6409
The temperature of the gas stream is controlled by a thermistor, mounted just below the nmr tube, which through a relay determines the rate of boiling of the N2. The temperature of the gas stream and HFSO, solution can be monitored with suitably. .placed thermocouples. In a typical experiment durene (20 mg) was dissolved in FS08H (0.5 ml) and irradiated at -90" using a thick window glass filter, cutoff 320 nm. The reaction was monitored by nmr and typically a photostationary state (60% conversion) was reached within 60
min of irradiation. After 150 min of irradiation ca. 18% of 7 was
detected.
Acknowledgment. Support of this work by the National ScienceFoundation and the U. S. Army Research office (UCLA) and by the National ~~~~~~~h council Of Canada and the Department Of University Affairs (Ontario) (McMaster) is gratefully adknowledged.
Circumambulatory Hexa- and Heptamethylbicyclo[3.1 .O]hexenyl Cations R. F. Childs* le. and S . Winsteinlb Contribution f r o m the Departments of Chemistry, University of California at Los Angeles, California 90024, and McMaster University, Hamilton, Ontario, Canada. Received October 2, 1973 Abstract: The low temperature photoisomerization of hepta-, 5, and hexamethylbenzenonium ions, 6,in HFS03 gave respectively the hepta-, 4, and 6-endo-hexamethylbicyclo[3.1.O]hexenyl cations, 7. Alternatively, 7 and its exo-isomer 10 could be obtained by protonation of the corresponding homofulvenes. These bicyclic cations were
stable at low temperatures but rearranged on heating to the corresponding benzenonium ions. The rate of this isomerization was shown to be dependent upon the c6 substituents. Both 4 and 10 exhibited variable temperature nmr spectra that indicated a degenerate migration of the cyclopropane around the periphery of the five-membered ring was occurring. This migration was shown to proceed by successive 1,4-shifts and not to involve the intermediacy of a bicyclo[2.1.llhexenyl cation. A very high stereoselectivity was observed in these rearrangements, the c6 substituents always remaining in the same relative positions. Thus for every 8 X lo8migrations of the cyclopropyl of 10, less than one took place with an overall inversion of stereochemistry to give 7. This observed stereoselectivity is that required by orbital symmetry considerations if the rearrangement proceeds by a concerted 1,4sigmatropic shift. The barriers to these degenerate rearrangements were found to be 10.1 kcal/mol for 4, 12.0 kcal/mol for 10, and 17.5 kcal/mol for 7. A consideration of the marked sensitivity of this isomerization to the nature and position of the c6 substituents suggests that the transition state may be represented as a substituted cyclopentadiene-5-methyl cation in which both lobes of the formally vacant p orbital on c6 are interacting symmetrically with the diene. A c6 endo substituent would appear to reduce the importance of this overlap by sterically interacting with Czand C3 and thus preventing c6 from moving toward the diene. The isomerizations of the 2hydroxy-1,3,4,5,6,6-hexamethylbicyclo[3.1.O]hexenyl cation were investigated and compared with the rearrangements of 4.
T
he preceding paper in this series has emphasized in particular two aspects of the chemistry of the bicycl0[3.1 .O]hexenyl cations, namely their structure and their relationship to the isomeric benzenonium cations. An important feature of these ions is that charge delocalization involves the two external cyclopropane bonds, rather than the internal Cl-Cj bond. For example, the nmr spectrum of the pentamethyl cation 1 showed that there was considerable leakage of positive charge onto c6. Structures such as l a were proposed to account for the properties of this cation.
la
la
la
(1) (a) Address all correspondence to this author at McMaster University; (b) deceased November 23, 1969.
(2) R. F. Childs, M. Sakai, B. D. Parrington, and S . Winstein, J. Amer. Chem. Soc., 96,6403 (1974).
One of the classic probes for an electron deficient carbon is to examine the effect of alkyl substitution upon its stability and reactivity. The elegant study of Swatton and Hart indeed suggested that methyl substitution at C6 of these systems might have a profound effect upon their reactions. These authors showed through the use of suitably deuterated materials that there was a migration of cyclopropane around at least four of the five sides of the cyclopentenyl ring of the hydroxy cation 2 (Scheme I). No such comparable reaction was detected with 1 nor with the tetramethylbicyclo[3.1.O]hexenyl ions. With the discovery of a novel photochemical route to 1 from the pentamethylbenzenonium ion, it seemed worthwhile to attempt the preparation of derivatives of 1 bearing methyl groups upon c6. In this paper we report the synthesis and observation of the hexa- and heptamethylbicyclo[3.1.O]hexenyl cations. (3) D. W. Swatton and H. Hart, J. Amer. Chem. Soc., 89, 5075 (1967). (4) R. F. Childs and B. Parrington, J . Chem. SOC.D , 1540 (1970). (5) Part of this work has appeared in preliminary form, R. F. Childs and S. Winstein, J. Amer. Chem. SOC.,90, 7146 (1968).
