phys. Chem. 69, 3043 (1965). [-10] A. DE MEIJEREand W. LOTTKE, Tetrahedron 25, 2047 (1969). [-11] M. TRAETTEBERG,P. BAKKEN,A. ALMENINGENand W. LOTTKE, to be published. [-12] A. B. NEASEand C. J. WURREY,J. phys. Chem. 83, 2135 (1979). [13] G. SCHRUMPFandT. ALSHUTH,Spectrochim. Acta 41A, 1335 (1985).