Several substituted propyl tosylates with γ-silicon groups have been studied. For the 2,2 -dimet

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Several substituted propyl tosylates with γ-silicon groups...

Several substituted propyl tosylates with $\gamma$-silicon groups have been studied. For the 2,2 -dimethyl derivatives 24-B, the solvolysis rates are $10^{3}$ to $10^{4}$ greater than for the nonsilyl analogs. The products are rearranged 1,1 -dimethyl derivatives. The reaction shows modest sensitivity to substituents in the aryl group, correlating with a Hammett $\rho$ value of $-1.0$. When the parent system 24-A (without the 2,2-dimethyl substituents) was studied in the nonnucleophilic solvent $97 \%$ TFE, cyclopropane was formed, ranging in yield from $0 \%$ with $\mathrm{EWG}\left(\mathrm{CF}_{3}\right)$ to $100 \%$ with ERG $\left(\mathrm{CH}_{3} \mathrm{O}\right)$. The Hammett correlation gave $\rho=-1.1$ for cyclopropane formation but no significant substituent effect for substitution. Describe a mechanism that is consistent with this information. a. The stabilization of vinyl cations tends to be somewhat larger than for the corresponding ethyl cation. b. $\mathrm{F}, \mathrm{OH}$, and $\mathrm{NH}_{2}$ provide less stabilization of vinyl cations than of ethyl cations. c. $\mathrm{CF}_{3}$ and $\mathrm{CN}$ are less destabilizing of vinyl cations than of ethyl cations. d. What factors dominate the effect of the $\mathrm{CH}_{2}-\mathrm{X}$ substituents? e. Compare the $\pi$-donor and polar effects of the $\mathrm{OH}, \mathrm{NH}_{2}$, and $\mathrm{SH}$ substituents.

Several substituted propyl tosylates with $\gamma$-silicon groups have been studied. For the 2,2 -dimethyl derivatives 24-B, the solvolysis rates are $10^{3}$ to $10^{4}$ greater than for the nonsilyl analogs. The products are rearranged 1,1 -dimethyl derivatives. The reaction shows modest sensitivity to substituents in the aryl group, correlating with a Hammett $\rho$ value of $-1.0$. When the parent system 24-A (without the 2,2-dimethyl substituents) was studied in the nonnucleophilic solvent $97 \%$ TFE, cyclopropane was formed, ranging in yield from $0 \%$ with $\mathrm{EWG}\left(\mathrm{CF}_{3}\right)$ to $100 \%$ with ERG $\left(\mathrm{CH}_{3} \mathrm{O}\right)$. The Hammett correlation gave $\rho=-1.1$ for cyclopropane formation but no significant substituent effect for substitution. Describe a mechanism that is consistent with this information.
a. The stabilization of vinyl cations tends to be somewhat larger than for the corresponding ethyl cation.
b. $\mathrm{F}, \mathrm{OH}$, and $\mathrm{NH}_{2}$ provide less stabilization of vinyl cations than of ethyl cations.
c. $\mathrm{CF}_{3}$ and $\mathrm{CN}$ are less destabilizing of vinyl cations than of ethyl cations.
d. What factors dominate the effect of the $\mathrm{CH}_{2}-\mathrm{X}$ substituents?
e. Compare the $\pi$-donor and polar effects of the $\mathrm{OH}, \mathrm{NH}_{2}$, and $\mathrm{SH}$ substituents.

Key Concepts

Cyclopropane Formation Mechanism

The formation of cyclopropane rings in solvolysis reactions can be explained by an intramolecular nucleophilic attack on a carbocationic center, leading to ring closure. This process is often sensitive to substituent electronic effects, as indicated by the variable yields observed with different aryl substituents and the corresponding Hammett correlations, emphasizing the interplay between cation stabilization and ring strain release.

Hammett Correlation and Substituent Effects

The Hammett equation is a quantitative tool used to relate the effects of substituents on reaction rates and equilibria, relying on the sigma constants. A negative ? value in this context indicates that electron?donating substituents accelerate the reaction, supportive of a mechanism involving a positively charged transition state. This is critical in understanding how different aryl substituents influence the solvolysis and rearrangement behavior.

Neighboring Group Participation (Anchimeric Assistance)

This key concept involves the assistance of a neighboring group in stabilizing an intermediate or transition state during a reaction. For the systems described, the gamma silicon group acts as such a neighboring group, stabilizing developing positive charge and facilitating rearrangements or cyclizations, such as cyclopropane formation under the appropriate conditions.

Beta?Silicon Effect

This concept refers to the stabilization that silicon atoms can impart on adjacent carbocations through hyperconjugation and through d-?–? or ?–? interactions. In reactions where a silicon substituent is positioned suitably (often at the ??position), these interactions can lead to enhanced stabilization of a positive charge, thereby accelerating solvolysis rates and enabling unusual rearrangements.

Carbocation Rearrangement and Stabilization

Carbocation rearrangements involve shifts or migrations of groups to form a more stable carbocation intermediate, which often occur during solvolysis reactions. In the context of these tosylate systems, the formation of rearranged 1,1?dimethyl derivatives reflects the tendency of the intermediate to achieve greater stabilization, mediated by effects such as hyperconjugation and inductive stabilization.

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