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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.
01:17Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:16Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)
Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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3,4-Ethylenedioxythiophene in planarizable push-pull oligothiophenes.
Marta Dal Molin1, Stefan Matile
1Department of Organic Chemistry, University of Geneva, Geneva, Switzerland.
Organic & Biomolecular Chemistry
|February 8, 2013
Summary
New fluorescent probes called push-pull quaterthiophene amphiphiles, featuring 3,4-ethylenedioxythiophene (EDOT), offer sensitive detection of lipid bilayer membrane fluidity and phase transitions. These probes show unique responses to changes in their structure and environment.
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Area of Science:
- Organic Chemistry
- Materials Science
- Biophysical Chemistry
Background:
- Planarizable push-pull oligothiophenes are innovative fluorescent probes for lipid bilayer membranes.
- These probes are designed to sense membrane fluidity and potential.
Purpose of the Study:
- To design, synthesize, and evaluate novel push-pull quaterthiophene amphiphiles with a 3,4-ethylenedioxythiophene (EDOT) unit.
- To investigate the response of these "hyper-twisted" EDOT probes to planarization and restricted rotational freedom.
- To assess their sensitivity in detecting lipid bilayer membrane properties, including fluidity, phase transitions, and potentials.
Main Methods:
- Synthesis of push-pull quaterthiophene amphiphiles incorporating EDOT.
- Spectroscopic analysis (excitation spectra) to observe shifts and vibrational fine structure changes.
- Evaluation of probe response to varying lipid bilayer membrane conditions (fluidity, potential).
Main Results:
- The "hyper-twisted" EDOT probes exhibit a red shift and changes in vibrational fine structure upon planarization and restricted rotation.
- These probes demonstrate high sensitivity to lipid bilayer membrane nature and fluidity, enabling ratiometric detection of phase transitions.
- While sensing of membrane potentials is somewhat diminished, it remains feasible.
Conclusions:
- Novel EDOT-containing push-pull quaterthiophene amphiphiles are effective fluorescent probes for lipid bilayers.
- The probes' sensitivity to membrane fluidity and phase transitions is significantly enhanced by their unique structural features.
- These findings pave the way for advanced molecular probes in biophysical studies.