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Diethyl (2-(tosyloxy)cyclohexyl)phosphonates have been prepared to test the hypothesis that the strongly electron-withdrawing phosphonate group can stabilize the formation of a /3 carbocation through hyperconjugation. Systems were constructed with the 180" dihedral angle between phosphonate and tosyloxy that is optimal for such participation and with the 60" dihedral angle that minimizes it. Reactions were carried out in aqueous mixtures of ethanol, trifluoroethanol, and hexafluoro-2-propanol. The 60" case had the standard profile for bimolecular reaction with solvent (kB), with a slower rate that is sensitive to solvent nucleophilicity. The 180" case had the standard profile for a carbocation pathway (kc), with a faster rate that is independent of solvent nucleophilicity and with products that are possible only with a carbocation intermediate. These results suggest that the phosphonate group is capable of stabilizing a carbocation, presumably through hyperconjugation, relative to the expectation based solely on its polar effect.In recent years there have been numerous studies of the effect of electron-withdrawing groups attached directly to carbocations (a effects).2 Such groups, including trifluoromethyl, sulfonyl, and nitro, in general have a profound destabilizing influence through their polar effects on the already electron deficient positive charge center. In some cases, however, the destabilization is less than expected from polar effects as measured for example by u constants. For cyan^,^,^ the destabilization is partially offset by positive charge delocalization, >C+C=N

The mechanism of stabilization of a j 3 positive charge by silicon in the antiperiplanar stereochemistry has been clarified by the 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 electrophilic 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 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 lonepair-bearing atoms (internal sN2) and involves closure of a three-membered ring to form a siliconium ion, 2, in the ratedetermining 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 at 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.In order to assess the relative importance of the two mechanisms, we previously prepared systems with ground-state dihedral angles of Oo, 60°, 120°, and 180...

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Deoxymercuration was examined in the antiperiplanar stereochemistry of trans‐2‐acetoxycyclohexylmercury(II) acetate (6). The presence of the mercury group accelerates the departure of the beta leaving group by a factor of about 1010. This beta effect is larger than that of silicon and comparable to that of germanium. In contrast to these latter cases, the rate for 6 was found to increase with the nucleophilicity of the solvent. It is concluded that a molecule of solvent is coordinated with mercury in the transition state. This nucleophilic assistance raises the polarizability and nucleophilicity of the mercury group, enhancing its ability to stabilize the developing positive charge or to form a bridged intermediate.