Deoxymetalation reactions.the mechanisms of deoxysilylation of mono-trimethylsilyl-and bis-trimethylsilyl-substituted alcohols and a comparison to the mechanism of deoxystannylation and deoxyplumbylation

Generality and Strength of Transition Metal β-Effects

Using computation, we examine the generality and strength of β-effects from transition metal centers on β-elimination. In particular, we find that a β-Pd(II) substituent imparts over twice the stabilization to a carbocation as a Si substituent, representative of the well-known β-silicon effect. We established efficient and practical computational parameters to investigate the σσ conjugation in an experimentally relevant system: N, N-picolinamide vinyl metalacycles with β-substituents that can undergo elimination. We have found that the β-Pd effect depends on the nature of the C_β substituent (X): This effect is negligible for X = H, Me, OH, and F, but is significant for X = Cl, Br, and I. We have also extended these studies to the β-effect in N, N-picolinamide vinyl metalacycles with β-substituents of other transition metals-Fe(II), Ru(II), Os(II), Co(III), Rh(III), Ir(III), Ni(II), Pd(II), Pt(II), Cu(III), Ag(III), and Au(III). We found that the electronegativity of the metals correlates reasonably well with the relative β-effects, with first-row transition metals exerting the strongest influence. Overall, it is our anticipation that a more profound appreciation of transition metal β-effects will facilitate the design of novel reactions, including new variants of transition metal catalyzed C-H functionalization.

3-Trimethylsilylcycloalkylidenes. γ-Silyl vs γ-Hydrogen Migration to Carbene Centers

A series of γ-trimethylsilyl-substituted carbenes have been studied experimentally and by computational methods. In an acyclic system, 1,3-trimethylsilyl migration successfully competes with 1,3-hydrogen migration to the carbene center. The behavior of cyclic 3-trimethylsilyl-substituted carbenes contrasts with that of the acyclic system. Only 1,2-hydrogen migration processes are observed in the five-membered ring due to the high barrier to 1,3-hydrogen migration. In the cyclohexyl system, a small amount of a cyclopropane derived from 1,3-hydrogen migration occurs, as shown by a labeling study. In the cycloheptyl carbene system, a labeling study again showed that 1,3-hydrogen migration to the carbene center leads to the major product. Computational studies suggest that the cyclic carbenes all have lower energy conformations where the trimethylsilyl group is in a pseudo equatorial conformation where it cannot migrate to the carbene center. Computational studies also suggest that cyclohexyl and cycloheptyl carbene systems are slightly stabilized by a rear lobe interaction of the Si-C bond with the carbene center.

γ-Trimethylsilylcyclobutyl carbocation stabilization

A series of isomeric 3-trimethylsilyl-1-arylcyclobutyl carbocations, 10 and 11, where the cross-ring 3-trimethylsilyl group has the potential to interact with the cationic center, have been generated under solvolytic conditions. When the cationic center can interact with the rear lobe of the carbon-silicon bond, rate enhancements become progressively larger as the substituent on the aryl group becomes more electron-withdrawing. When the potential interaction with the trimethylsilyl group is via a front lobe interaction, there is minimal rate enhancement over the range of substituents. Computational studies have also been carried out on these cations 10 and 11. Calculated trimethylsilyl stabilization energies progressively increase with electron-withdrawing character of the aryl groups when the trimethylsilyl interaction is via the rear lobe. By way of contrast, there are minimal changes in stabilization energies when the potential trimethylsilyl interaction is via the front lobe of the carbon-silicon bond. These computational studies, along with the solvolytic studies, point to a significant rear lobe 3-trimethylsilyl stabilization of arylcyclobutyl cations. They also argue against any front lobe stabilization of the isomeric arylcyclobutyl cations.

