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Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule02:17

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
16.1K
Halogenation of Alkenes02:46

Halogenation of Alkenes

18.3K
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.
18.3K
Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

4.1K
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...
4.1K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

4.7K
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...
4.7K
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

11.2K
Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
11.2K
Formation of Halohydrin from Alkenes02:41

Formation of Halohydrin from Alkenes

14.6K
An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.
14.6K

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Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS
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Evolved Aliphatic Halogenases Enable Regiocomplementary C-H Functionalization of a Pharmaceutically Relevant

Takahiro Hayashi1, Mathieu Ligibel2, Emine Sager2

  • 1Competence Center for Biocatalysis, Institute of Chemistry and Biotechnology, Zürich University of Applied Sciences, Einsiedlerstrasse 31, 8820, Wädenswil, Switzerland.

Angewandte Chemie (International Ed. in English)
|October 8, 2019
PubMed
Summary

Engineered a non-heme iron halogenase enzyme for enhanced chlorination of complex molecules. This breakthrough expands enzyme capabilities for precise late-stage halogenation in synthetic chemistry.

Keywords:
C−H activationbiocatalysisdirected evolutionhalogenasesnon-heme iron enzymes

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Area of Science:

  • Biocatalysis
  • Enzyme Engineering
  • Organic Chemistry

Background:

  • Non-heme iron halogenases are biocatalysts for selective C-H halogenation.
  • Current limitations include a narrow substrate scope, mainly natural products.

Purpose of the Study:

  • Engineer the halogenase WelO5* to broaden its substrate scope.
  • Improve catalytic efficiency for non-native substrates.

Main Methods:

  • Utilized structure-guided evolution for enzyme engineering.
  • Focused on chlorination of a martinelline-derived fragment.

Main Results:

  • Developed a WelO5* variant with >290-fold higher total turnover and 400-fold higher kcat.
  • Identified key active site residues for directing halogenation.
  • Demonstrated expanded substrate scope beyond native indole-alkaloid substrates.

Conclusions:

  • Enzyme engineering successfully broadened the substrate scope of non-heme iron halogenases.
  • WelO5* is a highly evolvable platform for late-stage halogenation.
  • This work enables new synthetic applications for biocatalytic halogenation.