The computer analysis of the PMR spectra of several partially deuterated benzocycloheptene derivatives at -120" provides values for all the coupling constants about the C,-C,bond. An interpretation using the Karplus equation shows unambiguously that the seven-membered ring exists as a chair conformation. The relationship between the coupling constants determined shows that the form of the Karplus equation established empirically for cyclohexane is applicable to this seven-membered ring. A comparison of coupling constants shows that the benzocycloheptene chair is more puckered than the cyclohexane chair.

The p.m.r. spectrum of 4,4,5,6,6-pentadeuteriocycloheptene (3) has been studied down to -171 "C. Analysis of the spectral change below -160 "C yielded a AF* value of 5.0 kcal/mol at -165 "C. Possible energy profiles for the ring inversion of cycloheptene are discussed.On a CtudiC le spectre r.m.p. du pentadeuterio-4,4,5,6,6 cyclohepttne (3) jusqu'a -171 "C. L'analyse des changements spectraux en bas de -160 "C permet d'ivaluer a 5.0 kcal/rnol la valeur du AF* a -165 "C. DiffCrents profils CnergCtiques pour l'interconversion du cycloheptirne sont discutCs.Canadian Journal of Chemistry, 49, 1256Chemistry, 49, (1971 Ring inversion has been studied in several due to the olefinic protons. Figure 1 shows the seven-membered cyclic compounds (1). Out of behavior of the two upfield bands as the temthe compounds investigated, 5,5-difluorocyclo-perature is decreased. The upfield singlet broadens heptene (1) and 4,4,6,6-tetradeuteriobenzocyclo-and splits into a doublet whose upfield component heptene (2) have a carbon ring skeleton closely is clearly visible at 6 1.40, the lowfield component related to cycloheptene and yet the free energy barriers (AF*) to ring inversion are significantly different, being 7.4 (2)' and 10.9 kcal/mol (3) respectively. We wish to report the results of a p.m.r. study of 4,4,5,6,6-pentadeuteriocycloheptene (3) which shows that AF* for ring inversion is appreciably lower for this parent compound than for 1 and 2. I \ Results and DiscussionCompound 3 was prepared from bcycloheptenone (4)2 by the following sequence of reactions: (1) base-catalyzed exchange of a-protons by deuterium atoms, (2) protons on C-3, and -7), and a lowfield multiplet 'The AF* value was calculated from the data at -165 "C (2).'We are very thankful to Professor G. For personal use only.

Free energy barriers to ring inversion of 3,3-dimethyl-, 4,4-dimethyl-, and 5,s-dimethylcycloheptene were found to be much smaller than those of corresponding benzocycloheptene derivatives but show the same trend. This observation strongly suggests that the chair conformationof both types of molecules interconverts through the same mechanism. On the other hand, the AGf value for 4,4,6,6-tetramethylcycloheptene is about 3 kcal/mol out of line from that expected from a consideration of the behavior of the corresponding benzocycloheptene derivative. An explanation is offered.Les barrikres d'tnergie libre pour l'inversion des dimethyl-3,3, dimethyl-4,4 et dimethyl-5,5 cycloheptimes, tout en conservant la m&me tendance, sont plus faibles que celles qui caractkrisent les derives correspondants du benzocycloheptkne. Cette observation indique que la conformation chaise des deux classes de molecules s'inverse par le m&me mecanisme. Par contre, la valeur de AG+ pour le tetramethyl-4,4,6,6 cycloheptene est environ 3 kcal/mol plus elevke que celle attendue aprks examen des benzocycloheptknes correspondants. Une explication est presentee.

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