γ-Silyl-substituted norbornyl carbocations and carbenes

endo-2-Trimethylsilyl-anti-7-norbornyl triflate undergoes solvolysis reactions 1.8 × 10(4) faster than 7-norbornyl triflate in CD3CO2D and 1.3 × 10(5) times faster in CF3CH2OH. The exclusive substitution products with retained stereochemistry point to a significantly stabilized γ-trimethylsilyl carbocation intermediate. The endo-2-trimethylsilyl-7-norbornyl carbene gives a major rearrangement product where the trimethylsilyl-activated hydrogen migrates to the carbenic center. This rearrangement product implies stabilization of the carbene by the γ-trimethylsilyl group. Isodesmic computational studies (M062X/6-311+G**) indicate that the endo-2-trimethylsilyl-7-norbornyl cation is stabilized by 16.2 kcal/mol and that the endo-2-trimethylsilyl-7-norbornyl carbene is stabilized by a smaller factor of 1.8 kcal/mol. By way of contrast, anti-7-trimethylsilyl-endo-2-norbornyl mesylate undergoes solvolysis in CD3CO2D only 2.6 times faster than endo-2-norbornyl mesylate and 9.4 times faster in CF3CH2OH. The substitution products have only partially retained stereochemistry, and significant rearrangements are observed. The anti-7-trimethylsilyl-2-norbornyl carbene gives a rearrangement product via 1,3-hydrogen migration of the C6 hydrogen, which is completely analogous to the behavior of the unsubstituted 2-norbornyl carbene. Isodesmic calculations show that the anti-7-trimethylsilyl-2-norbornyl cation is stabilized by only 3.2 kcal/mol relative to the 2-norbornyl cation, and the corresponding anti-7-trimethylsilyl-2-norbornyl carbene is stabilized by a negligible 0.9 kcal/mol.

3-Trimethylsilylcyclobutylidene. The γ-effect of silicon on carbenes

3-Trimethylsilylcyclobutylidene was generated by pyrolysis of the sodium salt of the tosylhydrazone derivative of 3-trimethylsilylcyclobutanone. This carbene converts to 1-trimethylsilylbicyclobutane as the major product. A labeling study shows that this intramolecular rearrangement product comes from 1,3-hydrogen migration to the carbenic center and not 1,3-silyl migration. Computational studies show two carbene minimum energy conformations, with the lower energy conformation displaying a large stabilizing interaction of the carbene center with the rear lobe of the C3-Si bond. In this conformation, the trimethylsilyl group cannot migrate to the carbene center, and the most favorable process is 1,3-hydrogen migration. When the carbene is generated photochemically in methanol, it reacts by a protonation mechanism giving the highly stabilized 3-trimethylsilylcyclobutyl carbocation as an intermediate. When generated in dimethylamine as solvent, the carbene undergoes preferred attack of this nucleophilic solvent from the back of this C-Si rear lobe stabilized carbene.

Gamma-silyl cyclobutyl carbocations

A series of 3-trimethylsilyl-1-substituted cyclobutyl trifluoroacetates have been prepared and reacted in CD(3)CO(2)D. Rate data indicate that the substrates with the trimethylsilyl group cis to the leaving group react with assistance due to gamma-silyl participation. Rate enhancements range from a factor of 209 for alpha-phenyl-substituted cations to 4.6 x 10(4) for alpha-methyl-substituted cations to >10(10) for the unsubstituted gamma-trimethylsilylcyclobutyl cation. Acetate substitution products are formed with net retention of stereochemistry. These experimental studies, as well as B3LYP/6-31G* computational studies, are consistent with the involvement of carbocations where the rear lobe of the gamma-Si-C bond interacts strongly with the developing cationic center. Solvolytic rate studies, as well as computational studies, suggest that the secondary gamma-trimethylsilylcyclobutyl cation is even more stable than the beta-trimethylsilylcyclobutyl cation, i.e., the gamma-silyl effect actually outweighs the potent beta-silyl effect. Although computational studies suggest the existence of certain isomeric cations, where the front lobe of the Si-C bond interacts with the cationic center, solvolytic evidence for the involvement of these front lobe stabilized cations is less compelling.