J. Am. Chem. SOC. zyxwvu 1993, 115, zyxwvu 1317-1320 1317 The Question of Vertical or Nonvertical Participation of Silicon zyxwvu ,l? to a Cation in the Antiperiplanar Stereochemistry Joseph B. Lambert,*,' Robert W. Emblidge, and Siobhan Malany Contribution from the Department of Chemistry, Northwestern University, Evanston, Illinois 60208-31 13. Received July 24, 1992. Revised Manuscript Received November 20, 1992 Abstract: The mechanism of stabilization of a zyxwvut j3 positive charge by silicon in the antiperiplanar stereochemistry has been clarified by the zyxwvutsrq a secondary deuterium kinetic isotope effect. A kH/kD of 1.17 * 0.01 has been observed for the trifluoroethanolysis of r-5-rerr-butyl-c-2-(trimethylsilyl)cyclohex-~-yl- 1,3,3-d, 3,5-dinitrobenzoate (3) at 25 OC. This value is consistent with vertical (hyperconjugative) stabilization (1) probably with rate-determining conversion of an intimate ion pair to a solvent-separated ion pair. The isotope effect is inconsistent with nonvertical participation to form a bridged intermediate (2). The ability of silicon to stabilize positive charge two carbons away, R3Si-CH2-CH2+, has been termed the j3 effect and may be the most important electronic interaction of silicon in organic chemistry.2 It has been exploited, for example, to direct elec- trophilic additions to double or triple bonds. Although the ability of silicon to stabilize j3 positive charge is ultimately related to its high polarizability and low electronegativity, the j3 effect has been discussed in terms of two possible mechanism^.^ (1) Vertical stabilization or hyperconjugation involves donation of C-Si zyxwvu u electrons to the empty carbon p orbital without significant movement of atomic positions, as in 1. (2) Nonvertical stabi- R3 Si+ /\ c ti,- c H R ' 2 1 lization is analogous to neighboring group participation of lone- pair-bearing atoms (internal sN2) and involves closure of a three-membered ring to form a siliconium ion, 2, in the rate- determining step. In addition, silicon may stabilize positive charge by through-bond u induction. Jorgensen and co-workers4 found that the nonvertical or bridged form 2 is more stable by about 2.4 kcal mol-' than the vertical or open form when the carbocation is primary (R, R' = H). When it is secondary (R' = CH3), however, the vertical form is more stable by about 4 kcal mol-', and the bridged form moreover is not an energy minimum. In principle these two mechanisms may be distinguished by their dependences on the stereochemical relationship between silicon and the leaving group (X) in the ground state antecedent to the carbocation, R3Si-CH2-CH2-X. The vertical mechanism should vary as a cosine-squared function of the Si-C-C-X dihedral angle, in order to have maximum overlap when the C-Si and p orbitals are parallel and minimum overlap when they are orthogonal. Thus a plot of the dihedral angle vs j3 stabilization as measured, for example, by a rate acceleration of the Si system compared with a structurally analogous Si-free system (ksi/kH) should resemble a Karplus curve, with maxima at 0' and 1 80' and a minimum a t 90'. On the other hand, the nonvertical mechanism should have a maximum only at 180', corresponding to the best geometry for backside displacement of X by Si, and a monotonically and rapidly decreasing effect as the dihedral angle decreases. (1) This work was supported by National Science Foundation Grant No. (2) Lambert, J. B. Tetrahedron 1990.46, 2677-2689. Jarvie, A. W. P. (3) Traylor, T. G.; Hanstein, W.; Berwin, H. J.; Clinton, N. A,; Brown, (4) Wierschke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. J. Am. Chem. (5) Ibrahim, M. R.; Jorgensen, W. L. J. Am. Chem. SOC. 1989, zyxwvutsrq 111, CHE-8910841 and by a grant from the Dow Corning Corporation. Organomet. Chem. Rev., Sect. A 1970,6, 153-170. R. S. J. Am. Chem. SOC. 1971, 93, 5715-5725. SOC. 1985, 107, 1496-1500. 819-824. In order to assess the relative importance of the two mecha- nisms, we previously prepared systems with ground-state dihedral angles of Oo, 60°, 120°, and 180°.6.7 The antiperiplanar system (180O) exhibited an extremely large j3 effect, as indicated by a ksi/kH of 10l2. The clinal geometries (60°, 120') exhibited much smaller accelerations, circa 1 04, as expected. A symmetrical cosine-squared plot would predict a synperiplanar (0') acceleration of magnitude similar to that of the antiperiplanar geometry, but we observed' an acceleration of only about lo5. Thus the j3 effect is extremely strong at 180' and significant but essentially constant at the other angles examined to date. This asymmetricplot may be explained in at least two different ways, and it was the objective of the present study to distinguish these two possibilities. (1) If the mechanism of stabilization is entirely vertical, then the reduced 0' arm must be caused by a nonelectronic phenomenon. We suggested7 that the syn leaving group prevents the C-Si bond from overlapping optimally with the developing p orbital, whereas in the anti geometry the leaving group is not in the way and optimal overlap may be achieved. By this scenario, the stereochemical dependence has'a highly asym- metric cosine-squared form. (2) Alternatively, the observations could be the result of the superposition of both vertical and nonvertical effects. Vertical stabilization then provides nearly symmetrical, cosine-squared stabilization with a maximum of about lo5 at both 0' and 180'. In addition, nonvertical stabi- lization enhances the 180' stabilization by another factor of 10'. This scenario means that both mechanisms contribute at the antiperiplanar geometry, with nonvertical stabilization dominant, and below about 1 50' only vertical stabilization contributes. These two mechanistic explanations agree that hyperconjugation is the key element in providing stabilization from Oo up to about 1 50' (induction may contribute as well). It is at the antiperiplanar geometry, then, that there are conflicting explanations. One scenario attributes all of the acceleration at this geometry to vertical interactions, and the other attributes the acceleration to a combination of vertical and nonvertical, but with a prepon- derance of the latter. After numerous studies, no clear experi- mental distinction to date has emerged between the vertical and nonvertical contributions at the antiperiplanar geometriesa2 Moreover, theory has found the energies of the respective inter- mediates (1 and 2) to be of similar energy, but with a small preference for vertical in secondary system^.^,^ In the present study, we have endeavored to provide a mech- anistic resolution by means of the a secondary hydrogen/deu- terium kinetic isotope effect. In the vertical mechanism, the hybridization of the carbon atom changes from sp3 to sp2, as in the sN1 reaction, for which the a hydrogen/deuterium kinetic isotope effect is normally in the range 1.15-1.25.* The nonvertical ~~ (6) Lambert, J. B.; Wang, G.-t.; Finzel, R. B.; Teramura, D. H. J. Am. (7) Lambert, J. B.; Chelius, E. C. J. Am. Chem. Soc. 1990, 112, Chem. SOC. 1987, 109, 7838-7845. 812C-8 126. 0002-7863/93/1515-1317$04.00/0 0 1993 American Chemical Society