The question of vertical or nonvertical participation of silicon .beta. to a cation in the antiperiplanar stereochemistry

Abstract

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...

The β‐silicon effect. 4: substituent effects on the solvolysis of 1‐alkyl‐2‐(aryldimethylsilyl)ethyl trifluoroacetates

“…The vertical ( L ) and the non‐vertical ( E ) mechanisms can be distinguished by the magnitudes of ρ . It is to be noted that Lambert originally proposed the vertical and non‐vertical mechanisms on finding β ‐silyl participation in the secondary systems (especially, cyclic system) 2–4…”

“…The so‐called β ‐silicon effect is referred to as the most powerful participating mechanism in the solvolytic reactions. The β ‐silicon effect has been elucidated in terms of either the Lambert (vertical) pathway via the Lambert intermediate ( 1L + ) of open cation form that is stabilized by σ ‐ π hyperconjugation or the Eaborn (non‐vertical) pathway via the Eaborn intermediate ( 1E + ) of a Si‐bridged cation form (Scheme ) 2–7…”

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

“…[64] Solvolysis of a series of constrained trifluoroacetate esters indicates that ionization is considerably faster for the silylsubstituted ester 54 than for the sulfide 52 or selenide 53 (Scheme 14). Ionization of the silyl ester 54 involves only hyperconjugation, [65] but the sulfide and selenide could benefit from anchimeric assistance. The lone pairs of sulfur and selenium, however, do not accelerate ionization.…”

Mechanism of β-Silyl Diacyl Peroxide Decomposition: A Mild and Stereoselective Synthesis of β-Silyl Esters

“…This procedure may find use for the in situ preparation of substrates used in the study of -silyl cations. [23][24][25][26] …”