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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

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Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
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By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
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Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
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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...
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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.
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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.
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Monitoring the Reductive and Oxidative Half-Reactions of a Flavin-Dependent Monooxygenase using Stopped-Flow Spectrophotometry
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Asymmetric catalysis by flavin-dependent halogenases.

Yuhua Jiang1, Jared C Lewis1

  • 1Department of Chemistry, Indiana University, Bloomington, Indiana, USA.

Chirality
|March 14, 2023
PubMed
Summary
This summary is machine-generated.

Flavin-dependent halogenases (FDHs) can be engineered for enantioselective halogenation, a novel application not seen in nature. The enzyme AetF shows high activity and selectivity in these asymmetric transformations.

Keywords:
atroposelectivedesymmetrizationhalocyclizationhalogenasekinetic resolution

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Area of Science:

  • Biocatalysis
  • Enzyme Engineering
  • Organic Chemistry

Background:

  • Flavin-dependent halogenases (FDHs) naturally perform site-selective halogenation of aromatic compounds.
  • FDHs are unique in their catalyst-controlled site selectivity, but enantioselective catalysis has not been observed in nature.
  • Previous research focused on engineering FDHs for selective chlorination, bromination, and iodination of aromatic compounds.

Purpose of the Study:

  • To establish and explore the potential of FDHs for enantioselective halogenation reactions.
  • To investigate FDH-catalyzed asymmetric transformations including desymmetrization, atroposelective halogenation, and halocyclization.
  • To optimize FDH activity and selectivity through mutagenesis and address challenges from hypohalous acid generation.

Main Methods:

  • Enzyme engineering and mutagenesis of FDHs.
  • Investigating FDH activity and selectivity in various asymmetric transformations.
  • Characterization of the single-component flavin reductase/FDH AetF.

Main Results:

  • FDHs were successfully engineered to catalyze enantioselective reactions like desymmetrization, atroposelective halogenation, and halocyclization.
  • Achieving high enantioselectivity required extensive mutagenesis and managing hypohalous acid byproducts.
  • The single-component FDH AetF demonstrated high activity and selectivity for several asymmetric transformations.

Conclusions:

  • FDHs can be engineered to perform enantioselective halogenations, expanding their synthetic utility beyond natural functions.
  • FDH active sites exhibit tolerance for diverse substrate structures, suggesting broad applicability in oxidative halogenations.
  • The enzyme AetF is a promising biocatalyst for asymmetric halogenation due to its high activity and selectivity.