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Solvolytic Reactivity of 4-Tricyclyl, 1-Apocamphyl,and 1-Methyl-4-tricyclo[ 2.2 .2.0”6]octyl Trifluoromethanesulfonates. Observations on the Interaction of the Face of the Cyclopropane Ring with Electrophiles and on the Interpretation of Activation Parameters for Solvolytic Reactions’ Shelby A. Sherrod,z&Robert G. Bergman,*zb Gerald J. Gleicher,zcand David G. Morriszd
Contribution No. 4376 f r o m the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91 109, the Department of Chemistry, Oregon State Unioersity, Coroallis, Oregon 97331, and the Chemistry Department, University of Glasgow, Glasgow, Scotland. Received November 12, 1971 Abstract: Ionization of 4-tricyclyl derivatives 13 produces a carbonium ion 14 in which the empty, charge-carrying p orbital is situated directly above the face of the cyclopropane ring on the opposite side of the molecule. The ionization of 4-tricyclyl trifluoromethanesulfonate (25) has been studied in buffered aqueous ethanol in the neighborhood of 230”; this compound is the least reactive tert-alkyl system whose solvolysis rate has ever been measured. The corresponding tosylate is estimated to undergo ionization with a half-life of 4 X lo9years at 25”, and times as reactive as its bicyclic analog apocamphyl trifluoromethanesulfonate (30). These studies 25 is show that no stabilization isgained by the interaction of a positively charged center with the face of a cyclopropane ring. Strain energy calculations completely account for the slow rate of solvolysis of 25, and indicate that the three-membered ring has a negligible inductive effect when oriented as in 14. In addition, the activation entropy for the ionization of 25 is quite negative at 230”; it is suggested that this may be due to the effect of an appreciably negative heat capacity of activation, and a caaeat is expressed regarding mechanistic interpretation of activation parameters which are measured at unusual temperatures.
T
hree-membered carbon rings have long been known t o interact readily with electrophiles. 3-5 A large number of intermolecular reactions are known in which ring opening has been achieved with halogens, 3 a acylium ions, 3b and protons. 3cm Protonated cyclopropanes have also been established as intermediates in a number of ionization reactions of n-propyl derivatives. (1) (a) Some of the results described in this paper have been reported in preliminary form: S. A. Sherrod, R. G. Bergman, G. J. Gleicher, and D. G. Morris, J. Amer. Chem. SOC.,92,3469 (1970); (b) for complementary observations, see R. C. Bingham, W. F. Sliwinski, and P. v. R. Schleyer, ibid., 92, 3471 (1970). (2) (a) National Science Foundation Predoctoral Fellow, 1967-1970; Caltech; (b) Alfred P. Sloan Foundation Fellow, 1970-1972; Camille and Henry Dreyfus Foundation Teacher-Scholar Grant Awardee, 1970-1975; Caltech; address correspondence to this author at California Institute of Technology; (c) Alfred P. Sloan Foundation Fellow, 1969-1971 ; Oregon State University; (d) University of Glasgow. (3) (a) N. C. Den0 and D. N. Lincoln, J . Amer. Chem. SOC.,88, 5357 (1966); (b) H. Hart and R. H. Schlosberg, ibid., 88, 5030 (1966); 90, 5189 (1968), and earlier papers; (c) R . L. Baird and A. A. Aboderin, Tetrahedron Lett., 235 (1963); J. Amer. Chem. SOC.,86, 252 (1964);
(d) R. T. LaLonde and J. J. Batelka, Tetrahedron Lett., 445 (1964); (e) R. T, LaLonde and L. S . Forney, J. Amer. Chem. SOC.,85, 3767 (1963); (f) R. T. LaLonde and M. A. Tobias, ibid., 85,3771 (1963); 86, 4068 (1964); (g) E. J. Corey and R. F. Atkinson, J . Org. Chem., 29, 3703 (1964); (h) C. H. DePuy, H. L. Jones, and D. H. Gibson, J . Amer. Chem. Soc., 90, 1830 (1968); (i) G. A. Olah and J. Lukas, ibid., 90, 933 (1968); (j) N. C. Deno, W. E. Billups, D. LaVietes, P. C. Scholl, and S. Schneider, ibid., 92,3700 (1970), and references cited therein; (k) P. E. Peterson and G. Thompson,J. Org. Chem., 33,968 (1968); (1) C. C. Lee, Progr. Phys. Org. Chem., 7, 129 (1970); (m) C. J. Collins, Chem. Reu., 69, 541 (1969). (4) (a) G. J. Karabatsos, C. Zioudrou, and S . Meyerson, J . Amer. Chem. SOC.,92, 5996 (1970); (b) G. J. Karabatsos, M. Anand, D. 0. Rickter, and S. Meyerson, ibid., 92, 1254 (1970), and earlier papers; (c) C. C. Lee and D. J. Woodcock, ibid., 92, 5992 (1970). ( 5 ) (a) P. v. R. Schleyer and G. W. Van Dine, ibid., 88, 2321 (1966); (b) R. G. Bergman and D. R . Kelsey, ibid., 93, 1953 (1971); (c) R. G. Bergman and D. R. Kelsey, ibid., 93, 1941 (1971); (d) R . G. Bergman and S. A. Sherrod, ibid., 93, 1925 (1971); (e) M. J. S . Dewar and J. M. Harris, ibid., 90, 4468 (1968). and references cited therein; (f) Y . E. Rhodes and T. Takino, ibid., 90,4469 (1968), and references cited therein.
Intramolecular examples of cyclopropane-electrophile interactions are provided by the well-established stability of cyclopropylcarbiny1,j” l-cyclopropylvinyl,5b-d and homocyclopropylcarbiny15e~f cations. Although cyclopropane rings are often considered to be intermediate between single and double bonds in many types of chemical behavior, they react in some cases even more rapidly with electrophiles than do double bonds. 3k Intuitively, it is relatively easy t o predict the most favored approach of an electrophile to a double bond. If one assumes that attack occurs at the region of highest electron density in the substrate, it seems reasonable that an electrophile will approach perpendicular t o ( z axis, Figure 1) rather than in the plane of the molecule (x or y axis, Figure l), since the latter approach involves steric interference from the vinyl hydrogens as well as bonding to a region in which there is a node in the filled 7r molecular orbitaL6 The cyclopropane situation is less clear, however. Two extreme avenues of electrophilic approach are toward the “face” of the ring ( z axis, Figure 2) or the “edge” of the ring ( y axis, Figure 2). The Walsh model of cyclopropane (Figure 3) predicts’ relatively high electron density in both locations, the first associated with the spz orbitals directed toward the center of the ring, and the latter associated with the p orbitals on the periphery of the ring. A number of theoretical studies have predicted that attack upon the face of a cyclopropane ring is clearly less favored than other modes of approach.8 Testing (6) M. J. S . Dewar, J . Chim. Phys., 46, 268 (1949), and subsequent papers. (7) A. D. Walsh, Trans. Faradaj’ SOC.,45, 179 (1949). (8) (a) R. Sustmann, J. E. Williams, M. J. S . Dewar, L. C. Allen, and P. v. R. Schleyer, J. Amer. Chem. SOC.,91, 5350 (1969); (b) J . D .
Bergman, et al. 1 Solaolytic Reactiaity of Trifuoromethanesuvonates
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