5364 J. Am. Chem. Sot. 1983, 105, 5364-5368 The 7-Norbornyl Cation. Structure and Interactions Dionis E. Sunko,* Hrvoj Vancik,'" Vjera Deljac, and Milorad Milunlb Contributionfrom the Laboratory of Organic Chemistry, Faculty of Science, University of Zagreb, Strossmayerov trg 14, 41000 Zagreb, Yugoslavia. Received December 21, I982 Abstract: In an attempt to detect possibly hyperconjugative, homohyperconjugative and/or steric interactions in the title compound specifically deuterated 7-norbornyl triflates 3-8-OTf were solvolyzed in aqueous trifluoroethanol. 3-OTf and 4-6-OTf showed normal, positive isotope effects (kH/kD > 1). The a-effect of 1.13 in 3OTf is interpreted as being due to incomplete rehybridization and strained geometry at C,. y-Effects were observed only in the exo-deuterated derivatives 4-6-OTf (kH/kD = 1.024-1 .lo) and can be ascribed to a net relief of steric strain during the progression from initial state to activated complex. The absence of isotope effects in the solvolysis of endo-deuterated derivatives 7-OTf and 8-OTf precludes any hyperconjugativeor neighboring bond interaction as the one implied in the nonclassical structure 2. Products obtained under solvolytic conditions in solvents of different nucleophilicities and ionizing power show almost identical, predominantly (- 90%) retained configuration. This result can be rationalized in terms of bent structures for the intermediate cation such as those predicted by MIND0 calculations. A greater proportion of inverted products (up to 35%) is obtained under SN2 conditions in aprotic solvents (DMF, benz- enell8-crown-6). Here direct back-side displacement competes with a possible front-side attack on the sulfur atom of the leaving group. An alternative, but highly speculative mechanism involving pseudorotation cannot be dismissed. Among secondary carbocations, the 7-norbornyl cation (1) stands out for its unusual properties. Its solvolytic precursors are extremely unreactive; the tosylate in acetic acid has a half-life of 3.35 X lo6 years at 25 "C2 Attempts to generate 1 in Sb- FS-S02 solutions failed due to its rapid rearrangement to the 2-norbornyl ~ a t i o n . ~ This rearrangement has not been observed under solvolytic conditions. Solvolysis at elevated temperature (at 205 "C in AcOH and at 100 "C in HC02H for 2 and 16 h, respectively) afforded -97% of unrearranged alcohol 1-OH with predominantly (-90%) retained config~ration.4.~ The methyl/H and phenyl/H rate ratios of 7-norbornyl derivatives are among the highest These observations together with the Hammett p value of -5.646 are taken as evidence for an enor- mously high electron demand of l.9 Quantum mechanical and molecular mechanical factors have been invoked to explain the destabilization of the 7-norbornyl cation. The C7C1C4 angle of 93.7" in norbornaneiO is considered partly responsible for the solvolytic inertness of 7-norbornyl de- rivatives. This is in accordance with the Footel' and Schleyer12 correlations. For comparison, 2-adamantyl tosylate which has a normal tetrahedral angle at C2 solvolyzes lo6 times faster than 1-OTs.I) Implied in this reasoning is the understanding that the cationic center in 1 is planar. When a C, symmetry was imposed on the carbon skeleton the planarity of the cationic center was also sustained by ab initio calculation^.^^ ~~~~ ~~~~ ~ ~~~ (1) (a) Taken in part from the Ph.D. Thesis of H.V., University of Zagreb, 1981. (b) Institute of Physics, University of Zagreb. (2) Winstein, S.; Shatavsky, M.;,Norton, C.; Woodward, R. B. J. Am. Chem. SOC. 1955, 77, 4183. (3) Schleyer, P. v. R.; Watts, W. E.; Fort, R. C., Jr.; Comisarow, M. B.; Olah, G. A. J. Am. Chem. SOC. 1964, 86, 5679-5680. (4) (a) Gassman, P. G.; Hornback, J. M. J. Am. Chem. Soc. 1967, 89, 2487-2488. (b) Gassman, P. G.; Homback, J. M.; Marshall, J. L. Ibid. 1968, 90.62384239. --. ---- (5) (a) Miles, F. B. J. Am. Chem. SO~. 1967, 89, 2388-2489. (b) Miles, (6) Tanida, H.; Tsushima, T. J. Am. Chem. SOC. 1970, 92, 3397-3403. (7) Sliwinski, W. F.; Su, T. M.; Schleyer, P. v. R. J. Am. Chem. SOC. 1972, F. B. Ibid. 1968, 90, 1265-1268. 94, 133-145. (8) Sunko, D. E.; Szele, I.; TomiE, M. Tetrahedron Lett. 1972,1827-1830. (9) Hoffmann, R.; Mollere, Ph. D.; Heilbronner, E. J. Am. Chem. SOC. (10) Allinger, N. I.; Hirsch, J. A.; Miller, M. A,; Tyminski, I. J.; Van- (11) Foote, C. S. J. Am. Chem. SOC. 1964, 86, 1853-1854. (12) Schleyer, P. v. R. J. Am. Chem. Soc. 1964,86, 1854-1856. (13) Schleyer, P. v. R.; Nicholas, R. D. J. Am. Chem. SOC. 1961, 83, (14) (a) Wenke, G.; Lenoir, D. Tetrahedron 1979, 35, 489-498. (b) 1973,95,4a60-4a62. Catledge, F. A. J. Am. Chem. SOC. 1968, 90, 1199-1210. 182-187. Dewar, M. J. S.; Schoeller, W. W. Tetrahedron 1971, 27, 4401-4406. Chart I U 1 2 3 4 5 7 8 Electronic, quantum mechanical effects which could stabilize 1 also seem to be absent. Here two different interactions should be taken into account: (a) carbon-carbon hyperconjugation with the underlying cyclohexane ring skeletong and (b) carbon-hy- drogen homohyperconjugation with the pseudo-n orbitals of the methylene carbon-hydrogen bonds.I5 Symmetry arguments preclude hyperconjugative interaction with the degenerate set of ribbon orbitals of the u bonds.9 It is however this type of in- teraction, illustrated by the nonclassical cation 2, which apparently has to be taken into account in order to rationalize both the formation of rearranged products with the bicyclo[3.2.0] heptane skeleton and the predominance of unrearranged products with retained configuration at C7.4b,5b In order to find an answer to these questions we decided to carry out a detailed investigation of the solvolysis of specifically deuterated 7-norbornyl triflates 3-8-OTf in a series of solvents of different nucleophilicities and ionizing power. The use of the more reactive triflates16 has the advantage of working at lower temperatures (65 "C vs. 100 "C) than was the case in the earlier work with tosylates.2 The specific aim was to search for hyperconjugative interactions mentioned above and to gain additional information on the geometry of the incipient cation 1. Methods and Materials Synthetic Procedures. The alcoholic precursors of triflates 3-8 were prepared by using standard synthetic procedures described in detail in the Experimental Section. The endo-deuterated norbornanols 7-OH and 8-OH could not be separated but were used as a 1:l mixture. (15) Sunko, D. E. Croat. Chem. Acta 1980, 53, 525-543. (16) (a) Su, T. A,; Sliwinski, W. F.; Schleyer, P. v. R. J. Am. Chem. SOC. 1969,91,5386-5388. (b) Creary, X. J. Am. Chem. Soc. 1976,98,6608-6613. 0002-7863/83/1505-5364$01.50/0 Q 1983 American Chemical Societv