Childs, Winstein / Circumambulatory Heptabicyclo[S.l .O]hexenyl Cations
6410
-395"
- 49" -59" -64"
-69" -79"
-866" I
75
a0
85
90
Figure 1. Variable temperature pmr spectra of 4 obtained by photoisomerization of 5.
Scheme I
OH
0
2
3
~IFAST
The cation 5, which has a long wavelength absorption at 400 nm (log 6 4.04),* was irradiated at -78" in HFSO,. While it was quite clear from the changes occurring in the nmr spectrum that a photochemical reaction was taking place, the product had a different spectrum to that expected on the basis of 1. The photoproduct derived from 5 exhibited a reversible temperature dependent spectrum, although the freezing point of H F S 0 3 (- 88.98")" precluded the observation of the limiting low temperature spectrum. On warming the solution to - 9 O a thermally irreversible isomerization took place to regenerate 5, the rate constant for this first-order process being 2.2 X sec-l at -9" (AF* = 19.8 kcal/mol). To characterize this photoproduct it was desirable to record the limiting low temperature spectrum and to this end it was prepared in a HFS03-S0,ClF (2: 1) solution. The nmr spectrum could now be recorded over the range - 110 to -9". Sample spectra are shown in Figure 1, Some viscosity broadening of all the peaks is occurring at the lower temperatures. Resonances attributable to a small amount of the benzenonium ion 5 can be detected in these spectra. r859
5
4
Below -90" the nmr spectrum of the photoproduct is essentially frozen, no further major change occurring on cooling the sample below this point. Apart from the two high field methyl resonances the methyl signals in this frozen spectrum occur in very similar positions I I to those observed for 1 and are quite consistent with the FAST formation of 4.2 The two resonances at T 8.87 and 8.43 can be attributed to the C6 methyl groups, both ocOH 0 curring in the region expected for similarly situated groups upon a cyclopropyl allyl cation.12 On warming the HFSO3-SO2C1F solution of 4 above - 90", the resonances attributed to the cyclopentenyl ring methyls broadened and coalesced at about -60" and above this temperature became a singlet. For Results and Discussion example, at -39.5" there are only three signals observed for 4 with the ratio 15:3:3. The observed Synthesis and Characterization of 4. The cation reposition of the averaged ring methyls is 'T 7.95, which quired for the photochemical preparation of 4 is the is very close to the position expected for the mean of heptamethylbenzenonium ion 5, which has been chartheir low temperature positions ('T 7.94). On cooling acterized by Doering, et aZ.6 Protonation of the methylthe sample, the original five-line spectrum was restored. enecyclohexadiene derivative 6' in H F S 0 3 gave 5, with a very similar nmr spectrum to that previously r e p ~ r t e d . ~ During this averaging of the ring methyls there was no detectable change in the signals attributable to the c6 gem-dimethyl group. The averaging of the five cyclopentenyl methyl groups of 4 clearly means that there must be some process which has the effect of moving the cyclopropane group around the periphery of the five-membered ring, without 4 5 6 the destruction of the cation (Scheme 11). The very
1
(6) W. v . E. Doering, M. Saunders, H. G. Boyton, H. W. Earhart, E. F. Wadley, W. R. Edwards, and G. Laber, Tetrahedron, 4, 178 ( 1958). (7) A. convenient route to 6 has been described.8 (8) H. Hart, P. M. Collins, and A. J. Waring, J. Amer. Chem. Soc., 88, 1005 (1966). (9) The signal positions originally reported by Doering are incorrect.8s10
Journal of the American Chemical Society 1 96:20
(10) V. A. Koptyug, V. G . Shubin, A. I. Rezbukin, D. V. Korchagina, V. P. Tret'yakov, and E. S. Rudakov, Dokl. Akad. Nauk SSSR, 171, 1109 (1966); M. Saunders in "Magnetic Resonance in Biological Systems," Pergamon Press, New York, N. Y., 1967, p 85. (11) R. J. Gillespie and T. E. Peel, Adoan. Phys. Org. Chem., 9, 1 (1971). (12) K. E. Hine and R. F. Childs, J. Amer. Chem. SOC.,93, 2323 (1971); I
