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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...

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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.

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Dieckmann cyclization is an intramolecular Claisen condensation of diesters. The reaction occurs in the presence of a base and generates a cyclic β-ketoester as the final product. Commonly, 1, 6 and 1, 7-diesters are preferred substrates for the reaction since the generated five, and six-membered cyclic β-keto esters are particularly more stable.

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Selection Rules: Thermal Activation
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Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
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Thermal and Photochemical Electrocyclic Reactions: Overview

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

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Carbocationic n-endo-trig cyclizations.

Lei Shi¹, Markus Horn¹, Shinjiro Kobayashi¹

¹Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München (Germany), Fax: (+49) 89-2180-77717.

Chemistry (Weinheim an Der Bergstrasse, Germany)

|July 25, 2009

Summary

Unsaturated benzyl cations react with pi-nucleophiles similarly to saturated analogs. However, a specific unsaturated cation undergoes rapid cyclization, forming a bridged carbocation and influencing solvolysis reactions.

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Area of Science:

Organic Chemistry
Physical Chemistry

Background:

Unsaturated benzyl cations are reactive intermediates.
Understanding their reaction mechanisms is crucial for organic synthesis.

Purpose of the Study:

To investigate the reactivity of unsaturated benzyl cations with pi-nucleophiles.
To elucidate the role of pi-participation in carbocation cyclization and solvolysis reactions.

Main Methods:

Laser-flash photolysis was used to generate unsaturated benzyl cations.
Reactions with enol ethers and 2-methylfuran were studied.
Quantum-chemical calculations (B3LYP and MP2) were performed.

Main Results:

Intermolecular reactions of unsaturated cations with pi-nucleophiles followed second-order kinetics.
A specific unsaturated cation (1 d) underwent rapid 6-endo-trig cyclization, forming a bridged carbocation.
This cyclization was significantly faster than intermolecular reactions.

Conclusions:

Pi-participation plays a critical role in the cyclization of unsaturated benzyl cations.
The rapid cyclization of cation 1 d influences its reactivity in solvolysis.
Findings provide insights into the mechanisms of solvolysis reactions involving carbocations.