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Carbocations

Carbocations

Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...

Cycloaddition Reactions: Overview

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Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

Cycloaddition Reactions: MO Requirements for Thermal Activation

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

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Radicals: Electronic Structure and Geometry

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

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Gamma-silyl cyclobutyl carbocations.

Xavier Creary¹, Elizabeth D Kochly

¹Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA. creary.1@nd.edu

The Journal of Organic Chemistry

|December 3, 2009

Summary

Gamma-silyl participation significantly enhances reactions in cyclobutyl trifluoroacetates. This effect, observed in solvolysis studies, stabilizes carbocations, with the secondary gamma-trimethylsilylcyclobutyl cation proving more stable than its beta-isomer.

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Area of Science:

Organic Chemistry
Computational Chemistry
Reaction Mechanisms

Background:

Investigating the influence of silicon substituents on carbocation stability and reactivity.
Understanding neighboring group participation in solvolysis reactions.

Purpose of the Study:

To explore the phenomenon of gamma-silyl participation in cyclobutyl systems.
To quantify the rate enhancements associated with this effect.
To elucidate the stereochemical outcomes and mechanistic pathways.

Main Methods:

Synthesis of 3-trimethylsilyl-1-substituted cyclobutyl trifluoroacetates.
Solvolysis reactions conducted in CD(3)CO(2)D with rate data collection.
Computational studies using B3LYP/6-31G* level of theory.

Main Results:

Significant rate enhancements observed due to gamma-silyl participation, particularly for the unsubstituted cation (>10^10).
Acetate substitution products formed with net retention of stereochemistry.
Computational and experimental data support the involvement of carbocations stabilized by the rear lobe of the gamma-Si-C bond.

Conclusions:

Gamma-silyl participation is a potent stabilizing effect in cyclobutyl systems.
The secondary gamma-trimethylsilylcyclobutyl cation is more stable than the beta-isomer, indicating the gamma-silyl effect can outweigh the beta-silyl effect.
Evidence for front-lobe stabilized cations is less compelling compared to rear-lobe interactions.