Photochemical proton-coupled C-H activation: an example using aliphatic fluorination.
Mackenzie J Field1, Soumalya Sinha1, Jeffrey J Warren2
1Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. j.warren@sfu.ca.
Physical Chemistry Chemical Physics : PCCP
|November 8, 2016
Summary
This study reveals that photochemical C-H activation occurs efficiently when excited sensitizers accept hydrogen radicals (H˙). This finding advances understanding of proton-coupled electron transfer (PCET) mechanisms in fluorination reactions.
Area of Science:
- Organic Chemistry
- Photochemistry
- Reaction Mechanisms
Background:
- Selective functionalization of unactivated C-H bonds remains a significant challenge in chemistry.
- Proton-coupled electron transfer (PCET) reactions, involving the transfer of H+ and e-, are crucial for C-H activation.
- Photochemical PCET reactions offer promising routes for synthesizing novel compounds, particularly in C-H fluorination.
Purpose of the Study:
- To investigate the mechanisms of electron transfer (ET) and PCET reactions involving excited 1,2,4,5-tetracyanobenzene and anthraquinone in fluorination reactions.
- To elucidate the role of excited sensitizers in C-H activation processes.
- To systematically define discrete PCET mechanisms in photochemical C-H fluorination.
Main Methods:
- Utilized kinetic and thermodynamic modeling.
- Employed steady-state and time-resolved fluorescence spectroscopy.
- Investigated reactions of electronically excited 1,2,4,5-tetracyanobenzene and anthraquinone with fluorination reagents.
Main Results:
- Analysis suggests C-H activation proceeds efficiently.
- Electron transfer and PCET pathways were examined.
- The study identified that excited sensitizers accepting hydrogen radicals (H˙) facilitates C-H activation.
Conclusions:
- Photochemical C-H activation is effectively mediated by excited sensitizers accepting H˙.
- This work provides insights into the mechanistic pathways of photochemical C-H fluorination.
- Understanding these PCET mechanisms is key for developing new synthetic methodologies.
More Related Videos
Related Concept Videos
01:21Acid-Catalyzed α-Halogenation of Aldehydes and Ketones
5.1K
By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
5.1K
01:13Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene
7.8K
Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
7.8K
02:46Halogenation of Alkenes
20.7K
Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
20.7K
00:51Base-Promoted α-Halogenation of Aldehydes and Ketones
4.3K
α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base. The reaction begins with the abstraction of α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...
4.3K
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
2.8K
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.
2.8K
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
9.3K
Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
9.3K