7622 J. Org. Chem. zyxwvuts 1993,58, 7622-7623 2-Methylbicyclo[ 3.2.21non-3-en-2-yl Cation Involving Rearrangement of zyxwv 1,3'-Spirocyclopropylbicyclo[ 2.2.2]oct-2'-yl Cation' V. Prakash Reddy, George A. Olah,' and G. K. Surya Prakash' The Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661 zyxwvut Received July 27, 1993 The 2-norbornyl cation has captured the attention of the majority of the physical organic community for over half a century. However, the higher homologue, secondary 2-bicyclo[2.2.2loctyl cation, has received relatively little attention. The nature of the cation and the u-participation of the adjacent 1,6 and 1,7 C-C bonds could be inferred only from solvolysis studiesS2 The tertiary 2-bicyclo[2.2.2loctyl cations have been studied in superacids at low temperatures. On the other hand, all attempts of preparation of the secondary 2-bicyclo[2.2.2locty1cation resulted only in the rearrange- ment to the thermodynamically more stable bicyclo[3.3.01- oct-1-yl cation? which further rearranges to 2-methyl-2- norbornyl cation at higher temperatures. H -80 "c CI A spirocyclopropyl group adjacent to the cation center was shown to significantly stabilize carbocations. Thus, for example, the presence of the a-spirocyclopropyl group makes possible the observation of an otherwise unstable secondary cyclohexyl cation, which rearranges to bicycle- [ 3.3.0loct-1-yl cation at higher temperatures? Attempted preparation of the 1,3'-spirocyclopropyl-2'-norbornyl cat- ion, on the other hand, resulted in the formation of the allyl cation, 2-methylbicyclo[3.2.lloct-3-en-2-yl cations6 We have now undertaken the preparation of the expectedly stabilized 1,3'-spirocyclopropylbicylo[2.2.2] oct- 2'-yl cation (6) by ionizing the corresponding 1,3'- spirocyclopropylbicyclo[2.2.21octan-2'-01 (5). However, (1) Stable Carbocatione. 292. For part 291 see: Olah, G. A., Liao, Q., Caaanova, J., Bau, R., Prakash, G. K. S. J. Am. Chem. zyxwvutsr SOC., submitted. (2) Walborsky, H. M.; Baum, M. E.; Youssef, A. A. J. Am. Chem. SOC. 1959,81,4709-4713; 1971,93,988. Walborsky, H. M.; Webb, J.; Pitt, C. G. J. Org. Chem. 1969,28,3214-3216. Goering, H. L.; Sloan, M. F. J. Am. Chem. SOC. 1961,1992-1999. Goering, H. L.; "hies, R. W. J. Am. Chem. SOC. 1968,90,2967-2968. Goering, H. L.; Fickes, G. N. J. Am. Chem. SOC. 1968,90,2848-2856,2862-2868. Grob, C. A.; Sawlewicz, P. Helu. Chim. Acta. 1984,67,1906-1917. Spurlock, L. A.; Schultz, R. J. J. Am. Chem. SOC. 1970,92,6302-9. Kwart, H.; Imine, J. L. J. Am. Chem. SOC. 1969, 91,5541-6. (3) Olah, G. A.; Gao, L. J. Am. Chem. SOC. 1971,93,6873-6877. Oiah, G. A.; Bollinger, J.; Martin, K.; Patterson, D. J. Am. Chem. SOC. 1970, 92,1432-1434. (4) Olah, G. A.; Fung, A. P.; Rawdah, T. N.; Prakaah, G. K. S. J. Am. Chem. SOC. 1981,103,4646-4847. See also: Wiberg, K. B.; Hiatt, J. E.; Hseih, K. J. Am. Chem. SOC. 1970,92, 544-552. Wiberg, K. B.; Pfeiffer, J. G. J. Am. Chem. SOC. 1970,92,653-564. (5) Prakash, G. K. S.; Fung, A. P.; Olah, G. A.; Rawdah, T. N. hoc. Natl. Acad. Sci. U.S.A. 1987,84, 5092-5095. Scheme I" C02Me 0 C02Me 4$+&c 'Key: 1. Na/K; (a) ether, MesSiCl, reflux, 30 min; 2. Brdether- hexane, 0 "C, 5 min; (b) CHyPPh3/THF-DMSO, 0 "C, 5 min; (c) CHzIz/Zn-Cu-ether, reflux; (d) LiAlWether, 0 "C to rt. cation 6 could not be observed, and facile rearrangement resulted in the formation of the allylic 2-methylbicyclo- 13.2.21non-3-en-2-yl cation (7). Results and Discussion A standard procedure for the preparation of a-spiro- cyclopropyl alcohols involvesthe Simmons-Smith reaction of the corresponding enones followed by the reduction of the carbonyl group. However, the precursor enone, 3-methylenebicyclo[2.2.2loctan-2-one (31, is unreported (Scheme I). We have developed a convenient synthesis of the enone by a modified acyloin condensation6 of dimethyl 1,4-cyclohexanedicarboxylate and selectiveWittig reaction of the bicyclo[2.2.2loctane-2,3-dione (2). The acyloin condensation using a dispersion of sodium in toluene gave poor yields of the diketone 2. However, using a sodium- potassium alloy in ether solution provided 2 in high yield under mild conditions. The Simmons-Smith reaction of 3 using Zn/Cu in refluxing ether gave the a-spirocyclo- propyl ketone 4. The alcohol 5 was obtained by the reduction of the ketone using lithium aluminum hydride. Ionization of the alcohol 5 in FS03H in low nucleophi- licity solvent, sulfuryl chloride fluoride, at -95 OC gave the allyl cation, 2-methylbicyclo[3.2.2lnon-3-en-2-yl cation (7). The proton-coupled 13C NMR spectrum confirmed OH -950~ 6 the structure and aided in the assignments of the chemical shifts to the individual carbons. The tertiary cationic center (C2) is significantly more positively charged com- pared to C4, as evidenced by the more deshielded C2 carbon (6C2 = 249, 6C4 = 206). The absorption for C1 is correspondingly more deshielded than that of the C5 (6W 45.4 and 36.6, respectively). The allylic cation 7 shows 13CNMRsignals similar to those of 2-methylbicyclo[3.2.1]- oct-3-en-2-yl ~ a t i o n . ~ The formation of the 2-methylbicyclo[3.2.2]non-3-en- 2-yl cation implies the involvement of the 1,3'-spiro- cyclopropylbicyclo[2.2.2loct-2'-yl cation (6) as a highly energetic intermediate. Several consecutive intramolec- ular 1,2-hydride and alkyl shifts may intervene in the rearrangement of this cation to the observed allyl cation, 7. A similar mechanism may be operative in the case of the lower homologous 2-norbornyl deri~ative.~ (6) Ruhlmann, K. Synthesis 1971, 236-253. 0022-3263/93/1958-7622$04.00/0 0 1993 American Chemical Society