Pericyclic reactions in an aqueous molecular flask
Takashi Murase1, Makoto Fujita
1Department of Applied Chemistry, School of Engineering, The University of Tokyo, and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
Chemical Record (New York, N.Y.)
|September 21, 2010
Summary
Molecular flasks create unique nano-environments for reactions. These self-assembled cages enhance reactivity and control selectivity for challenging cycloadditions, enabling new synthetic pathways.
Area of Science:
- Supramolecular Chemistry
- Organic Synthesis
- Nanotechnology
Background:
- Traditional organic reactions in solution face limitations in reactivity and selectivity.
- Developing novel reaction environments is crucial for accessing challenging chemical transformations.
- Self-assembled supramolecular structures offer unique platforms for molecular recognition and catalysis.
Purpose of the Study:
- To investigate the utility of a self-assembled molecular flask as a confined reaction environment.
- To explore the performance of Diels-Alder reactions and [2+2] photoadditions within the nanometer-sized cavity.
- To understand how the cage structure influences substrate organization, reactivity, and selectivity.
Main Methods:
- Construction of a self-assembled molecular flask with a restricted nanometer-sized cavity.
- Encapsulation of hydrophobic substrate pairs within the molecular flask.
- Performing Diels-Alder reactions and [2+2] photoadditions of aromatic molecules inside the cage.
- Utilizing chiral auxiliaries to induce asymmetry within the confined environment.
Main Results:
- The molecular flask facilitated unusual Diels-Alder reactions and [2+2] photoadditions of unreactive aromatic molecules.
- Naphthalene underwent Diels-Alder reactions smoothly under mild conditions within the cage.
- Aceanthrylene exhibited reactivity for both [2+2] and [2+4] cycloadditions via a ternary host-guest complex.
- Enhanced reactivity was attributed to increased local concentration and pre-organization, mimicking intramolecular reactions.
- Controlled regio- and stereo-selectivities were achieved due to reinforced substrate orientation.
- Asymmetric induction up to 50% enantiomeric excess (ee) was observed using external chiral auxiliaries.
Conclusions:
- Self-assembled molecular flasks provide a powerful platform for novel organic reactions.
- The confined environment significantly enhances substrate reactivity and controls reaction selectivity.
- This approach offers a promising strategy for developing new synthetic methodologies and asymmetric catalysis.
Related Concept Videos
01:17Pericyclic Reactions: Introduction
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.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are...
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are...
01:16Cycloaddition Reactions: Overview
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.
01:16Cycloaddition Reactions: MO Requirements for Thermal Activation
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.
01:34Woodward–Hoffmann Selection Rules and Microscopic Reversibility
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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.
01:17Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.